Speaker 1: Your digestive system is, in the quietest and most literal sense,
Speaker 1: a tube. A long, winding, astonishingly complex tube that runs
Speaker 1: from your mouth all the way to the other end
Speaker 1: of you. If you were to take that tube and
Speaker 1: stretch it out flat, it would be roughly nine meters long.
Speaker 1: Nine meters. That is about 30 feet of biology. Coiled
Speaker 1: and folded inside a space. No bigger than your torso.
Speaker 1: And inside that tube, right now, something is moving. Digestion
Speaker 1: does not wait for you to be awake. It does
Speaker 1: not pause when you sleep. It does not take a holiday.
Speaker 1: from the moment you were born until the moment your
Speaker 1: life ends, your digestive system is always working, always squeezing,
Speaker 1: always churning, always quietly transforming whatever you give it into
Speaker 1: something your body can use. There is something almost meditative
Speaker 1: about that. A system so faithful, It never asks for
Speaker 1: a break. Let us start at the very beginning. Before
Speaker 1: food ever reaches your stomach, before it travels anywhere at all,
Speaker 1: the process of digestion has already started. It starts in
Speaker 1: your mouth. And it starts, remarkably, before you even take
Speaker 1: a bite. The moment you smell something cooking, or see
Speaker 1: a plate of food across the room, or even just
Speaker 1: think about eating, your brain sends a signal, and your
Speaker 1: mouth begins to water. That watering is not just a reflex.
Speaker 1: It is preparation. Your body is getting ready. Saliva is
Speaker 1: being produced. In small glands tucked beneath your tongue and
Speaker 1: along the inside of your jaw, and it is flowing gently, softly,
Speaker 1: into the space of your mouth, you produce somewhere between
Speaker 1: one liter and one and a half liters of saliva
Speaker 1: every single day. That is quietly remarkable. All of it
Speaker 1: made by your body. All of it. serving a purpose.
Speaker 1: Saliva is not simply water. It is a carefully assembled
Speaker 1: liquid containing enzymes, proteins, minerals, and a compound called amylase,
Speaker 1: which begins breaking down carbohydrates the instant they touch your tongue.
Speaker 1: So that piece of bread you chewed this morning, the
Speaker 1: digestion of it In the very first second, it was
Speaker 1: in your mouth. The stomach had not even been involved yet.
Speaker 1: It was your saliva doing the first quiet work. Chewing,
Speaker 1: which we tend to rush through without thinking, is also
Speaker 1: far more important than it might seem. Every time you chew,
Speaker 1: you are doing something your stomach cannot do. Your stomach
Speaker 1: can churn and squeeze and mix food with acid, but
Speaker 1: it cannot grind. That grinding, that mechanical breaking down of
Speaker 1: food into smaller and smaller pieces, is the job of
Speaker 1: your teeth and your jaw. And the more thoroughly you chew,
Speaker 1: the less work everything downstream has to do. Your digestive
Speaker 1: system is, in a quiet way, grateful when you chew slowly.
Speaker 1: Once food is chewed, moistened with saliva, and shaped by
Speaker 1: your tongue into a soft little bundle, it is called
Speaker 1: a bolus. That word comes from a Greek word meaning lump,
Speaker 1: or clod. Not the most elegant name, but that small,
Speaker 1: soft lump is what you swallow. And swallowing, when you
Speaker 1: stop and think about it, is one of the most
Speaker 1: coordinated acts your body performs. In the fraction of a
Speaker 1: second it takes to swallow, over 30 muscles work in
Speaker 1: perfect sequence. Your tongue pushes the bolus to the back
Speaker 1: of your throat. A small flap of tissue called the
Speaker 1: epiglottis folds down over your windpipe to make absolutely sure
Speaker 1: the food does not go the wrong way. Your throat
Speaker 1: contracts in a wave and the food slides smoothly and
Speaker 1: efficiently into the tube. that connects your mouth to your stomach.
Speaker 1: That tube is called the oesophagus. The oesophagus is roughly
Speaker 1: 25 centimeters long. It sits just behind your windpipe, tucked
Speaker 1: in the chest between your heart and your spine. And
Speaker 1: it does something that surprises many people. When they first
Speaker 1: learn it, It does not simply drop food downward with gravity.
Speaker 1: It actively squeezes food toward the stomach using rhythmic muscular waves.
Speaker 1: These waves are called peristalsis. And peristalsis is the fundamental
Speaker 1: movement of your entire digestive system. Peristalsis happens whether you
Speaker 1: are standing, sitting, or lying completely flat. Astronauts in orbit
Speaker 1: can eat and swallow normally, because their oesophagus does not
Speaker 1: rely on gravity at all. The food moves because of muscle,
Speaker 1: because of rhythm, because of a quiet and reliable biological
Speaker 1: wave that has been moving through you your whole life.
Speaker 1: The journey from your mouth to your stomach takes only
Speaker 1: a few seconds. Just 5 to 10 seconds for a
Speaker 1: soft piece of food, up to about 30 seconds for
Speaker 1: something harder and drier. And then the food arrives at
Speaker 1: a remarkable muscle called the lower esophageal sphincter. A sphincter
Speaker 1: is a ring of muscle that can tighten and relax
Speaker 1: on command. And this one is the gatekeeper of your stomach.
Speaker 1: It stays closed most of the time. It opens only
Speaker 1: to let food in, and then closes firmly again, to
Speaker 1: keep the stomach's contents from flowing back upward. When it
Speaker 1: does not close firmly enough, you get what we call heartburn.
Speaker 1: not a heart problem at all. Just that gatekeeper, not
Speaker 1: quite doing its job. And then there is the stomach itself.
Speaker 1: Most people picture the stomach as a large, hollow bag
Speaker 1: somewhere in the middle of the abdomen. But when it
Speaker 1: is empty, the stomach is actually not much bigger than
Speaker 1: your fist. It collapses into folds, like a deflated balloon,
Speaker 1: with walls layered and ridged, waiting. And when food arrives,
Speaker 1: it expands. The stomach can stretch to hold about one
Speaker 1: litre comfortably and up to four litres. If you push it,
Speaker 1: four litres. That is roughly the same as four large
Speaker 1: bottles of water. fitting inside that one small space. The
Speaker 1: stomach is, in its own quiet way, one of the
Speaker 1: most chemically aggressive environments in the human body. The lining
Speaker 1: of your stomach produces hydrochloric acid, the same kind of
Speaker 1: acid used in some industrial cleaning processes. It is strong
Speaker 1: enough to dissolve metal, if given enough time. And it sits,
Speaker 1: right now, inside you. The acid kills most of the
Speaker 1: bacteria that enter your digestive system with food. It unravels proteins.
Speaker 1: It activates enzymes. It begins breaking food down into a
Speaker 1: semi-liquid mixture called chyme. You might wonder quite reasonably, why
Speaker 1: your stomach does not simply digest itself. And the answer
Speaker 1: is that it tries to. Every few days, the entire
Speaker 1: lining of your stomach renews itself. New cells replace the
Speaker 1: old ones before the acid can eat through. The stomach
Speaker 1: is in a constant quiet war with its own contents,
Speaker 1: and it wins by shedding and regrowing faster than it
Speaker 1: can be damaged. It is, in the most literal sense,
Speaker 1: continuously remaking itself. Your stomach also does something beyond chemistry.
Speaker 1: It churns. The walls of your stomach contain three layers
Speaker 1: of muscle arranged in different directions, and they squeeze and
Speaker 1: mix and roll the food from all angles simultaneously. This
Speaker 1: mechanical mixing turns solid food into that soft, acidic, partially
Speaker 1: digested liquid, the chyme, which is then slowly released into
Speaker 1: the next part of the journey. The stomach usually empties itself,
Speaker 1: Over the course of two to four hours, depending on
Speaker 1: what you ate, a light meal passes through quickly. A heavy,
Speaker 1: fatty meal stays longer. The stomach is deliberate. It does
Speaker 1: not rush. The exit from the stomach leads into the
Speaker 1: small intestine. And here the word small is almost comically misleading.
Speaker 1: The small intestine is called small only because it is
Speaker 1: narrow in diameter, not because it is short. In length,
Speaker 1: the small intestine is the longest part of your digestive system.
Speaker 1: It is somewhere between 6 and 7 meters long. 6
Speaker 1: to 7 meters. folded back and forth, coiled upon itself,
Speaker 1: packed into your abdomen, in loops and curves. If you
Speaker 1: unraveled it, it would stretch the length of a small room.
Speaker 1: The small intestine is where the real work of digestion happens.
Speaker 1: By the time food leaves the stomach, it is partially
Speaker 1: broken down, But it is in the small intestine that
Speaker 1: the useful parts are actually separated out and absorbed into
Speaker 1: your body. Nutrients, vitamins, minerals, glucose, amino acids, fatty acids.
Speaker 1: All of them pulled through the wall of the small
Speaker 1: intestine and into the bloodstream. What enters the small intestine
Speaker 1: is food. What leaves it going into the blood is you.
Speaker 1: To absorb nutrients as efficiently as possible, the small intestine
Speaker 1: has a surface area that would stagger you if you
Speaker 1: saw it. The inside wall of the small intestine is
Speaker 1: not smooth. It is covered in tiny finger-like projections, called villi.
Speaker 1: And each villus is covered in even tinier projections, called microvilli.
Speaker 1: So many layers of surface. So many folds within folds.
Speaker 1: When you account for all of them, the total surface
Speaker 1: area of the inside of your small intestine is roughly
Speaker 1: the size of a tennis court. A tennis court, folded
Speaker 1: inside your abdomen. These villi are not decorative. Each one
Speaker 1: contains a network of tiny blood vessels, and a special
Speaker 1: channel for carrying fat. When nutrients pass through the wall
Speaker 1: of the villus, they step directly into this network, and
Speaker 1: are carried away in the blood, to wherever the body
Speaker 1: needs them most. To your liver first, where they are
Speaker 1: processed and sorted, then out through the body, to your muscles,
Speaker 1: your organs, your brain. The meal you ate hours ago
Speaker 1: is already becoming part of your thoughts, your movements, your heartbeat,
Speaker 1: The small intestine also receives help from two other organs
Speaker 1: that live just nearby. The pancreas and the liver. The
Speaker 1: pancreas is a long, soft organ tucked behind the stomach.
Speaker 1: It produces a remarkable fluid containing digestive enzymes that flow
Speaker 1: into the small intestine. through a small duct. These enzymes
Speaker 1: break down proteins, fats, and carbohydrates that the stomach's acid
Speaker 1: could not fully dissolve. The pancreas also neutralizes the acid
Speaker 1: that arrives from the stomach, making the environment of the
Speaker 1: small intestine far gentler, safer for the delicate wall's doing
Speaker 1: the absorbing. The liver is the largest internal organ in
Speaker 1: your body. It sits beneath your right ribcage, weighing about
Speaker 1: one and a half kilograms, dense and dark red. The
Speaker 1: liver performs over 500 different functions. 500. Among them, it
Speaker 1: produces bile, Bile is a greenish-yellow fluid that is stored
Speaker 1: in a small pouch beneath the liver called the gallbladder.
Speaker 1: And when fat enters the small intestine, the gallbladder squeezes,
Speaker 1: releasing bile into the intestine to help break the fat
Speaker 1: apart into tiny droplets that enzymes can then digest and
Speaker 1: Bile is why your stools have color. Without it, they
Speaker 1: would be pale and gray. Even something so ordinary has
Speaker 1: a story behind it. After the small intestine has done
Speaker 1: its work, what remains continues onward. And what remains at
Speaker 1: this point is not much that your body can use. water,
Speaker 1: fiber that cannot be digested, some dead cells shed from
Speaker 1: the intestinal lining, and a vast living community of microorganisms.
Speaker 1: All of this passes into the large intestine, which is,
Speaker 1: despite its name, shorter than the small intestine. The large
Speaker 1: intestine is only about one and a half meters long.
Speaker 1: But it is wider, and its job is different. The
Speaker 1: large intestine's primary task is to absorb water. The chyme
Speaker 1: that arrives from the small intestine is still quite liquid.
Speaker 1: As it moves slowly through the large intestine, water is
Speaker 1: pulled back into the bloodstream. What was liquid becomes softer,
Speaker 1: then firmer, then firmer still, as more and more water
Speaker 1: is reclaimed. The large intestine is efficient in this way.
Speaker 1: It recycles, it takes back what the body gave, and
Speaker 1: returns it to circulation. the large intestine is also home
Speaker 1: to a remarkable population of organisms. Roughly 100 trillion bacteria.
Speaker 1: live in your gut. 100 trillion. That number is so large,
Speaker 1: it is almost meaningless when spoken aloud. But imagine this.
Speaker 1: Your body contains roughly 37 trillion human cells. And the
Speaker 1: number of bacteria in your gut alone outnumbers those cells
Speaker 1: by nearly three to one. You are, in a very
Speaker 1: real numerical sense, more microorganism than human. These bacteria are
Speaker 1: not invaders. They are residents. Partners. They have evolved alongside
Speaker 1: humans for millions of years, and both sides of that
Speaker 1: relationship have come to depend on the other. The bacteria
Speaker 1: help digest things. Your own enzymes cannot break down. Certain
Speaker 1: types of fiber, for example, pass through the small intestine
Speaker 1: untouched and arrive in the large intestine intact. And there,
Speaker 1: the bacteria feast on them, fermenting them, and producing short-chain
Speaker 1: fatty acids that feed the cells of the colon wall.
Speaker 1: They are, in a sense, farming themselves a meal out
Speaker 1: of your leftovers. The bacteria in your gut also produce vitamins.
Speaker 1: Vitamin K, which is essential for blood clotting, is partly
Speaker 1: synthesized by gut bacteria. So is a type of vitamin B,
Speaker 1: called biotin. Your gut microbes are, among other things, a
Speaker 1: tiny internal pharmacy, quietly producing things your body needs. Without them,
Speaker 1: you would be less than you are. With them, you
Speaker 1: are more. The collection of all these microorganisms is called
Speaker 1: the gut microbiome. And in recent decades, scientists have begun
Speaker 1: to understand just how influential it is. Your gut microbiome
Speaker 1: affects your immune system. It influences how your body responds
Speaker 1: to inflammation. It may affect your mood. It may even
Speaker 1: play a role in how clearly you think. The gut
Speaker 1: and the brain are connected by a long pathway called
Speaker 1: the vagus nerve and signals flow in both directions along it.
Speaker 1: What happens in your gut is never entirely separate from
Speaker 1: what happens in your mind. This connection between the gut
Speaker 1: and the brain is deep and ancient. Your gut contains
Speaker 1: its own nervous system. It is called the enteric nervous system,
Speaker 1: and it contains roughly 100 million nerve cells. 100 million.
Speaker 1: More than the spinal cord. The enteric nervous system can
Speaker 1: operate independently, without any instruction from the brain. It is
Speaker 1: sometimes called the second brain. And it is always listening,
Speaker 1: always adjusting, always reading the environment inside you, and responding.
Speaker 1: Have you ever felt butterflies in your stomach when you
Speaker 1: were nervous? That is the gut-brain connection in action. Stress
Speaker 1: signals from the brain travel down the vagus nerve and
Speaker 1: cause the gut to change its behavior. Blood flow shifts
Speaker 1: away from digestion. Muscle contractions change. The gut, in a
Speaker 1: state of anxiety, behaves differently than the gut at rest.
Speaker 1: And signals travel the other way too. The state of
Speaker 1: your gut influences your emotional state. A healthy, thriving microbiome
Speaker 1: has been associated with better mood, lower anxiety, and calmer
Speaker 1: responses to stress. Your gut is, in a very real way,
Speaker 1: feeling things. About 95% of the body's serotonin is produced
Speaker 1: in the gut. Not in the brain. In the gut.
Speaker 1: Serotonin is the neurotransmitter most associated with feelings of well-being
Speaker 1: and calm. The fact that the majority of it is
Speaker 1: made in your digestive system says something quietly profound. about
Speaker 1: the relationship between how you eat, how you live, and
Speaker 1: how you feel. Your gut is not just digesting your food.
Speaker 1: It is, in some measure, shaping your experience of being alive.
Speaker 1: Let us return to the journey. The large intestine moves
Speaker 1: material slowly, Slowly is the right word. The small intestine
Speaker 1: moves food relatively quickly, but the large intestine takes its time.
Speaker 1: It can take anywhere from 10 hours to 50 hours
Speaker 1: for material to travel the length of the large intestine.
Speaker 1: 50 hours. The food you ate yesterday or perhaps the
Speaker 1: day before may still be completing its journey inside you
Speaker 1: right now. The large intestine is divided into sections. It
Speaker 1: begins with the cacum, a small pouch where the small
Speaker 1: intestine connects. Attached to the cacum is the appendix. For
Speaker 1: a long time, the appendix was dismissed as a vestigial organ,
Speaker 1: a leftover from an earlier stage of evolution with no
Speaker 1: meaningful function. But more recent thinking suggests it may play
Speaker 1: a role as a kind of reservoir for beneficial bacteria,
Speaker 1: a backup supply. If the gut is severely disrupted, perhaps
Speaker 1: by illness or infection, the appendix may help repopulate it
Speaker 1: with the right microorganisms. An organ once thought useless quietly
Speaker 1: turned out to have a purpose after all. From the caecum,
Speaker 1: the large intestine rises on the right side of your body,
Speaker 1: crosses over to the left, and descends again. This path
Speaker 1: is called the ascending colon, the transverse colon, and the
Speaker 1: descending colon. It ends in the sigmoid colon, a gentle-shaped
Speaker 1: curve that leads to the rectum. The rectum holds waste
Speaker 1: until the body is ready. And then, in the most
Speaker 1: uncelebrated step of the entire digestive process, The waste exits
Speaker 1: the body. The whole journey from first bite to final
Speaker 1: exit takes somewhere between 24 and 72 hours on average.
Speaker 1: Your body has been quietly running this process every day
Speaker 1: of your entire life. But let us go back because
Speaker 1: there is so much more To say about the small intestine.
Speaker 1: About the way it coordinates with the rest of the body.
Speaker 1: About the hormones it releases. About the way it knows, somehow,
Speaker 1: what it is receiving and what to do about it.
Speaker 1: Your small intestine produces hormones. This surprises many people. who
Speaker 1: think of hormones as something produced only by glands. But
Speaker 1: the digestive system is one of the largest hormone-producing systems
Speaker 1: in the body. When food enters the small intestine, specialized
Speaker 1: cells in the lining detect what kind of food it is.
Speaker 1: Is it fatty? Sugary? Full of protein? Based on what
Speaker 1: it detects, the small intestine releases different hormones to orchestrate
Speaker 1: the response. Cholecystokinin tells the gallbladder to release bile. Secretin
Speaker 1: tells the pancreas to send more digestive enzymes. And other
Speaker 1: hormones travel all the way to the brain To say,
Speaker 1: in the quietest way, we have had enough. The sensation
Speaker 1: of fullness is partly generated by the gut, not the brain.
Speaker 1: It takes about 20 minutes, after you begin eating, for
Speaker 1: the gut hormones to reach the brain and signal satiety.
Speaker 1: 20 MINUTES which is why eating slowly so often leads
Speaker 1: to eating less. The gut is communicating. It just needs
Speaker 1: time for the message to arrive. There is also a
Speaker 1: hormone called ghrelin. Ghrelin is produced primarily in the stomach,
Speaker 1: when it is empty, and is one of the main
Speaker 1: drivers of hunger. It rises before meals and falls after eating.
Speaker 1: It signals to the brain, it is time to eat.
Speaker 1: Ghrelin also increases when you are sleep deprived. This is
Speaker 1: part of why, when you have not slept enough, you
Speaker 1: feel hungrier than usual. Your gut and your sleep are
Speaker 1: in conversation. Even right now, as you drift toward rest,
Speaker 1: your digestive system is adjusting, slowing in some ways, continuing
Speaker 1: in others, never truly still. Let us think about the
Speaker 1: stomach for a moment more. The stomach produces roughly 2
Speaker 1: litres of gastric acid every day. 2 litres of acid
Speaker 1: carefully managed, carefully contained. The lining of the stomach is
Speaker 1: protected from this acid by a thick layer of mucus
Speaker 1: about half a millimeter thick. That half millimeter of mucus
Speaker 1: is the only thing standing between the acid and the
Speaker 1: tissue beneath. And it is renewed constantly. Stomach cells that
Speaker 1: produce mucus are among the fastest dividing cells in the body.
Speaker 1: They have to be. The acid would win otherwise. When
Speaker 1: that mucus layer is compromised, perhaps by a certain type
Speaker 1: of bacteria, called Helicobacter pylori, or by long-term use of
Speaker 1: certain pain medications, the result is an ulcer. a painful
Speaker 1: erosion of the stomach lining, where the acid reaches tissue
Speaker 1: it was never meant to touch. For decades, doctors believed
Speaker 1: stomach ulcers were caused by stress and diet. Then, in
Speaker 1: the 1980s, an Australian researcher named Barry Marshall proposed that
Speaker 1: many ulcers were actually caused by a bacterial infection Nobody
Speaker 1: believed him. So he drank a solution of the bacteria himself,
Speaker 1: to prove the point. He developed gastritis within days, treated
Speaker 1: himself with antibiotics, and eventually shared the Nobel Prize in
Speaker 1: Medicine for the discovery. Sometimes the most important ideas in
Speaker 1: science begin with someone willing to be proven right. in
Speaker 1: the most uncomfortable way possible. The digestive system is also
Speaker 1: deeply involved in your immune response. About 70% of the
Speaker 1: body's immune system is located in the gut. 70%. This
Speaker 1: makes sense when you consider that the gut is one
Speaker 1: of the main entry points for the outside world. Everything
Speaker 1: you eat and drink arrives from an environment full of pathogens.
Speaker 1: The gut has to be able to tell the difference
Speaker 1: between food, helpful bacteria, and harmful invaders. And it does
Speaker 1: this with remarkable precision. Specialized immune cells line the gut wall,
Speaker 1: constantly sampling what passes through Learning. Remembering. Building a record
Speaker 1: of what belongs and what does not. Your gut is
Speaker 1: also responsible for the gradual education of your immune system
Speaker 1: from the very beginning of your life. When a baby
Speaker 1: is born, their gut is essentially sterile. In the first hours, days,
Speaker 1: and weeks of life, bacteria begin to colonize the gut.
Speaker 1: Which bacteria settle in first depends on many things. How
Speaker 1: the baby was born, whether they were breastfed, what environment
Speaker 1: they are in. And this early microbiome has consequences that
Speaker 1: ripple through decades of life. The foundation of the gut's
Speaker 1: immune education is laid in the first months. The bacteria
Speaker 1: you carry now are, in part, a legacy of those
Speaker 1: early days. There is something called the hygiene hypothesis. It
Speaker 1: suggests that children who grow up in environments that are
Speaker 1: too clean Too free of bacteria and minor infections may
Speaker 1: develop immune systems that have not had enough to practice on.
Speaker 1: Without early exposure to a wide variety of microorganisms, the
Speaker 1: immune system may become confused. It may begin to overreact
Speaker 1: to things that are not actually dangerous. Pollen certain foods,
Speaker 1: its own tissue. The rise in allergies and autoimmune conditions
Speaker 1: in the modern world may be, at least in part,
Speaker 1: a story about a gut microbiome that has not been
Speaker 1: given enough to do. The gut, in some ways, needs
Speaker 1: to be challenged to become wise Let us consider fiber
Speaker 1: for a moment. Dietary fiber is the part of plant
Speaker 1: food that your own digestive enzymes cannot break down. It
Speaker 1: passes through the small intestine without being absorbed. And when
Speaker 1: it reaches the large intestine, the bacteria there are ready
Speaker 1: for it. They ferment it. They use it as fuel.
Speaker 1: And in doing so, they produce compounds that protect the
Speaker 1: lining of the colon, reduce inflammation, and even influence genes
Speaker 1: in ways that are still being studied. Fiber also acts
Speaker 1: like a slow-moving broom. It adds bulk to the contents
Speaker 1: of the intestine, and helps move things along. Without enough fiber,
Speaker 1: the gut contents slow down. They linger. And the longer
Speaker 1: they linger, the more water is absorbed, and the more
Speaker 1: compacted things become. Constipation is at its simplest. What happens
Speaker 1: when the gut's contents stay too long. And it is
Speaker 1: largely preventable with enough fiber and enough water. Water, in fact,
Speaker 1: is essential to every part of digestion. Saliva is mostly water.
Speaker 1: Stomach acid is dissolved in water. The enzymes of the
Speaker 1: small intestine work in a watery medium. The large intestine
Speaker 1: absorbs water. Without adequate hydration, the entire system slows. Food
Speaker 1: moves less smoothly. Absorption becomes less efficient The microbiome is affected. Water.
Speaker 1: Plain and quiet. Is perhaps the single most important thing.
Speaker 1: You can give your digestive system. More than any supplement
Speaker 1: or special diet. Just water. Steady and consistent. Throughout the day.
Speaker 1: Now let us talk about the liver. a little more deeply.
Speaker 1: Because the liver is not simply a producer of bile.
Speaker 1: It is a metabolic command center of extraordinary complexity. After
Speaker 1: the small intestine absorbs nutrients and they enter the bloodstream,
Speaker 1: almost everything goes to the liver first. The liver inspects everything.
Speaker 1: It processes glucose and decides whether to use it immediately,
Speaker 1: store it as glycogen, or convert it to fat. It
Speaker 1: processes amino acids and builds proteins from them, including the
Speaker 1: proteins that carry substances through the blood and the proteins
Speaker 1: that help clot it. The liver also detoxifies. Drugs, Alcohol,
Speaker 1: toxins that are absorbed through the gut wall, must pass
Speaker 1: through the liver before entering the general circulation. The liver
Speaker 1: breaks them down, converts them, makes them safer. A liver cell,
Speaker 1: called a hepatocyte, can perform over 500 different biochemical reactions
Speaker 1: and 500 distinct tasks in a single cell. And the
Speaker 1: liver contains billions of these cells, all working in parallel,
Speaker 1: all day, all night, without rest. The liver is, in
Speaker 1: its quiet way, one of the most remarkable organs in
Speaker 1: the body. and the liver can regenerate. This is the
Speaker 1: detail that tends to make people go still when they
Speaker 1: hear it. If up to 70% of the liver is removed,
Speaker 1: in a healthy person, it will grow back to its
Speaker 1: original size within a few weeks. No other solid organ
Speaker 1: in the human body can do this. The liver simply
Speaker 1: knows how to rebuild itself. It is as if there
Speaker 1: is a blueprint inside it, that it returns to again
Speaker 1: and again, regardless of how much is taken away. This
Speaker 1: is what made partial liver donation possible. A living person
Speaker 1: can give a portion of their liver to someone who
Speaker 1: needs it, and both pieces will grow. back to full
Speaker 1: size let us think about the pancreas again beyond its
Speaker 1: role in digestion the pancreas has another function that most
Speaker 1: people are more familiar with it produces insulin and glucagon
Speaker 1: these are hormones that regulate the level of glucose in
Speaker 1: the blood after a meal When glucose rises in the bloodstream,
Speaker 1: the pancreas releases insulin. Insulin unlocks cells, allowing glucose to
Speaker 1: enter and be used as fuel. When blood sugar falls
Speaker 1: too low, glucagon signals the liver to release stored glucose
Speaker 1: back into the blood. The pancreas is always listening to
Speaker 1: the blood always adjusting. A quiet regulator, keeping the balance
Speaker 1: between too much and too little. When the pancreas loses
Speaker 1: its ability to produce enough insulin, or when cells stop
Speaker 1: responding to it properly, the result is diabetes. Type 1
Speaker 1: diabetes occurs when the immune system mistakenly destroys the insulin-producing
Speaker 1: cells of the pancreas. Type 2 diabetes develops when the
Speaker 1: body becomes less responsive to insulin over time. Both conditions are,
Speaker 1: at their root, disruptions in the quiet conversation between the
Speaker 1: pancreas and the rest of the body. The glucose that
Speaker 1: should become fuel Accumulates instead, unable to find its way in.
Speaker 1: Let us drift back to something more peaceful. Something that
Speaker 1: happens every day, without drama or disruption. The simple, rhythmic
Speaker 1: process of peristalsis. We mentioned it earlier, but it deserves
Speaker 1: more time. Peristalsis is the wave of muscular contraction that
Speaker 1: moves food through your digestive tract. It is not a
Speaker 1: single wave. It is many overlapping waves, each one coordinated
Speaker 1: by the enteric nervous system. The muscles of the digestive
Speaker 1: tract do not all contract at once. They contract in sequence,
Speaker 1: one section relaxing just ahead of the food, while the
Speaker 1: section behind it contracts, squeezing it forward. It is a
Speaker 1: beautifully coordinated movement, like a crowd doing a slow wave
Speaker 1: at a stadium, but more purposeful, more gentle, more continuous.
Speaker 1: At certain times, usually between meals, the digestive system performs
Speaker 1: what is called the migrating motor complex. It is a
Speaker 1: strong wave of muscular contraction that moves slowly along the
Speaker 1: entire length of the small intestine, sweeping the lumen clean,
Speaker 1: pushing any remaining debris further down the tract, preventing bacteria
Speaker 1: from overgrowing. in places they should not. And this process
Speaker 1: is what makes the quiet, gurgling sounds you sometimes hear
Speaker 1: from your stomach when you have not eaten for a while.
Speaker 1: That rumbling is not hunger, exactly. It is your digestive
Speaker 1: system doing its housekeeping, And now let us speak about
Speaker 1: something that does not get enough credit. The mucus of
Speaker 1: the digestive system. Mucus is not glamorous. It is not
Speaker 1: something people tend to think about with appreciation. But without mucus,
Speaker 1: digestion would be painful, inefficient, and dangerous. mucus coats the
Speaker 1: entire inner surface of your digestive tract. In the stomach,
Speaker 1: as we discussed, it protects the lining from acid. In
Speaker 1: the small intestine, it helps nutrients slide smoothly along the wall.
Speaker 1: In the large intestine, it helps compacted material move more easily.
Speaker 1: It is the quiet, lubricant, of your inner world. The
Speaker 1: mucus lining of the gut. is also home to a
Speaker 1: rich community of microorganisms. Not just in the open space
Speaker 1: of the intestine, but nestled right against the wall in
Speaker 1: a layer that acts as both a habitat and a buffer.
Speaker 1: This layer of mucus-dwelling bacteria forms part of the first
Speaker 1: line of defense against harmful pathogens. They crowd out the
Speaker 1: bad actors simply by being there first. They occupy the space.
Speaker 1: They produce compounds that make the environment hostile to invaders.
Speaker 1: The beneficial bacteria of the gut are, in a very
Speaker 1: real sense, your first army. Let us talk about the
Speaker 1: speed of digestion in different parts of the system. The
Speaker 1: mouth does its work in second. The esophagus in under
Speaker 1: a minute. The stomach takes two to four hours on
Speaker 1: a typical meal. The small intestine takes two to six hours.
Speaker 1: And the large intestine takes as we noted, can take
Speaker 1: 10 to 50 hours. So when you eat breakfast in
Speaker 1: the morning, the nutrients from it begin entering your bloodstream
Speaker 1: within an hour or two. But the fiber and the
Speaker 1: remnants of that same breakfast may not complete their journey
Speaker 1: until the following day. The digestive system is not in
Speaker 1: a hurry. It has learned over millions of years of
Speaker 1: evolution that thoroughness is more valuable than speed. Now let
Speaker 1: us spend a little time with something quietly extraordinary. The
Speaker 1: surface cells of the small intestine are among the most
Speaker 1: rapidly renewed cells in the body. The cells lining the
Speaker 1: villi are replaced roughly every three to five days. Every
Speaker 1: three to five days, the entire absorptive surface of your
Speaker 1: small intestine is new. These old cells, shed from the lining,
Speaker 1: become part of what moves through the large intestine, a
Speaker 1: significant proportion of the solid material that eventually leaves the
Speaker 1: body is made up of shed intestinal cells. You are,
Speaker 1: quietly and continuously, shedding and regrowing yourself from the inside.
Speaker 1: The bacteria in your gut also have a life cycle.
Speaker 1: They live, reproduce, and die, and their remains add to
Speaker 1: the composition. of the material in the large intestine. In fact,
Speaker 1: bacteria make up a substantial portion of the typical stool.
Speaker 1: Some estimates suggest that by dry weight, bacteria account for
Speaker 1: roughly half of the solid mass. What leaves your body
Speaker 1: is partly the remains of trillions of tiny organisms that
Speaker 1: lived and worked inside you. There is something quietly remarkable
Speaker 1: in that if you let yourself sit with it, let
Speaker 1: us move gently toward the subject of digestion and sleep
Speaker 1: because they are more intertwined than most people realize. Sleep
Speaker 1: is not a neutral state for the digestive system. It
Speaker 1: is an active time. The gut does not simply stop
Speaker 1: working when you drift off. It shifts into a different mode.
Speaker 1: Certain processes slow. The muscles of the digestive tract relax
Speaker 1: into gentler rhythms. Gastric acid production decreases and the migrating
Speaker 1: motor complex, that housekeeping wave, may become more active. The
Speaker 1: gut takes the quiet hours to tidy up. Some people
Speaker 1: experience heartburn at night because lying down makes it easier
Speaker 1: for stomach acid to slide back through the lower esophageal sphincter.
Speaker 1: Gravity which helps keep the contents of the stomach down
Speaker 1: during the day, is no longer working in the same
Speaker 1: direction when you are horizontal. This is why advice about
Speaker 1: not eating too close to bedtime is rooted in real biology.
Speaker 1: Giving the stomach a few hours to empty before lying
Speaker 1: down reduces the opportunity for acid to travel upward. Sleep
Speaker 1: also affects the gut microbiome. Research suggests that disrupted sleep
Speaker 1: can alter the composition of gut bacteria, reducing diversity and
Speaker 1: potentially increasing populations of less beneficial strains. And a less
Speaker 1: diverse microbiome has been linked to all sorts of downstream effects.
Speaker 1: On mood, on inflammation, on metabolism. Good sleep, then, is
Speaker 1: not just rest for the brain. It is rest and
Speaker 1: repair for the gut as well. The two systems are
Speaker 1: looking after each other in the dark. Let us think
Speaker 1: about the remarkable diversity of the gut microbiome for a moment.
Speaker 1: A healthy gut typically contains hundreds of different species of bacteria. Hundreds.
Speaker 1: Each species playing a slightly different role. Each one occupying
Speaker 1: a slightly different niche in the complex ecology of the
Speaker 1: intestine This diversity is considered a sign of health. A
Speaker 1: less diverse microbiome, one that has been simplified by poor
Speaker 1: diet or repeated antibiotic use, is associated with a range
Speaker 1: of conditions. Inflammatory bowel disease, obesity, depression. The connection between
Speaker 1: gut diversity and overall health is one of the most
Speaker 1: active areas of research in medicine today. What you eat
Speaker 1: shapes your microbiome. This is perhaps the most direct and
Speaker 1: powerful way that diet affects health. Not just through the
Speaker 1: nutrients it provides to you, but through the food it provides.
Speaker 1: to the trillions of organisms living inside you. A diet
Speaker 1: rich in a variety of plant foods feeds a wider
Speaker 1: variety of bacteria. A diet high in ultra-processed food feeds
Speaker 1: a narrower range and tends to favor species associated with inflammation.
Speaker 1: You are not just eating for yourself. You are eating
Speaker 1: for an entire community. Fermented foods deserve a quiet mention here.
Speaker 1: Foods like yogurt, kefir, sauerkraut, kimchi, and miso contain living bacteria.
Speaker 1: When you eat them, you are introducing new microorganisms directly
Speaker 1: into your gut. Some of them survive the journey through
Speaker 1: the stomach. Some do not. But even those that do
Speaker 1: not survive may have beneficial effects because their remains can
Speaker 1: still interact with and support the bacteria already there. Fermented
Speaker 1: foods are, in a sense, a gift to your gut.
Speaker 1: one that humans have been giving themselves for thousands of years,
Speaker 1: long before they understood. the biology behind it. The history
Speaker 1: of fermentation is ancient. Humans have been fermenting food for
Speaker 1: at least 10,000 years, perhaps much longer. They did not
Speaker 1: know about bacteria, They did not understand microbiomes. But they
Speaker 1: learned through observation and practice that certain foods preserved in
Speaker 1: certain ways made people feel well. That yogurt kept travelers healthy.
Speaker 1: That fermented vegetables sustained communities through long winters. The body
Speaker 1: knew what the mind had not yet named. The gut
Speaker 1: was teaching humans how to care for it, long before
Speaker 1: science could explain why. Let us talk about something most
Speaker 1: of us take completely for granted. The act of not
Speaker 1: being poisoned by what you eat. Every day, you consume
Speaker 1: food from an environment. teeming with microorganisms. Some of those
Speaker 1: microorganisms are benign. Some are helpful. And some, if they
Speaker 1: manage to get through, would make you very ill. Your
Speaker 1: digestive system is an elaborate filtration system that separates what
Speaker 1: can safely enter your body from what cannot. The stomach
Speaker 1: acid kills many pathogens before they ever reach the intestine.
Speaker 1: The mucus layer traps others. The immune cells along the
Speaker 1: gut wall neutralize those that make it through. The resident
Speaker 1: bacteria compete with and suppress harmful visitors. The system is layered, redundant,
Speaker 1: built with many backup systems precisely because the stakes are high.
Speaker 1: And yet it works so smoothly most of the time
Speaker 1: that we never notice it happening. We eat. We are nourished.
Speaker 1: We are protected. And the system asks for nothing except
Speaker 1: to be given something decent to work with. When the
Speaker 1: system does fail, it fails loudly. Nausea and vomiting are
Speaker 1: among the most ancient protective reflexes in the body. The
Speaker 1: detection of a harmful substance in the stomach triggers an
Speaker 1: immediate response. The diaphragm and abdominal muscles contract sharply. The
Speaker 1: lower esophageal sphincter relaxes and the contents of the stomach
Speaker 1: are expelled upward as quickly as possible. Unpleasant as it is,
Speaker 1: vomiting is the gut protecting you. It is the system saying,
Speaker 1: whatever this is, it must not go further. Even the
Speaker 1: discomfort has a purpose. Diarrhoea works the same way, from
Speaker 1: the other direction. When something harmful reaches the intestine, the
Speaker 1: gut floods the area with fluid and accelerates movement dramatically.
Speaker 1: The goal is to flush the pathogen out before it
Speaker 1: can do more damage. The unpleasantness is intentional. The speed
Speaker 1: is intentional. the body is prioritizing elimination over comfort. It
Speaker 1: is making a sacrifice of your immediate well-being in service
Speaker 1: of your longer-term survival. Once again, the system that seems
Speaker 1: to be failing is actually working. There is something quietly
Speaker 1: moving about the fact that every part of your digestive
Speaker 1: system is doing exactly what it was designed to do.
Speaker 1: Not by anyone who planned it, but by the long,
Speaker 1: slow process of natural selection over hundreds of millions of years.
Speaker 1: Digestive systems existed long before humans did. Even the simplest
Speaker 1: animals on Earth have some form of internal processing for
Speaker 1: breaking down food The earthworm has a digestive tract. The
Speaker 1: jellyfish has a digestive cavity. The bacteria themselves have their
Speaker 1: own internal processes for extracting energy from their environment. Digestion
Speaker 1: is one of the most ancient strategies in life. Take
Speaker 1: what is outside. Transform it into something you can use.
Speaker 1: become more from what was less. Let us speak about
Speaker 1: the role of the digestive system in the experience of taste.
Speaker 1: We think of taste as something that happens in the mouth,
Speaker 1: and it does begin there. The tongue contains taste receptor
Speaker 1: cells that detect sweet, sour, salty, bitter, and a fifth
Speaker 1: category called umami, which is the savory depth found in
Speaker 1: foods like mushrooms, aged cheese, and slow-cooked meats. But these
Speaker 1: same taste receptors have been found lining the digestive tract.
Speaker 1: The stomach has bitter receptors. The small intestine has sweet receptors.
Speaker 1: The gut is, in a sense, tasting what you eat
Speaker 1: at every stage of the journey. These internal taste receptors
Speaker 1: are not connected to conscious experience. You do not consciously
Speaker 1: taste your food with your small intestine. But the receptors
Speaker 1: still function. They detect compounds in the food. and trigger responses.
Speaker 1: Sweet receptors in the small intestine, for example, help regulate
Speaker 1: the absorption of glucose. Bitter receptors in the stomach may
Speaker 1: trigger a slowing of gastric emptying to allow more time
Speaker 1: for digestion. The gut is not passive. It is reading
Speaker 1: the food. and making decisions about it every step of
Speaker 1: the way. The experience of hunger is worth sitting with
Speaker 1: for a moment. We think of hunger as a simple thing,
Speaker 1: an empty feeling, a growling stomach. But hunger is actually
Speaker 1: a complex signal generated by multiple systems at once. ghrelin rises.
Speaker 1: Blood sugar falls slightly. The enteric nervous system signals the brain.
Speaker 1: Memories of food activate. The body coordinates a response that
Speaker 1: draws your attention, your motivation and eventually your behavior toward
Speaker 1: finding and consuming food. And when you find it When
Speaker 1: you sit down and eat, the system shifts again. Pleasure
Speaker 1: signals from the gut. Satisfaction building slowly. A gradual return
Speaker 1: to stillness. The body at peace because its most fundamental
Speaker 1: need has been met. There is a concept in nutrition
Speaker 1: called the thermic effect of food It refers to the
Speaker 1: energy your body expends simply in the act of digesting.
Speaker 1: It costs something to break food down. Digestive enzymes must
Speaker 1: be made. Muscles must contract. Cells must transport nutrients. All
Speaker 1: of this requires energy. energy. Protein requires the most energy
Speaker 1: to digest. Roughly 20 to 30% of the energy it
Speaker 1: contains is used just in the process of breaking it down.
Speaker 1: Fat requires the least energy. around 5 to 10%. Carbohydrates
Speaker 1: fall somewhere in between. Digestion is not free. Your body
Speaker 1: is spending something to get something. And it is always
Speaker 1: making the calculation about whether the investment is worth the return.
Speaker 1: Let us take a gentle turn. toward the digestive system
Speaker 1: at the level of the cell. Because what happens at
Speaker 1: the cellular level is where the real magic occurs. When
Speaker 1: glucose is absorbed through the intestinal wall and enters a cell,
Speaker 1: it enters a process called cellular respiration. Inside the cell,
Speaker 1: in small structures called mitochondria glucose is broken down step
Speaker 1: by step in a series of chemical reactions that release energy.
Speaker 1: This energy is stored in a molecule called ATP, adenosine triphosphate.
Speaker 1: ATP is the currency of cellular energy. It is what
Speaker 1: allows your muscles to contract, your neurons to fire your
Speaker 1: heart to beat. The food you eat becomes, through this
Speaker 1: long and quiet chain of events, the energy of thought, movement,
Speaker 1: and life itself. And the byproducts of this process? Carbon
Speaker 1: dioxide and water. The carbon dioxide you breathe out with
Speaker 1: every exhale, is, in part, the chemical remnant of the
Speaker 1: food you ate. When you lose weight, most of what
Speaker 1: you lose leaves your body as carbon dioxide, breathed out
Speaker 1: through the lungs. Not sweated out, not shed some other way. Breathed.
Speaker 1: The food disappears into the air. Every exhale is, in
Speaker 1: a strange and beautiful way, the final chapter of digestion.
Speaker 1: Let us consider water absorption a little more carefully. The
Speaker 1: large intestine absorbs about one and a half liters of
Speaker 1: water every day from the material that passes through it.
Speaker 1: That water came from saliva, from gastric juice, from the
Speaker 1: secretions of the pancreas, from bile, and from fluids in
Speaker 1: the small intestine. The gut floods itself with liquid to
Speaker 1: facilitate digestion and then reclaims most of that liquid in
Speaker 1: the large intestine. The whole system is extraordinarily economical. Very
Speaker 1: little is wasted. Very little is lost. The gut knows
Speaker 1: how to take care of itself. Let us speak about
Speaker 1: the remarkable fact that your digestive system was already working
Speaker 1: before you were born. In the womb, a developing baby
Speaker 1: begins swallowing amniotic fluid from around the 15th week of pregnancy.
Speaker 1: The digestive system begins practicing. The stomach produces acid. The
Speaker 1: intestines begin moving. The liver begins producing bile. By the
Speaker 1: time a baby takes their first breath, the digestive system
Speaker 1: has already been running. For months. And almost immediately after birth,
Speaker 1: it begins the work of processing real food. First milk. Then, slowly, solids.
Speaker 1: Learning with every meal. Adapting with every bite. Breast milk
Speaker 1: is a particular wonder. In the context of the digestive system,
Speaker 1: it is perfectly calibrated for the infant gut. It contains
Speaker 1: not just nutrients, but also compounds called human milk oligosaccharides,
Speaker 1: which are complex carbohydrates that the infant cannot digest. They
Speaker 1: pass through the small intestine untouched, and arrive in the
Speaker 1: large intestine, where they feed the specific bacteria that should
Speaker 1: be establishing themselves there. The milk is not just feeding
Speaker 1: the baby. It is feeding the baby's microbiome, nurturing the
Speaker 1: ecosystem that will protect and support the child for the
Speaker 1: rest of their life. the gut microbiome continues to develop
Speaker 1: and shift throughout life. In childhood, it diversifies as the
Speaker 1: diet broadens. In adulthood, it stabilizes into a relatively consistent community,
Speaker 1: though it still responds to what you eat and how
Speaker 1: you live. In old age, it tends to become less
Speaker 1: diverse again. Certain key species decline. The gut becomes less resilient.
Speaker 1: And this decline in microbiome diversity in older adults may
Speaker 1: contribute to increased frailty, slower recovery from illness, and changes
Speaker 1: in cognitive function. The gut ages. not as visibly as
Speaker 1: the skin, not as noticeably as the joints, but it
Speaker 1: ages nonetheless. And caring for it is, in a real sense,
Speaker 1: a way of caring for your future self. Let us
Speaker 1: sit for a moment with the concept of digestion and
Speaker 1: its relationship to emotions. We speak of gut feelings, of
Speaker 1: following our gut, of butterflies in the stomach, of having
Speaker 1: no stomach for something. Language has always known, at some level,
Speaker 1: that the gut and the emotions are linked. And biology
Speaker 1: is beginning to understand why. The gut contains the same neurotransmitters,
Speaker 1: Found in the brain. Dopamine. Serotonin. GABA. These chemical messengers
Speaker 1: are not confined to the skull. They move through the
Speaker 1: gut as well. Shaping mood and behavior. From below as
Speaker 1: well as above. Stress changes the gut. This is not metaphor.
Speaker 1: It is physiology. Under stress, the body directs blood away
Speaker 1: from the digestive organs and toward the muscles and heart.
Speaker 1: Digestion slows or becomes erratic. The composition of the microbiome
Speaker 1: may shift. Inflammation may increase. People who live under chronic
Speaker 1: stress often experience chronic digestive problems, irritable bowel syndrome, bloating, discomfort.
Speaker 1: These are not imagined. They are the body's real response
Speaker 1: to a nervous system that never fully relaxes. And the
Speaker 1: reverse is also true. A calm body Deep, slow breathing
Speaker 1: activates the parasympathetic nervous system, which is the branch of
Speaker 1: the nervous system that governs rest and digestion. When you
Speaker 1: are calm, your digestive system works better. Blood flows more
Speaker 1: freely to the organs. Enzymes are produced more efficiently. The
Speaker 1: microbiome is more stable. There is wisdom Then, in the
Speaker 1: old idea of not rushing a meal, of sitting quietly
Speaker 1: with your food, of letting the moment of eating be
Speaker 1: something peaceful, let us think about what happens to different
Speaker 1: kinds of food as they travel through you. A simple sugar,
Speaker 1: like glucose in a spoonful of honey, barely needs digesting
Speaker 1: at all. It is absorbed very quickly into the bloodstream,
Speaker 1: often before it has even left the small intestine. Protein
Speaker 1: is more complex. It must be unraveled from its intricate
Speaker 1: three-dimensional structure before it can be broken into amino acids.
Speaker 1: This unraveling begins in the stomach. with acid, and continues
Speaker 1: in the small intestine, with specialized enzymes. Fat is the
Speaker 1: most complex of all. Fat does not dissolve in water,
Speaker 1: and the digestive system is a water-based environment. So the
Speaker 1: gut has developed a remarkable workaround. Bile, produced by the
Speaker 1: liver and released by the gallbladder, acts as an emulsifier.
Speaker 1: It breaks large fat globules into tiny droplets, vastly increasing
Speaker 1: the surface area, available for digestive enzymes to work on.
Speaker 1: Then the enzymes break the fat into fatty acids and glycerol.
Speaker 1: These are absorbed into the villi and packaged into small
Speaker 1: capsules that travel through the lymphatic system rather than directly
Speaker 1: into the blood. Fat takes a different route. It enters
Speaker 1: the circulation more slowly, which is part of why a
Speaker 1: fatty meal keeps you full for longer than a sugary one.
Speaker 1: The gallbladder is an interesting organ in its own right.
Speaker 1: It is a small pear-shaped sac tucked beneath the liver.
Speaker 1: Its sole function is to store bile until it is needed.
Speaker 1: When fat enters the small intestine, cholecystokinin signals the gallbladder
Speaker 1: to contract and squeezing a concentrated burst of bile into
Speaker 1: the intestine. Without fat in the diet, the gallbladder is
Speaker 1: barely used. And if bile is allowed to sit in
Speaker 1: the gallbladder too long, it can crystallize into gallstones. Hard,
Speaker 1: sometimes painful deposits made largely of cholesterol or bilirubin a
Speaker 1: pigment from broken-down red blood cells. The gallbladder is one
Speaker 1: of those organs that most people only discover when it
Speaker 1: causes them trouble. Small, unremarkable, and yet essential to the
Speaker 1: smooth handling of fat. Let us take a moment with
Speaker 1: the small intestine's remarkable transport systems Not all nutrients are
Speaker 1: absorbed by simple diffusion. Many require active transport. This means
Speaker 1: specialized protein channels in the intestinal cell walls selectively grab
Speaker 1: specific molecules and carry them across the cell membrane. This
Speaker 1: requires energy. The cell is working hard to pull in
Speaker 1: what the body needs. Iron is one example. Iron is
Speaker 1: tricky to absorb. The body absorbs only about 10 to
Speaker 1: 30% of the iron you consume in food. And the
Speaker 1: amount absorbed depends on how much the body already has.
Speaker 1: If iron stores are low, The gut absorbs more. If
Speaker 1: they are adequate, it absorbs less. The gut is not
Speaker 1: just absorbing. It is regulating. It is making intelligent decisions
Speaker 1: about what to let in. Calcium is similar. The absorption
Speaker 1: of calcium is regulated by vitamin D. Without enough vitamin D,
Speaker 1: the gut cannot efficiently absorb calcium, no matter how much
Speaker 1: of it is in your food. This is why vitamin
Speaker 1: D deficiency can lead to weak bones, even in people
Speaker 1: who eat plenty of dairy. The gut needs all its
Speaker 1: tools to do its job properly. A missing vitamin is
Speaker 1: not just a nutritional gap. It is a disruption in
Speaker 1: the quiet machinery of absorption. Zinc, magnesium, potassium, folate, vitamin B12.
Speaker 1: Each one has its own story. Its own absorption pathway.
Speaker 1: its own set of requirements, its own relationship with the
Speaker 1: other nutrients around it. Some nutrients compete. Too much calcium
Speaker 1: can interfere with iron absorption. Too much zinc can reduce
Speaker 1: copper levels. The gut is not just dealing with individual molecules.
Speaker 1: It is managing a complex, ever-changing mixture where everything interacts
Speaker 1: with everything else. The chemistry of digestion is like a
Speaker 1: conversation in which every participant is affecting every other. Let
Speaker 1: us think about gluten for a moment. Gluten is a
Speaker 1: protein found in wheat, barley and rye. For most people,
Speaker 1: the gut digests it without incident. But for people with
Speaker 1: coeliac disease, the presence of gluten triggers an immune response
Speaker 1: that attacks the lining of the small intestine. The villi
Speaker 1: become inflamed and flattened. The surface area for absorption shrinks dramatically.
Speaker 1: Nutrients pass through without being absorbed. A person with untreated
Speaker 1: coeliac disease can eat plenty of food and still be malnourished.
Speaker 1: Because the gut, under attack, cannot do what it is
Speaker 1: meant to do. Coeliac disease affects roughly one in a
Speaker 1: hundred people worldwide. It is an autoimmune condition, meaning the
Speaker 1: immune system turns against the body's own tissue. In this case,
Speaker 1: the small intestine becomes the site of that mistaken assault.
Speaker 1: For centuries, people with Coeliac Disease were simply ill without
Speaker 1: knowing why. Malnourished despite eating. despite resting. The cause only
Speaker 1: became understood in the 20th century. A Dutch physician named
Speaker 1: Willem Carol Dick noticed during the Second World War that
Speaker 1: children with the condition improved dramatically when wheat bread became
Speaker 1: scarce and worsened again when the war ended and wheat returned.
Speaker 1: A terrible historical moment became a medical breakthrough. Inflammatory bowel
Speaker 1: disease is different from coeliac disease. though both involve the
Speaker 1: immune system. Crohn's disease and ulcerative colitis are the two
Speaker 1: main forms. In Crohn's disease, inflammation can occur anywhere along
Speaker 1: the digestive tract, from the to the end of the
Speaker 1: large intestine. In ulcerative colitis, the inflammation is confined to
Speaker 1: the large intestine and rectum. Both conditions involve the immune
Speaker 1: system mounting an excessive response to the bacteria and contents
Speaker 1: of the gut. They are chronic, meaning they persist over time,
Speaker 1: flaring and remitting shaped by stress, diet, and factors still
Speaker 1: being investigated. The gut, in these conditions, is in conflict
Speaker 1: with itself. A system designed for peace, caught in a
Speaker 1: cycle of inflammation. But let us move away from illness
Speaker 1: and return to the quiet functioning of a healthy gut.
Speaker 1: because there is still much to explore there. Let us
Speaker 1: talk about the role of the digestive system in producing warmth.
Speaker 1: Digestion is an exothermic process. It releases heat. When you
Speaker 1: eat a large meal, particularly a meal rich in protein,
Speaker 1: your body temperature rises slightly. The metabolic work of breaking
Speaker 1: down food generates warmth. This is the thermic effect of
Speaker 1: food again, but felt differently. That comforting warmth after a
Speaker 1: hot meal is partly the warmth of digestion itself, your
Speaker 1: body working, your chemistry humming. The meal is warming you.
Speaker 1: from the inside. The enteric nervous system we mentioned earlier
Speaker 1: is worth pausing on once more. It is estimated to
Speaker 1: contain between 100 million and 500 million neurons. This is
Speaker 1: more than the entire peripheral nervous system of many simpler
Speaker 1: animals and it functions almost entirely independently of the brain.
Speaker 1: If you were to sever the vagus nerve, the connection
Speaker 1: between brain and gut, the gut would continue to function.
Speaker 1: It would still produce peristaltic waves. It would still regulate
Speaker 1: acid secretion. It would still manage the movement of food.
Speaker 1: The brain would no longer know what was happening down there.
Speaker 1: but the gut would not care. It would continue, quietly,
Speaker 1: on its own. This independence is ancient. In the very
Speaker 1: earliest animals with nervous systems, the gut nervous system came first.
Speaker 1: Before there was a brain, there was a gut. Some
Speaker 1: evolutionary biologists suggest that that the brain evolved, in part,
Speaker 1: from the gut-nervous system. That the sophisticated, centralized brain was
Speaker 1: built upon a foundation that began in the intestine. Which
Speaker 1: means that what you think of as your mind, your consciousness,
Speaker 1: your sense of self, may have its deepest roots not
Speaker 1: in the skull, but in the belly. let us think
Speaker 1: about something quite specific and quietly marvelous. The fact that
Speaker 1: your digestive system can tell what kind of food is
Speaker 1: entering it and respond appropriately with remarkable precision. When a
Speaker 1: fatty meal enters the small intestine, specific cells in the
Speaker 1: duodenum detect the fat and release cholecystokinin, The gallbladder contracts.
Speaker 1: Bile flows. The pancreas releases lipase, the enzyme for fat.
Speaker 1: When a carbohydrate-rich meal arrives, different enzymes are prioritized. When
Speaker 1: protein dominates, different signals are sent. The gut is not
Speaker 1: simply a passive channel. It is an active, responsive, intelligent
Speaker 1: system that adapts to what it receives. And here is
Speaker 1: something worth sitting with. The digestive system does not work
Speaker 1: in isolation. It is connected to the lungs, to the heart,
Speaker 1: to the kidneys, to the skin. To every organ in
Speaker 1: the body. What the gut absorbs enters the bloodstream and
Speaker 1: reaches every cell. What the gut does not absorb is
Speaker 1: not available to the rest of the body. The gut
Speaker 1: is the gatekeeper. The first processor. The interface between the
Speaker 1: outside world and the interior of you. Everything that becomes
Speaker 1: part of you passes through it first. Think about that
Speaker 1: for a moment. Every cell in your body, every molecule
Speaker 1: of protein in your muscles, every lipid in your cell membranes,
Speaker 1: every iron molecule in your blood came through your gut.
Speaker 1: Your gut is the original architect. of your body. It decided,
Speaker 1: day by day and meal by meal, what to let through,
Speaker 1: what to keep, what to discard. And in doing so,
Speaker 1: it has built you, quietly, continuously, without fanfare, from the
Speaker 1: very first moment of your life, Let us take a
Speaker 1: quiet turn and speak about something that might surprise you.
Speaker 1: The role of gas in the digestive system. Gas is
Speaker 1: produced in several ways. Air is swallowed during eating and drinking.
Speaker 1: Carbon dioxide is released when stomach acid meets the bicarbonate
Speaker 1: from the pancreas in the small intestine and bacteria in
Speaker 1: the large intestine, produce gas as a byproduct of fermentation.
Speaker 1: The gases produced include hydrogen, methane, and carbon dioxide, along
Speaker 1: with small amounts of other compounds that give gas its
Speaker 1: characteristic odor. The odor, incidentally, is produced by compounds containing sulfur.
Speaker 1: These compounds are produced primarily by the fermentation of protein.
Speaker 1: A diet high in protein tends to produce more odorous gas.
Speaker 1: One more quiet piece of information that the body has
Speaker 1: been keeping to itself. The average adult passes gas between
Speaker 1: 10 and 20 times per day. This is completely normal.
Speaker 1: It is, in fact, a sign that the gut bacteria
Speaker 1: are working. Fermenting. Processing. Doing their quiet biological work. A
Speaker 1: complete absence of gas would be more concerning than the
Speaker 1: presence of it The gut is alive and living things
Speaker 1: make noise. Let us talk about the digestive system across
Speaker 1: different ages of life. In infancy, the gut is learning.
Speaker 1: In adolescence, it is adapting to a rapidly changing body.
Speaker 1: In adulthood, it settles into relatively stable patterns. In old age,
Speaker 1: the gut changes in ways that are only now becoming understood.
Speaker 1: Gastric acid production may decline. The movement of the gut
Speaker 1: may slow. the microbiome shifts in composition. Many older adults
Speaker 1: experience constipation, reduced appetite, or difficulty absorbing certain nutrients. These
Speaker 1: are not failures of the body. They are the expected
Speaker 1: consequences of a system that has been working without pause
Speaker 1: for seven, eight, nine decades. The gut, like the rest
Speaker 1: of us, changes with time. The digestive system is also
Speaker 1: affected by the medications we take. Antibiotics, for example, do
Speaker 1: not distinguish between harmful bacteria and the beneficial residents of
Speaker 1: the gut. They reduce populations across the board. After a
Speaker 1: course of antibiotics, the microbiome may take weeks or months
Speaker 1: to return to its previous state. Some research suggests it
Speaker 1: may never fully recover exactly. That each course of antibiotics
Speaker 1: leaves a small, permanent mark on the ecology of the gut.
Speaker 1: This is not a reason to avoid antibiotics when they
Speaker 1: are needed. It is simply a reminder that every intervention
Speaker 1: has a cost and the gut bears some of that
Speaker 1: cost on our behalf. Proton pump inhibitors commonly used to
Speaker 1: reduce stomach acid also affect the gut. By reducing acid
Speaker 1: They change the environment of the stomach and the early
Speaker 1: part of the small intestine. Bacteria that would normally be
Speaker 1: killed by acid may survive further down the tract. The
Speaker 1: microbiome shifts. Absorption of certain nutrients, including vitamin B12 and magnesium,
Speaker 1: may be reduced. The gut is delicate in this way.
Speaker 1: Its chemistry is finely balanced. Changing one variable changes many others.
Speaker 1: And yet, the gut is also resilient. It recovers. It adapts.
Speaker 1: It rebuilds. When the microbiome is disrupted... It does not
Speaker 1: stay disrupted forever, if given the right conditions to recover.
Speaker 1: Varied food, fiber, fermented products, reduced stress, adequate sleep. These
Speaker 1: are not magical cures. They are the simple, steady nourishment
Speaker 1: that the gut has always responded to The gut does
Speaker 1: not ask for much. It asks for the basics, provided
Speaker 1: with some consistency, and it will do the rest. Let
Speaker 1: us think about digestion from an evolutionary perspective for a
Speaker 1: little while. The human digestive system evolved over millions of years.
Speaker 1: in an environment very different from the one most of
Speaker 1: us live in now. Our ancestors ate a highly varied diet. Roots, tubers, leaves, fruits, nuts, seeds, insects,
Speaker 1: meat when available. The gut evolved to handle this variety,
Speaker 1: to thrive on it, The microbiome evolved alongside this variety,
Speaker 1: shaped by the wide range of substrates. It was given
Speaker 1: to ferment. The modern diet, for many people in the
Speaker 1: industrialized world, is far less varied. Studies have found that
Speaker 1: the gut microbiome diversity of people living in industrialized societies
Speaker 1: is significantly lower than that of people living more traditional lives,
Speaker 1: eating more varied, less processed food. And lower microbiome diversity is,
Speaker 1: as we have discussed, associated with poorer health outcomes across
Speaker 1: many dimensions. The gut, in a sense, is homesick for
Speaker 1: the variety It evolved to expect. But this is not
Speaker 1: cause for despair. The gut is responsive. Research suggests that
Speaker 1: even a few weeks of increased dietary variety can meaningfully
Speaker 1: shift the microbiome. Adding more plant foods, more types of fiber,
Speaker 1: more colors and textures to the diet, can begin to
Speaker 1: restore some of that lost diversity. The gut is not static.
Speaker 1: It is always in the process of becoming. And it
Speaker 1: responds more quickly than you might expect to the choices
Speaker 1: you make. Let us think about the connection between the
Speaker 1: digestive system and the cardiovascular system. The gut processes the
Speaker 1: fats and cholesterol that eventually end up in the bloodstream.
Speaker 1: The liver regulates cholesterol levels. The microbiome affects how certain
Speaker 1: dietary compounds are metabolized into molecules that influence blood vessel health.
Speaker 1: A compound called TMAO Trimethylamine N-oxide is produced by certain
Speaker 1: gut bacteria when they metabolize compounds found in red meat
Speaker 1: and eggs. High levels of TMAO in the blood have
Speaker 1: been associated with increased cardiovascular risk. The connection between what
Speaker 1: we eat, how our gut processes it, and the health
Speaker 1: of our heart runs through the microbiome in ways that
Speaker 1: are only beginning to be understood. The gut and the
Speaker 1: brain share something else beyond chemistry. They both have circadian rhythms.
Speaker 1: Your gut has its own internal clock. The microbiome fluctuates
Speaker 1: over the course of the day, Different bacterial species are
Speaker 1: more active at different times. Digestive enzyme production is higher
Speaker 1: at certain times of day. Gut motility changes across the
Speaker 1: 24-hour cycle. Eating at times that are out of sync
Speaker 1: with the gut's internal clock, such as eating late at night,
Speaker 1: can disrupt these rhythms. and affect metabolic health. The gut,
Speaker 1: like the rest of you, has a sense of time.
Speaker 1: It prefers predictability. It thrives when your life has a
Speaker 1: rhythm it can follow. This is part of why shift
Speaker 1: workers and frequent travelers across time zones often experience digestive disruptions.
Speaker 1: The gut's clock has been thrown out of alignment. Meals
Speaker 1: arrive at unexpected times. The microbiome is out of step.
Speaker 1: The gut is doing its best to adapt to an
Speaker 1: unpredictable schedule. And over time, chronic circadian disruption may have
Speaker 1: lasting consequences for gut health. The gut, like a garden,
Speaker 1: does better with a consistent routine. Let us spend a
Speaker 1: little while with something gentle and interesting. The role of
Speaker 1: the digestive system in the experience of pleasure. Eating, for
Speaker 1: most animals, is not just about nutrition. It is pleasurable.
Speaker 1: And much of that pleasure is generated by the gut
Speaker 1: and the brain working together. Dopamine, the neurotransmitter associated with reward,
Speaker 1: is released when you eat something satisfying. The gut sends
Speaker 1: signals of satisfaction. The brain registers the pleasure. And a
Speaker 1: complex feedback loop reinforces the behavior, motivating you to eat
Speaker 1: again when you need to. Certain foods activate this system
Speaker 1: more strongly than others. Foods that combine fat and sugar
Speaker 1: in particular tend to be highly rewarding to the brain.
Speaker 1: This combination is relatively rare in nature, but common in
Speaker 1: processed food. The brain and gut, evolved to seek out
Speaker 1: high-energy food in an environment of scarcity, respond powerfully to
Speaker 1: this combination. They do not know that scarcity is no
Speaker 1: longer the problem. They are still running on ancient software.
Speaker 1: And the digestive system finds itself processing far more of
Speaker 1: these combinations than it was ever designed to handle. The
Speaker 1: body is not failing. It is doing exactly what evolution
Speaker 1: built it to do. In a world that evolution could
Speaker 1: not have anticipated. Let us return to the large intestine
Speaker 1: one more time. Because there is something happening there that
Speaker 1: is deeply important and rarely discussed. The fermentation of fiber
Speaker 1: by gut bacteria produces short-chain fatty acids. Specifically, acetate, propionate,
Speaker 1: and butyrate. These compounds have effects. that extend far beyond
Speaker 1: the colon. Butyrate is the primary fuel source for the
Speaker 1: cells lining the colon wall. Without enough butyrate, the colon
Speaker 1: cells begin to suffer. And a colon under nutritional stress
Speaker 1: is more vulnerable to disease. Propionate travels to the liver
Speaker 1: and helps regulate glucose production. Acetate enters the bloodstream and
Speaker 1: serves as an energy source for muscle and other tissues.
Speaker 1: Short-chain fatty acids also have anti-inflammatory effects. They signal to
Speaker 1: immune cells, in the gut wall, to remain calm, to
Speaker 1: stand down, to not overreact. They are, in a quiet way,
Speaker 1: peacemakers produced by bacteria from the fiber in your food.
Speaker 1: Another reason that fiber is so important. It is not
Speaker 1: just bulk. It is food for the bacteria that make
Speaker 1: the compounds that keep you calm and well. Now let
Speaker 1: us think about something that sits at the very beginning
Speaker 1: of the digestive process and is easy to overlook. The
Speaker 1: roll of the nose. Before food even enters the mouth,
Speaker 1: the smell of it begins preparing the digestive system. Olfactory
Speaker 1: signals The signals from scent receptors in the nose travel
Speaker 1: directly to the brain. The brain processes the smell as
Speaker 1: food and immediately begins sending preparation signals. Saliva increases. Gastric
Speaker 1: acid begins to rise. The pancreas may begin to release
Speaker 1: small amounts of insulin in anticipation of glucose, the digestive
Speaker 1: system is ready before you have even taken a bite.
Speaker 1: This anticipatory phase of digestion is called the cephalic phase.
Speaker 1: Cephalic meaning related to the head. It is governed by
Speaker 1: the sight, smell, and even the thought of food And
Speaker 1: it is significant. The cephalic phase accounts for roughly 20
Speaker 1: to 30% of the total digestive enzyme output in response
Speaker 1: to a meal. Almost a third of your digestive response
Speaker 1: has already happened before the food arrives. The gut has
Speaker 1: been listening to your thoughts and preparing accordingly. There is
Speaker 1: also what some researchers call the gastric phase. Once food
Speaker 1: is in the stomach, the stretching of the stomach wall
Speaker 1: triggers more acid production. The presence of proteins triggers pepsin release.
Speaker 1: The composition of the meal is constantly being sampled and
Speaker 1: responded to. and then the intestinal phase. When food reaches
Speaker 1: the small intestine, brings its own cascade of signals and responses.
Speaker 1: Digestion is not a single event. It is a long,
Speaker 1: layered series of responses, each one building on the last,
Speaker 1: each one tuned to what is actually there. Let us
Speaker 1: think about what happens when we fast. When there is
Speaker 1: no food, the digestive system does not sit idle. The
Speaker 1: migrating motor complex, as we discussed, runs its housekeeping sweeps.
Speaker 1: The microbiome shifts its composition slightly, adapting to the absence
Speaker 1: of incoming nutrients. gastric acid production decreases. The mucus layer
Speaker 1: is maintained. The gut is not sleeping when you fast.
Speaker 1: It is restoring, tidying, resetting. Some research suggests that periods
Speaker 1: of fasting allow the gut to perform repair processes that
Speaker 1: are harder to complete When digestion is continuously active, the
Speaker 1: cells of the intestinal lining may undergo a form of
Speaker 1: deep renewal when there is nothing arriving to process. Autophagy,
Speaker 1: the process by which cells recycle their own damaged components,
Speaker 1: is thought to increase during fasting. The body, given space,
Speaker 1: turns to its own housekeeping. Digestion, after all, is not
Speaker 1: only about what comes in. It is also about what
Speaker 1: is cleared away. The digestive system has its own experience
Speaker 1: of time. The stomach knows morning from night. The microbiome
Speaker 1: follows its circadian rhythms. The rate of cell renewal in
Speaker 1: the intestinal lining fluctuates across the day. The body is
Speaker 1: a clock in many places at once. And the gut
Speaker 1: is one of its most faithful timekeepers. It expects food
Speaker 1: at certain hours. It expects rest at others. When those
Speaker 1: expectations are met consistently, The system flows. When they are disrupted,
Speaker 1: the system adapts, but at a cost. There is one
Speaker 1: more thing worth dwelling on. quietly, before we begin our
Speaker 1: slow descent towards sleep. The fact that digestion is, at
Speaker 1: its most fundamental level, a story about transformation. Something from
Speaker 1: outside you, enters your body it is broken apart rearranged
Speaker 1: rebuilt and woven into the fabric of you the carbon
Speaker 1: atoms in the food you ate today will become the
Speaker 1: carbon atoms in your cells they will become part of
Speaker 1: your body part of your thoughts part of your breath
Speaker 1: until they leave you again, one way or another, to
Speaker 1: become part of something else. Everything flows. Everything transforms. The
Speaker 1: boundary between you and the world is more permeable than
Speaker 1: it appears. You are not a closed system. You are
Speaker 1: an open one. Constantly taking in. Constantly releasing. Constantly transforming.
Speaker 1: What you receive into what you are. And your digestive
Speaker 1: system is the great engine of that transformation. Quiet. Faithful. Continuous.
Speaker 1: The nine meters of tube inside you has been working
Speaker 1: since before you were born. It will work as long
Speaker 1: as you live. It does not ask for recognition. It
Speaker 1: does not need to be told. It simply receives what
Speaker 1: you give it and makes the best of it. And
Speaker 1: in the making, it makes you. Tonight, while you sleep,
Speaker 1: It will continue, gently moving whatever remains of today's meals
Speaker 1: along their slow and purposeful journey. The bacteria in your
Speaker 1: gut will ferment and produce. The cells of your intestinal
Speaker 1: lining will shed and regrow. The liver will process and store.
Speaker 1: The quiet machinery will hum beneath the stillness of your sleep.
Speaker 1: And you will not feel it. You will not need to.
Speaker 1: It has never needed you to manage it. It has
Speaker 1: only ever needed you to feed it, rest it, and
Speaker 1: trust it. So let that trust settle into you now.
Speaker 1: Your body knows what it is doing. It has been
Speaker 1: doing it. Far longer than your conscious mind has existed.
Speaker 1: Longer than you can remember. Longer than you can imagine.
Speaker 1: The wisdom inside you is ancient. The systems are precise.
Speaker 1: And tonight they are at work, gentle and reliable, as
Speaker 1: they have always been. Let your breathing slow. Let the
Speaker 1: weight of your body sink into wherever you are resting.
Speaker 1: Feel the warmth of your abdomen. The slow, soft movement
Speaker 1: of your breath. In and out. In and out. Somewhere
Speaker 1: inside you, a wave of gentle muscle moves through the dark.
Speaker 1: A bacteria settles into a fold of mucus and rests.
Speaker 1: A cell completes itself and begins again. Your gut is
Speaker 1: dreaming in its way. And you, who are made of
Speaker 1: all its work, can dream in yours. If you have
Speaker 1: not already closed your eyes, let them close now. There
Speaker 1: is nothing left to see tonight. Nothing left to think about.
Speaker 1: The world outside can wait. Your body is tending to itself.
Speaker 1: Every system is quiet and working. And all you have
Speaker 1: to do is nothing at all. Just be here. Just
Speaker 1: be still. Just breathe. The darkness is soft and kind
Speaker 1: and ready. Let yourself go.