Insulin: The Hormone That Runs Everything

Insulin: The Hormone That Runs Everything

There's a hormone your body produces every single day that most people only think about in the context of diabetes. It gets mentioned at the doctor's office when blood sugar is off. It shows up on nutrition labels in indirect ways. It's the punchline of carbohydrate debates and the villain in certain diet circles. None of that does it justice. Insulin is not the diabetes hormone. It is not a fat-storage switch. It is not something that only matters if your pancreas is failing. Insulin is the master signal that coordinates nearly every system in your body — your brain, your muscles, your nerves, your skin, your immune system, your gut bacteria, your mood, your heart rate, your cancer risk, and the speed at which you age. The story of insulin is the story of your metabolic health in its entirety. And the reason it matters — the reason understanding it changes things — is that almost everything capable of improving your health does so, at least in part, by improving how insulin works. Including, and especially, exercise. This article is the foundation. Everything else we write about in this series links back here.

Part 1: What Insulin Actually Is

Insulin is old. Far older than you might expect. Insulin-like proteins have been found in fungi, in single-celled organisms, in creatures that have been on this planet for over a billion years. This is not a hormone that mammals invented to manage their diet. It is one of the most ancient and conserved signaling molecules in the history of life on Earth. The fact that it exists in essentially the same form across wildly different species — from fish to humans — tells you something important: evolution decided very early that this particular signal was worth keeping. It has been refined, not replaced, across a billion years of biological development. In your body, insulin is produced in the pancreas — a small, unremarkable-looking organ tucked behind your stomach that most people couldn't point to on a diagram. Scattered throughout the pancreas are tiny clusters of cells called the islets of Langerhans, named after Paul Langerhans, a German medical student who first described them in 1869 at the age of 22, with no idea what they actually did. He just noticed they looked different from the surrounding tissue and wrote them up. Inside each islet is a community of different cell types, each producing a different hormone. The ones we're concerned with are the beta cells — and they have a singular, non-negotiable purpose: they make insulin. Every molecule of insulin in your body comes from them. No other cell type produces it in meaningful quantities. There is no backup system. There is no secondary factory. When beta cells are damaged or destroyed, the only response available to the body is to replace the insulin from outside — which is exactly what insulin-dependent diabetics do every day. That singularity matters. It means these cells are both irreplaceable and, as we'll see, more vulnerable than most people realise.

Part 2: The Surprise Before The First Bite

Here is something that most people — including most people with a professional interest in nutrition and metabolism — don't know. Your insulin response to a meal begins before you eat. The moment you see food, smell food, or even think seriously about food, your brain sends signals to your pancreas telling it to start preparing. Insulin begins to release before a single calorie has crossed your lips. This is called the cephalic phase of insulin secretion — cephalic meaning "of the head" — and it exists for a specific reason: your body is getting ahead of the glucose surge that's about to arrive. Think about what that means. Your brain is monitoring your environment, detecting signals that food is imminent, and issuing instructions to a gland in your abdomen to begin production. Before you've chewed anything. Before anything has entered your digestive system. This isn't reflex. This is anticipation. Your body doesn't just react to what's happening — it predicts what's about to happen and prepares accordingly. Research suggests this anticipatory response may become less precise as metabolic health declines — the pancreas potentially falling behind the glucose curve rather than getting ahead of it, and blood sugar management becoming more reactive than proactive. A body losing its metabolic precision stops anticipating and starts catching up. The difference between those two states, over years, is significant. It also means that the way you eat matters to your insulin response — not just what you eat. Eating while distracted, eating fast, eating without any sensory engagement with your food — these aren't just mindfulness concerns. They may affect the quality of the physiological preparation your body makes for the meal that follows.

Part 3: How Insulin Gets Made

Your pancreatic beta cells are running a manufacturing operation around the clock. When blood sugar rises — after a meal, after a snack, after anything that puts glucose into your bloodstream — glucose enters a beta cell through a specialised door that opens in response to rising blood sugar levels. The cell metabolises the glucose, and as it does, energy builds up inside. That energy buildup acts as a trigger: it closes a specific channel in the cell wall, which changes the cell's electrical charge, which opens a different channel, which allows calcium to flood in. The calcium is the final instruction. It tells thousands of tiny storage granules inside the cell to move to the surface, fuse with the outer wall, and release their contents into the bloodstream. From the moment the glucose signal fires to the moment insulin enters your blood: seconds. But what's inside those granules is worth understanding, because it changes how you think about what the body needs to function properly. The insulin sitting in those granules isn't floating loose, ready to be tipped out. It's locked. Six insulin molecules are bundled together around two zinc atoms, crystallised into a tight, stable, inactive structure. Think of it as a pallet of product in a warehouse, shrink-wrapped and strapped down. The product is finished and ready. But it doesn't move — it can't activate — until the release signal arrives and the zinc-bound structure dissolves. Zinc is the mechanism that makes this safe storage possible. Without adequate zinc, the packaging system is compromised. The beta cell can still produce insulin, but it struggles to store it in a controlled, stable form. The warehouse loses its inventory management. This is why zinc — a mineral most people associate with cold remedies or immune function — is also a foundational raw material for your insulin system. The body needs it not just to produce insulin but to hold it safely until the precise moment it's needed. We'll return to zinc's broader role in metabolic health in a later article. For now, the key point is this: metabolic health is not only a carbohydrate story. It is a raw materials story too.

Part 4: What Insulin Actually Does

Most people's understanding of insulin goes roughly like this: you eat carbohydrates, blood sugar rises, insulin is released, blood sugar comes back down. End of story. That understanding is approximately as complete as describing the internet as "the thing that sends emails." Insulin is the primary anabolic hormone in the human body. Anabolic means building — as opposed to catabolic, which means breaking down. When insulin is circulating, your body is in building and storing mode. When insulin is absent or low, your body shifts toward breaking down and burning. Every cell in your body is reading this signal constantly, adjusting its behaviour accordingly. Consider what's happening after an ordinary meal. You've just eaten a plate of food. As blood sugar rises and insulin follows, your liver receives a signal to stop releasing glucose into the bloodstream and start storing it. Your fat cells receive a signal to hold their contents and accept incoming fuel. Your muscle cells receive a signal to take up glucose and begin repair and rebuilding. Your blood vessels relax slightly as insulin stimulates nitric oxide production, improving circulation to the tissues that need it. Your immune cells receive metabolic support to do their work. Your brain receives signals influencing mood, appetite, and the sense of satiety. All of that from one hormone. All of it from a single meal. Insulin is not a sugar management tool with some secondary effects. It is a system-wide coordinator that uses blood sugar as its primary trigger, with downstream effects reaching almost every tissue in the body. The reason blood sugar matters is not because high blood sugar is the only problem — it's because blood sugar is the input signal, and what insulin does with that signal is the real story. Understanding this changes the conversation about insulin resistance. When cells begin to go deaf to insulin's signal, the consequences aren't limited to blood sugar management. They ripple outward into every system insulin was coordinating. That's what insulin resistance actually means. And that's what makes it one of the most consequential and underappreciated health problems of our time.

Part 5: Insulin and Your Muscles — The Brake on Breakdown When most people think about insulin and muscle, they think about building. Insulin is anabolic, therefore insulin builds muscle. That's the association. The reality is more nuanced — and for many people, particularly anyone over forty, anyone who has lost significant weight, and anyone managing a chronic health condition, the more important relationship is the one between insulin and muscle breakdown. Insulin's primary role in muscle tissue is not building. It is protecting. Insulin is the main brake on muscle protein breakdown. When insulin is present at normal physiological levels, it suppresses the pathways that break down muscle protein for energy. It keeps the muscle tissue intact. The actual building of new protein happens primarily when amino acids from food are available alongside insulin. But the protection from breakdown is constant, and it matters enormously for long-term muscle mass. When insulin resistance develops in muscle tissue — when the cells stop responding properly to insulin's signal — that brake weakens. Muscle protein breakdown accelerates. And because this process is slow and silent, most people don't notice it until the consequences are significant. The medical term for the progressive loss of muscle mass and function with ageing is sarcopenia. It is one of the strongest predictors of mortality, disability, and loss of independence in older adults. Falls become more likely. Recovery from illness slows. The ability to perform basic daily tasks erodes. While sarcopenia has multiple causes, insulin resistance in skeletal muscle is increasingly recognised as a key driver — not merely a consequence. Here is the part that almost nobody mentions: as we age, our muscles don't just become less responsive to exercise. They become less responsive to insulin. The same insulin concentration that would maintain muscle protein in a twenty-five-year-old produces a significantly weaker anabolic response in a sixty-five-year-old. The dose-response curve shifts. The cells require a stronger signal to produce the same effect. This has direct practical implications. The older you get, the more important it becomes to do the things that maintain or improve insulin sensitivity in muscle tissue — because a system becoming less responsive needs every advantage it can get. Chief among those things, as we'll come back to, is resistance exercise. Not because exercise builds muscle in some simple mechanical sense, but because movement is one of the most powerful known interventions for maintaining the sensitivity of muscle tissue to insulin's signal.

Part 6: Insulin and Your Skin — The Visible Warning

Your skin is an organ. Most people don't think of it that way — it's more familiar as a surface, the thing you moisturise and protect from the sun. But it is metabolically active tissue, and like most metabolically active tissue, it expresses insulin receptors. It responds to insulin. And when insulin signalling goes wrong, the skin shows it. Sometimes literally. There is a skin condition called acanthosis nigricans — dark, velvety patches of thickened skin that develop in the folds of the body, most commonly at the back of the neck, the armpits, and the groin. Most people who have it assume it's a hygiene issue, or a skin condition with a topical solution. It's neither. It is insulin resistance made visible on the surface of the body. When insulin resistance develops, the pancreas compensates by producing more insulin. Those chronically elevated insulin levels begin to bind to growth factor receptors on skin cells — receptors that respond to insulin at high concentrations by triggering cell proliferation. The skin thickens and darkens in the folds where friction and moisture concentrate this effect. No cream resolves it. No laser removes it permanently. The only effective treatment is addressing the underlying insulin resistance, at which point the skin often normalises on its own. This is one of the most underused diagnostic signals in medicine. Acanthosis nigricans can appear years before a formal diabetes diagnosis — years before blood sugar is significantly elevated, years before most standard markers flag anything unusual. It is the body announcing a problem that the conventional system often isn't looking for yet. But the skin connection goes deeper than that visible signal. When blood sugar runs chronically elevated — even at levels that don't yet qualify as diabetes — a process called glycation causes cumulative damage throughout the body. Glycation is what happens when glucose molecules attach to proteins in a way that was never designed to happen. In the skin specifically, the proteins most affected are collagen and elastin — the structural scaffolding that gives skin its firmness, elasticity, and ability to bounce back. Glycated collagen becomes cross-linked and brittle. It loses the pliability that makes skin resilient. The body's normal repair mechanisms — which continuously replace and rebuild collagen — become progressively less able to repair collagen that's been glycated, because the damage is structural rather than simply worn. The result, over time, is accelerated skin ageing: wrinkling, sagging, and loss of elasticity that happens faster than biological age would explain. There is also a well-documented impairment of wound healing in people with poor insulin signalling. Wounds close more slowly. Infections take hold more easily in compromised tissue. The healing cascade that normally coordinates immune response, cell migration, and tissue rebuilding requires functional insulin signalling at multiple steps. When the signal is degraded, the entire process slows. Your skin is functioning as a metabolic readout — reflecting, in visible and measurable ways, what is happening in your insulin system long before most other symptoms appear. There is an important inversion of this picture worth naming. If insulin resistance accelerates skin ageing through glycation, inflammation, and impaired collagen repair, then maintaining insulin sensitivity does the opposite. It slows those processes. And one of the most reliable ways to maintain insulin sensitivity is regular exercise. This is part of the reason people who exercise consistently tend to look younger than their chronological age would suggest. Research shows that exercise sends circulating signals through the bloodstream that directly stimulate skin cells to build and maintain their structural proteins — collagen, elastin, and the matrix that holds them together. Reduced systemic inflammation, which exercise produces over time, removes one of the primary drivers of collagen degradation. And better insulin sensitivity reduces the glycation that stiffens and ages the skin's scaffolding from the inside. None of those mechanisms work overnight. But they compound. A body that has been moving consistently for years is protecting its skin through at least three converging pathways simultaneously — and none of them require anything applied from the outside.

Part 7: When It Breaks — The Full Cascade of Insulin Resistance

By now you have a sense of how much insulin is managing. It coordinates muscle preservation, maintains skin integrity, regulates the liver, fat tissue, blood vessels, and dozens of other processes simultaneously. All of this depends on one thing working properly: the cells it's signalling need to actually respond. When they stop responding, we call it insulin resistance. And it's worth understanding precisely what that means — because the common explanation is incomplete in a way that limits how seriously people take it. Insulin resistance is not primarily a pancreas problem. It is not even primarily a blood sugar problem, at least not initially. It is a cellular communication failure — and it happens gradually, silently, across multiple organ systems at once. Here is the cellular picture. Insulin works by binding to receptors on the surface of cells — locks that insulin fits like a key. When the key fits the lock, a cascade of signals moves through the cell, opening glucose transporters, activating metabolic pathways, triggering protein synthesis. The cell receives the message and responds. In insulin resistance, the lock becomes stiff. The key still fits, but it no longer turns smoothly. The cellular response weakens. Glucose doesn't enter as efficiently. The metabolic signals downstream don't fire with the same strength. The cell is, in a real sense, going deaf to a signal it used to hear clearly. The body's first response is to shout louder. The pancreas produces more insulin — if the signal isn't getting through at normal volume, increase the volume. For a period of time, this compensatory hyperinsulinemia maintains relatively normal blood sugar levels even as the underlying resistance builds. Blood sugar looks normal on a test. Insulin is abnormally high, but that test is rarely ordered. The dysfunction is invisible to standard screening. Chronically elevated insulin has its own consequences — which we'll explore throughout this article. And over time, as resistance deepens and the beta cells are asked to produce more and more, the compensatory system begins to fail. Blood sugar rises. The clinical threshold for prediabetes or type 2 diabetes is crossed. But by that point, the insulin resistance itself has often been building for years — sometimes a decade or more. What drives insulin resistance in the first place? Multiple converging factors. Chronic caloric excess — particularly from highly processed foods — overwhelms the storage capacity of fat cells, which then begin releasing inflammatory signals that directly interfere with insulin receptor function. Physical inactivity removes one of the most powerful regulators of muscle insulin sensitivity. Sedentary muscle tissue becomes progressively less responsive to insulin's signal. Chronic sleep deprivation impairs insulin sensitivity measurably, even in healthy people, after a single night of poor sleep — a finding we'll examine in detail later. Chronic psychological stress elevates cortisol, which directly antagonises insulin action — a thread we'll follow in the next article in this series. Chronic low-grade inflammation — which obesity, inactivity, poor sleep, and poor diet all contribute to — disrupts insulin receptor signalling at the molecular level. These factors don't operate independently. They amplify each other. Poor sleep raises cortisol. Cortisol drives cravings for processed food. Processed food drives inflammation. Inflammation worsens insulin resistance. Insulin resistance disturbs sleep. The loop closes and tightens. This is why insulin resistance rarely has a single cause and rarely responds to a single intervention. It is a systems problem. Which also means it responds to systems-level changes — changes that address multiple inputs simultaneously. Which is, as it happens, exactly what regular exercise does.

Part 8: Insulin and the Brain — An Emerging Picture

Of all the places insulin turns out to matter, the brain is the most surprising — and arguably the most important. Most people carry a mental model of the brain as something separate from metabolic health. Blood sugar problems happen in the body. The brain is up there, protected, running on its own system. That model is wrong in a way that has significant consequences for how we think about cognitive decline, mood disorders, and one of the most feared diseases of ageing. Your brain has insulin receptors. Dense ones. They're distributed throughout the brain but concentrated most heavily in the hippocampus — the structure most centrally involved in memory formation and learning — and the hypothalamus, which coordinates appetite, energy balance, and the stress response. Insulin in the brain is not managing blood sugar. It is doing something else entirely: regulating synaptic function, supporting the survival of neurons, modulating the release of dopamine, and maintaining the cellular machinery that learning and memory depend on. When insulin signalling in the brain works properly, these processes run cleanly. When it doesn't — when the brain develops insulin resistance — the consequences are primarily neurological rather than metabolic. Brain insulin resistance disrupts dopamine regulation. Dopamine is the neurotransmitter most associated with motivation, reward, and the ability to feel pleasure in ordinary things. When insulin's signal to dopamine-producing neurons weakens, the system becomes dysregulated. Motivation drops. Pleasure from normal activities dims. The emotional landscape flattens. This is one of the direct neurochemical mechanisms connecting metabolic dysfunction to depression and anxiety — not a correlation, not an association, but a pathway. Insulin resistance in the brain impairs the very chemistry of mood and drive. The deeper concern is what happens over decades rather than years. Research shows that brain glucose metabolism — the brain's ability to use glucose as fuel — can begin declining measurably in certain brain regions more than a decade before the first symptoms of Alzheimer's disease appear. The neurons aren't dying yet. There's no detectable memory loss. But the energy supply to specific brain regions is already quietly faltering. A growing number of researchers have proposed that insulin resistance developing in brain tissue is a significant contributor to this early decline — a connection strong enough that some scientists have begun informally describing late-onset Alzheimer's as a form of brain-specific metabolic dysfunction, sometimes referred to in research literature as type 3 diabetes. This framing is not yet officially recognised but reflects a genuine and active area of investigation. The connection sits in the biology. Brain tissue from Alzheimer's patients shows hallmarks of impaired insulin receptor signalling alongside the amyloid plaques and tau tangles that have traditionally defined the disease. These features appear to be connected rather than coincidental. Researchers exploring ways to deliver insulin directly to the brain through nasal spray — bypassing the bloodstream entirely — have produced interesting early results. In one small imaging study, insulin delivered this way was taken up in multiple brain regions critical for memory and cognition, including the hippocampus, the amygdala, and the temporal lobe. In participants with mild cognitive impairment, the uptake pattern differed from healthy brains, suggesting altered insulin transport or processing in early cognitive decline. The research is preliminary and needs replication at scale, but it opens a potentially important window into understanding where brain insulin signalling is failing before clinical damage is done. The preventive implication sits quietly but powerfully behind all of this. If insulin resistance in the brain is a precursor to cognitive decline — and the evidence increasingly points in that direction — then the things that maintain insulin sensitivity are also, in a real sense, brain protective. Including the simple act of regular movement.

9: Insulin and Your Nervous System — The Wire Insulation Story

The nervous system conversation tends to start and end with the brain. But insulin's reach extends well beyond the central nervous system into the peripheral nervous system — the vast network of nerves that carries signals from the brain to every corner of the body and back again. Every nerve fibre that allows you to feel the ground beneath your feet, sense temperature, coordinate the fine movements of your hands, and register pain operates because of myelin — the insulating sheath that wraps around nerve fibres and allows signals to travel quickly and cleanly. Myelin is to your nervous system what insulation is to electrical wiring. Without it, signals degrade, slow, or fail to arrive at all. Maintaining that insulation requires healthy, functioning Schwann cells — the cells responsible for producing and maintaining myelin in peripheral nerves. And Schwann cells express insulin receptors. They depend on insulin signalling to function properly, to proliferate when needed, and to repair myelin when it's damaged. When insulin signalling fails in peripheral nerve tissue, the consequences follow a predictable and deeply unpleasant sequence. Myelin production declines. Existing myelin begins to thin and degrade. Nerve conduction slows. The first thing most people notice is sensory — a faint numbness or tingling, usually starting in the feet or hands. Over time, without intervention, it progresses to more significant loss of sensation, pain, and eventually loss of function. This is diabetic peripheral neuropathy — and in people who have had type 2 diabetes for more than ten years, prevalence rates in studies have exceeded 50%. It is one of the leading causes of non-traumatic limb amputation worldwide. Most people understand it as a consequence of high blood sugar damaging nerves. That explanation is true but incomplete. Here is the piece that changes the picture. When researchers blocked insulin signalling in the peripheral nerves of otherwise healthy, non-diabetic animals — without raising blood sugar at all — those animals developed nerve damage that looked identical to diabetic neuropathy. Slowed conduction. Thinning axons. Impaired sensory function. The damage was reproduced through the insulin pathway alone, without the high blood sugar typically assumed to be the cause. Conversely, when researchers delivered very small amounts of insulin directly to peripheral nerves in diabetic animals — doses too small to affect blood sugar — nerve conduction improved. Axonal atrophy reversed. The nerve tissue responded to the insulin signal directly, independently of glucose management. This has clinical implications that most people with diabetes — and many of their healthcare providers — are not fully aware of. The nerve damage is not purely a blood sugar story. It is an insulin signalling story. The nerves themselves need that signal to maintain their structure and repair capacity. For anyone managing insulin resistance, this means the nervous system is at risk along a timeline that begins earlier than blood sugar tests will show. And for anyone who exercises — which improves insulin sensitivity in peripheral tissues — there is a protective dimension to regular movement that extends down to the wire insulation of the nervous system itself.

Part 10: Insulin and the Autonomic Nervous System — Why You're Wired

There is a part of your nervous system that runs entirely without your input. You don't decide to make your heart beat faster when you stand up quickly. You don't choose to slow your digestion when you're under stress. You don't instruct your blood vessels to dilate during a meal. The autonomic nervous system manages all of it — and it operates in two distinct modes that are meant to balance each other. The first is fight-or-flight. Heart rate climbs, blood pressure rises, digestion pauses, and the body shifts into action mode. The second is rest-and-digest — the counterpart where heart rate slows, digestion activates, and the body moves into recovery, repair, and restoration. This is where sleep deepens. Where healing happens. Where the nervous system finally goes quiet. A healthy system moves fluidly between these two states. What most people don't know is that insulin helps regulate this movement. After a meal, rising insulin briefly activates the fight-or-flight mode. This sounds alarming but it's entirely normal — the brief activation helps maintain blood pressure as circulation shifts toward digestion. Insulin crosses into the brain, signals through the hypothalamus, and the adjustment happens. In a healthy metabolic state the signal resolves quickly, and the nervous system settles back to its resting balance. In insulin resistance, this elegant handoff breaks down. When cells stop responding to insulin, the pancreas produces more of it — and chronically elevated insulin means the fight-or-flight signal never fully switches off. The nervous system sits in a state of persistent low-grade activation that it was only designed to occupy temporarily. Over time the consequences accumulate. Blood pressure climbs. Resting heart rate increases. Heart rate variability — the subtle, healthy variation in the time between heartbeats that reflects a nervous system able to flex and adapt — decreases. The rest-and-digest mode, unable to fully assert itself against the chronic signal keeping the body on alert, loses its capacity to bring the body into genuine recovery. This is why insulin resistance and anxiety often travel together. Why people with metabolic dysfunction frequently describe sleep that never feels restorative. Why cardiovascular risk rises alongside insulin resistance. These aren't separate problems converging by coincidence. They are downstream effects of a nervous system held in low-grade fight-or-flight for months or years by a hormonal signal that won't resolve. This is not psychological. It is physiological — a specific, identifiable mechanism with a specific, identifiable cause. And it responds to exercise. Regular physical activity reduces resting fight-or-flight tone, improves heart rate variability, and shifts the autonomic balance toward greater rest-and-digest capacity. It does this in part by improving insulin sensitivity directly — reducing the chronic excess insulin that was driving the alert signal in the first place. The body gets quieter because the alarm keeping it loud has been turned down.

Part 11: Insulin and Your Immune System — The Missing Connection

Your immune system runs on energy. Every cell that mounts a defence against infection, clears damaged tissue, or resolves inflammation needs fuel to do its job. And the primary signal governing how that fuel is delivered and used is insulin. This is not a widely known relationship. Insulin tends to be discussed in metabolic and endocrine contexts. The immune system tends to be discussed in terms of pathogens and defences. But the two systems are deeply integrated — and the consequences of their interaction, when it breaks down, are significant. Immune cells — the soldiers, scouts, and cleanup crews of your body's defence network — all express insulin receptors. They read insulin's signal as part of regulating their own activity. When that signal is healthy, the immune system maintains a kind of disciplined precision: aggressive when a genuine threat exists, quiet and restorative when it doesn't. When insulin signalling breaks down, that precision goes with it. The first shift happens in macrophages — the immune system's primary cleanup crew, responsible for clearing pathogens, dead cells, and cellular debris. In healthy insulin signalling, macrophages do their job and then shift into a repair-and-resolve mode once the threat is cleared. In insulin resistance, particularly in macrophages living in fat tissue, they get stuck in attack mode. They release inflammatory signals continuously — not in response to any specific threat, but as a chronic background state. The immune system is perpetually on, not because there's always an enemy, but because the signal telling it to stand down isn't getting through. This chronic inflammation then feeds back to worsen insulin resistance, which further impairs immune regulation, which produces more inflammation. The loop is self-reinforcing. As insulin resistance deepens, it reaches into different branches of the immune system and compromises their particular functions. Natural killer cells — your body's rapid-response cells that identify and destroy infected or cancerous cells — lose their killing efficiency. They show up but are less able to do what they came to do. Neutrophils — the first responders that rush to sites of infection — lose two critical abilities in insulin-resistant states. The first is navigation: they become less effective at sensing and moving toward the source of a problem. The second is destruction: once they arrive, their ability to engulf and eliminate bacteria is impaired. A soldier who has trouble finding the battle and then struggles to fight once there. T cells — the immune system's strategic coordinators — shift out of regulatory balance, with inflammatory subtypes gaining dominance over the ones that keep responses calibrated and controlled. The result is a paradox that anyone working in chronic disease management will recognise: people with significant insulin resistance are simultaneously more inflamed and more vulnerable to infection. The immune system is busy — chronically, exhaustingly busy — but not busy doing the right things. It has lost its precision. Wound healing is where this failure becomes most visible. A wound that would close in days in a metabolically healthy person can linger for weeks when insulin signalling is impaired. The coordinated sequence of immune events that normally orchestrates healing — the initial response, the shift to repair mode, the rebuilding of tissue, the final resolution — requires functional insulin signalling at multiple steps. When the signal is degraded, each step slows or stalls. Chronic, non-healing wounds are not primarily a circulation problem. They are an insulin signalling problem expressed through the immune system. One final finding in this area is worth noting because it expands the picture in an unexpected direction. Researchers have found that insulin influences not just the immediate behaviour of immune cells but their longer-term programming — changing how those cells respond to future challenges in a way that persists beyond the original exposure. Your metabolic hormone appears to be actively shaping how your immune system remembers and responds to the world. What that means when chronic excess insulin is doing the programming — versus healthy, well-regulated insulin — is a question researchers are still working to fully answer. But the direction of the evidence is consistent: the metabolic and immune systems are not separate territories. They are the same conversation, conducted in the same language.

Part 12: Insulin and Your Gut — The Bacteria Running Part of the System

Everything discussed so far has treated insulin as a signal that originates in the pancreas and travels outward. That framing is accurate but it misses something important. There is a conversation happening in the other direction — from the gut, upward to the pancreas — that turns out to be a significant part of how the insulin system actually functions. Your gut contains somewhere in the region of 38 trillion microorganisms. Bacteria, primarily, but also fungi, viruses, and archaea — a community collectively called the microbiome that outnumbers your own cells and carries roughly a thousand times more genes than the human genome. This community is not simply a digestive aid. It is a metabolically active system that communicates with your immune system, your nervous system, your brain, and your pancreas. The connection to insulin runs in both directions, and both directions matter. From gut to pancreas: when healthy gut bacteria ferment dietary fibre — the kind found in vegetables, legumes, and whole grains — they produce short-chain fatty acids as byproducts. These molecules travel through the gut wall into the bloodstream and reach specialised cells in the intestinal lining that respond by releasing a hormone called GLP-1. GLP-1 travels to the pancreas and acts as an advance signal — telling the beta cells that food is arriving and amplifying the insulin response that follows. Healthy, diverse gut bacteria are, in a direct and measurable sense, supporting the efficiency and precision of your insulin response every time you eat. When the microbiome is disrupted — by a diet low in fibre, high in processed food, by chronic antibiotic use, by chronic stress — this advance signal weakens. The pancreas gets less preparation. Blood sugar management becomes less precise. From pancreas to gut: insulin resistance changes the gut environment in ways that further disrupt the microbiome. Elevated blood sugar and the inflammatory signals that accompany insulin resistance alter the composition of gut bacteria, favouring species that produce inflammatory metabolites over the ones producing beneficial short-chain fatty acids. The gut lining itself becomes more permeable. Bacterial byproducts that should stay inside the gut begin crossing into the bloodstream, triggering immune activation and worsening systemic inflammation. The most striking demonstration of this relationship came from a series of fecal transplant experiments. When researchers transferred gut microbiota from obese, insulin-resistant mice into mice raised with no gut bacteria of their own, the recipients developed insulin resistance. The microbial community alone was sufficient to transfer the metabolic dysfunction — the gut bacteria weren't just a consequence of the disease, they were a driver of it. It's worth noting this was demonstrated in an artificial experimental model, but the findings align with a growing body of human observational evidence pointing in the same direction. What shapes the microbiome toward the beneficial species? Dietary fibre is the most powerful lever — it is the primary food source for the bacteria that produce short-chain fatty acids. But exercise also matters. Regular physical activity consistently shifts the microbiome toward greater diversity and toward species associated with metabolic health. The gut, it turns out, is another place where movement leaves a beneficial fingerprint.

Part 13: The Sleep Bombshell

Everything discussed so far might create the impression that insulin resistance is a condition that develops over years or decades — the slow accumulation of poor dietary choices, insufficient movement, chronic stress. That is true. But there is a faster pathway into impaired insulin sensitivity that almost nobody talks about, and it happens to most people regularly. One bad night of sleep. This is not a metaphor or a gentle warning about lifestyle. It is a specific, documented, measurable physiological event. In a controlled study, healthy subjects — people with no metabolic dysfunction, normal weight, normal insulin sensitivity — were given a single night of partial sleep restriction, sleeping approximately four hours instead of their normal seven or eight. The following day, insulin sensitivity was measured across multiple metabolic pathways. The result was unambiguous. Partial sleep deprivation during only a single night induced insulin resistance in multiple metabolic pathways in people who were perfectly healthy going in. The mechanism involves several intersecting pathways. Sleep deprivation elevates cortisol — the stress hormone that directly antagonises insulin action, a relationship we'll examine in detail in the next article in this series. It activates the fight-or-flight nervous system, which further impairs insulin signalling. It increases circulating free fatty acids, which interfere with insulin receptor function in muscle and liver. And it disrupts the body's internal clock, which governs insulin sensitivity across the day. The rate at which the body processed and cleared glucose from the bloodstream dropped meaningfully after a single night of sleep restriction in that study. From one night. Most adults in the modern world are not sleeping enough most of the time. The chronic accumulation of that deficit — five hours here, six hours there, disrupted patterns through the week — is producing a persistent, measurable impairment of insulin sensitivity that most people have no idea is happening. They feel tired. They reach for food that spikes blood sugar. Their bodies respond to that spike less efficiently than they should. The blood sugar stays elevated longer. More insulin is required. The demand on the pancreas increases. The next morning they wake up marginally more insulin resistant than they were before. The cycle continues. There is a counterpoint that carries genuine hope. A single session of exercise has been shown to improve insulin sensitivity for up to 48 to 72 hours afterward. The damage from a night of poor sleep and the benefit of a session of physical activity operate on similar timescales. They are not equivalent in magnitude — sleep loss does real damage that needs real recovery. But the system is responsive, not fixed. It recovers. It responds to the right inputs. The sensitivity that is lost can be rebuilt. That responsiveness is the thread that runs through all of this.

Part 14: Cancer, Longevity, and the Surprising Endpoints

By this point the picture of insulin's reach is already broader than most people expect. But there are two more connections that extend the story into territory that tends to stop people when they first encounter it. The first is cancer. The second is how long you live. Insulin and Cancer The relationship between insulin and cancer is not a fringe hypothesis. It is an active and well-funded area of research, and the evidence connecting chronically elevated insulin to the growth of specific cancers has become substantial enough to change how oncologists think about the metabolic environment of tumours. Here is the core finding. Cancer cells are metabolically aggressive — they need enormous amounts of energy to fuel their rapid, uncontrolled division. And many cancer cells — particularly those associated with obesity, including colorectal, breast, and prostate cancers — express insulin receptors. They can read insulin's signal. When insulin is chronically elevated, those cells respond the way any insulin-responsive cell would: they take up more glucose, they accelerate growth, and they become harder to stop. Research from Yale demonstrated this in animal models. Tumour cell lines from obesity-associated cancers, when exposed to elevated insulin levels, grew faster and became more metabolically active. When drugs were used to lower circulating insulin in those animals — without changing the tumours directly, without chemotherapy — tumour growth slowed. The human epidemiological evidence broadly supports the direction of this finding, though the direct causal mechanism in humans is still being established. The mechanism is not that insulin causes cancer. It does not initiate the genetic mutations that turn a normal cell into a cancerous one. What the evidence suggests is that chronic hyperinsulinemia creates an environment in which certain cancers — the ones that can read and respond to insulin — grow faster, spread more aggressively, and may resist treatment more effectively. The elevated insulin is not the spark. It is the accelerant. This reframes the relationship between obesity and cancer risk in an important way. The question shifts from "does body fat cause cancer" to "does the metabolic dysfunction that often accompanies excess body fat create conditions that favour tumour growth." For specific cancer types, the answer increasingly appears to be yes. Insulin and Longevity The longevity connection approaches from the opposite direction — not disease, but the absence of it, sustained across an unusually long life. Centenarians — people who live past 100 — have been studied across different populations, different continents, and different diets. They don't share a single food pattern. They don't share a consistent exercise regimen. They don't share genetics in any simple, replicable way. But when researchers look for metabolic markers that distinguish them from people who died decades earlier, one signal comes up consistently. They maintain insulin sensitivity late into life. Not just adequate insulin sensitivity — the kind that keeps blood sugar from becoming diabetic. But a metabolic responsiveness to insulin that in most people has significantly degraded by their seventies or eighties. In centenarians, the cells are still listening. The signal is still getting through. This does not mean insulin sensitivity is the only determinant of a long life. Longevity is complex, and single-factor explanations for it are almost always incomplete. But the consistency of this finding across independent studies and different populations suggests it is capturing something real — that a body which maintains the ability to respond properly to insulin is a body whose fundamental metabolic machinery is still running cleanly. Insulin sensitivity is not just a marker of whether you're at risk for diabetes. It is a marker of how well your cells are communicating, how efficiently your energy is being managed, how much chronic inflammation your system is carrying, and how well your organs — brain, heart, kidneys, immune system — are being maintained at the cellular level. It is, in a meaningful sense, a proxy for how well the whole system is ageing. Maintaining it is not a narrow metabolic goal. It is one of the broadest, most upstream levers available for healthy ageing.

Part 15: Exercise — The Back Door

Everything in this article has been building toward a simple but consequential question. If insulin sensitivity is this central — if it touches the brain, the nerves, the immune system, the muscles, the skin, the gut, the risk of cancer, and the trajectory of ageing — then what actually maintains it? Diet matters. Sleep matters. Stress management matters. But there is one intervention that works on more of these systems simultaneously, more reliably, and with a faster onset of effect than any other. It is also free, has no prescription requirement, and the evidence base behind it is stronger than most pharmaceutical interventions in the metabolic space. It is exercise. When you move — when your muscles contract repeatedly, whether through walking, lifting, cycling, or any other form of physical activity — something happens in muscle tissue that bypasses the normal insulin signalling pathway entirely. An energy sensor inside the muscle cell activates, and glucose transporters move directly to the cell surface. No insulin required. The door to the cell opens through a completely different mechanism. This is why exercise improves blood sugar management even in people whose cells have become largely deaf to insulin. It doesn't fix the broken lock — it opens a second door. And that door stays open. The improvement in insulin sensitivity that follows a single exercise session persists for 24 to 72 hours. Not because the session is still happening, but because the muscle cells have been primed — the transporters are more available, the signalling machinery is more responsive, and the cell is better prepared to receive glucose efficiently. With regular exercise over weeks and months, the change becomes structural. Muscle tissue builds more of the machinery needed for glucose uptake. The energy-producing structures inside cells multiply and become more efficient. The muscle's capacity to handle glucose improves not just acutely but chronically. The insulin resistance that developed over years of inactivity begins, gradually but measurably, to reverse. But the exercise effect on insulin sensitivity is only the beginning of what movement does across the systems this article has covered. In the brain, exercise increases production of a growth factor that supports neuron survival and the formation of new neural connections. It raises dopamine and serotonin. It improves the brain's own glucose metabolism — directly addressing one of the earliest signals in the cognitive decline pathway we discussed. Regular exercisers show measurably better preserved memory-related brain structures into older age. In the peripheral nervous system, improved insulin sensitivity from exercise helps maintain the function of the cells that keep nerve insulation intact. The signal those cells need to repair and maintain their work is the same signal that exercise helps restore. In the autonomic nervous system, regular physical activity is one of the most reliable ways to shift the balance from chronic fight-or-flight activation toward genuine rest-and-recovery capacity. Heart rate variability improves. Resting heart rate drops. The nervous system becomes more adaptable. In the immune system, exercise has a dual effect that mirrors the dual problem we described. Acutely, it produces a brief, beneficial inflammatory response that trains immune cells and clears cellular debris. Chronically, it reduces the systemic low-grade inflammation that drives immune dysfunction in insulin resistance. In the gut, regular exercise consistently shifts the microbiome toward greater diversity and toward the species that produce the molecules that help your pancreas work properly. In the skin, exercise sends circulating signals that directly stimulate collagen and elastin production, reduces the systemic inflammation that accelerates skin ageing, and through improved insulin sensitivity, slows the glycation process that stiffens and damages the skin's structural scaffolding. In the muscles, regular resistance exercise is the most powerful known intervention for maintaining muscle insulin sensitivity into older age — directly addressing the muscle loss that drives loss of independence. In the cancer risk and longevity picture, the evidence consistently shows that regular exercisers maintain insulin sensitivity longer into life, carry lower levels of the chronic excess insulin associated with tumour-promoting environments, and show biological ageing markers that trail their chronological age. There is no pill that does all of this. There is no single dietary change that touches this many systems simultaneously. There is no supplement, no intervention, no pharmaceutical compound with an evidence base that spans the brain, the nerves, the immune system, the gut, the skin, the muscles, and the ageing process at once. There is a 20-minute walk. And what that walk actually does inside you — minute by minute, system by system — is a story worth telling properly.

A Note on Where to Start

Understanding all of this is one thing. Knowing what to do with it is another. If reading this article made you think seriously about your own metabolic health — if something here landed in a way that made the abstract feel personal — that's exactly where good change begins. Not with a complete overhaul. Not with a perfect plan. With a conversation about where you actually are and what a realistic first step looks like for your specific situation. I work with people ready to understand their bodies well enough to make real, lasting changes. Whether you're managing a chronic health condition, navigating life before or after bariatric surgery, or simply tired of advice that doesn't account for how your body actually works — reach out. Let's talk. geof@coachingforbariataricsuccess.com

Coming Next: Cortisol

Insulin doesn't operate in isolation. Its closest partner — and in chronic stress, its greatest adversary — is cortisol. The hormone that gets blamed for everything from belly fat to burnout is more nuanced, more essential, and more interesting than its reputation suggests. And the way cortisol and insulin interact explains a significant portion of why modern life is so metabolically hostile. That story is next.