The Hidden Cause of Insulin Resistance (It’s Not Just Sugar)

The Muscle "Sink" Effect: Left: Low muscle mass acts like a small bucket, it fills up quickly, causing glucose to "spill over" into the bloodstream and fat stores. Right: Building muscle creates a larger tank. You can handle the same amount of carbohydrates without the metabolic mess.

You might weigh the same as you did at 30, but you are not the same animal. You are softer. You have less muscle. Your GP has started eyeing your blood sugar levels with mild alarm, and you crash after lunch in a way that feels less like fatigue and more like a system failure.

This didn’t happen overnight. It is the cumulative debt of twenty years where muscle quietly atrophied and was replaced by fat. This shift in composition, effectively the changing ratio of engine to fuel tank, is creating metabolic havoc.

Specifically, your cells have stopped listening to insulin.

This condition, insulin resistance, is often painted as a consequence of obesity, but that is a dangerous oversimplification. You do not need to be overweight to suffer from it. You simply need to be under-muscled.

For many, the weight gain does drive the dysfunction. But for others, specifically the "skinny fat," the problem is a lack of storage capacity. Your muscles are the primary disposal site for blood glucose. When you let that tissue waste away, you shrink the tank. Even a normal meal eventually overflows the system because there is nowhere for the energy to go.

Whether driven by excess fat, insufficient muscle, or both, the outcome is the same: your metabolism is no longer efficient. It is broken. Once you enter this state, the rules change. Your body stops burning fat and starts hoarding it. Your energy creates peaks and troughs. You are effectively aging at double speed.

Understanding that this is a tissue problem, not just a weight problem, is the only way to fix it.

What Insulin Does and Why It Matters

Insulin is a hormone produced by your pancreas. When you eat carbohydrates, your digestive system breaks them down into glucose, which enters your bloodstream. Your pancreas detects the rise in blood sugar and releases insulin in response.

Insulin acts like a signalling molecule that tells your cells to absorb glucose from the blood. Muscle cells, fat cells, and liver cells all have insulin receptors on their surfaces. When insulin binds to these receptors, it triggers a cascade of events inside the cell that ultimately moves glucose transporters to the cell membrane, allowing glucose to enter.

Once inside your cells, glucose can be used immediately for energy or stored for later. Muscle and liver cells store glucose as glycogen, a form of quick-access energy. When glycogen stores are full, excess glucose gets converted to fat.

In a healthy system, this process is efficient. A small amount of insulin clears glucose from your blood quickly. Blood sugar rises after a meal, insulin is released, cells absorb the glucose, and blood sugar returns to baseline within a couple of hours. Energy stays stable throughout the day.

The problems begin when this system becomes less efficient.

How Insulin Resistance Develops

Insulin resistance means your cells have become less responsive to insulin's signal. The receptors still work, but the downstream signalling is impaired. It takes more insulin to achieve the same glucose uptake that used to happen easily.

Your pancreas compensates by producing more insulin. For a while, this works. Blood sugar stays in the normal range because insulin levels stay elevated. You might have insulin resistance for years without knowing it because your standard blood tests only measure glucose, not insulin. Your glucose looks fine. Meanwhile, your pancreas is working overtime.

This compensated state is called hyperinsulinemia, elevated insulin with normal blood sugar. It's the hidden phase of metabolic dysfunction that precedes type 2 diabetes by a decade or more.

The consequences of chronically elevated insulin are significant:

Fat becomes harder to access. Insulin is an anabolic hormone. It signals your body to store energy rather than release it. High insulin levels keep fat locked in adipose tissue and make it harder for your body to access stored fat for fuel. Eating more calories than you need creates the fat gain in the first place. Once you're carrying excess body fat and are insulin resistant, the elevated insulin makes accessing that stored fat more difficult during a calorie deficit.

Hunger signals can be disrupted. Insulin interacts with leptin, the hormone that signals satiety. Chronic hyperinsulinemia can impair leptin signalling in the brain. This doesn't cause overeating on its own, but it can make appetite regulation harder.

Energy becomes unstable. When cells are resistant to insulin, glucose clearance is slower and less predictable. Blood sugar spikes higher after meals and then crashes as the delayed insulin response overshoots. The result is the familiar cycle of energy peaks and troughs that many people experience after eating.

Inflammation increases. Insulin resistance is associated with chronic low-grade inflammation. This isn't the acute inflammation of an injury. It's a persistent, systemic inflammatory state that contributes to cardiovascular disease, accelerates ageing, and further impairs insulin signalling in a vicious cycle.

Eventually, the pancreas can't keep up. Insulin production plateaus or declines, and blood sugar starts rising. This is when type 2 diabetes is diagnosed. But the metabolic dysfunction started years earlier, during the long silent phase when insulin was high but glucose was still controlled.

Why Muscle Mass Matters for Blood Sugar Control

Here's something your GP probably hasn't explained, your skeletal muscle is the primary site of glucose disposal in your body.

Under insulin-stimulated conditions, skeletal muscle accounts for 70-80% of glucose uptake. Muscle cells are dense with mitochondria, the cellular machinery that burns fuel for energy. They have high metabolic activity. They need glucose to function, and they have the capacity to store substantial amounts as glycogen.

When you have more muscle mass, you have more metabolic real estate available to absorb glucose. More muscle means more insulin receptors, more glucose transporters, more storage capacity. Your body can handle a larger glucose load without needing to produce excessive insulin.

When you have less muscle mass, that metabolic real estate shrinks. The same meal produces the same glucose load, but you have fewer cells available to absorb it. Your pancreas has to produce more insulin to push glucose into a smaller volume of tissue. The system works harder to achieve the same outcome.

This is why sarcopenia, the age-related loss of muscle mass, is so metabolically dangerous. You're not just losing strength. You're losing the tissue that handles your blood sugar. Every kilogram of muscle lost reduces your body's capacity to dispose of glucose efficiently.

Studies consistently show that muscle mass is inversely associated with insulin resistance. More muscle, better insulin sensitivity. Less muscle, worse insulin sensitivity. A 2011 study in the Journal of Clinical Endocrinology and Metabolism found that each 10% increase in skeletal muscle index was associated with an 11% reduction in insulin resistance.

This relationship is causal, not just correlational. When you build muscle through resistance training, insulin sensitivity improves. When you lose muscle through inactivity or ageing, insulin sensitivity declines. The muscle tissue itself is driving the effect.

Why Body Fat, Especially Visceral Fat, Makes Everything Worse

Excess body fat contributes to insulin resistance through several mechanisms. But not all fat is equal in this regard. Where the fat is stored matters as much as how much you have.

Subcutaneous fat, the fat stored just under your skin, is relatively metabolically benign. It's not ideal to carry excess amounts, but it's primarily a storage depot. It holds energy and releases it when needed. It's doing the job fat is supposed to do.

Visceral fat is different. This is the fat stored deep in your abdominal cavity, surrounding your organs. It's metabolically active in ways that subcutaneous fat is not, and that activity is harmful.

Visceral fat cells release inflammatory cytokines, signalling molecules that promote systemic inflammation. They release free fatty acids directly into the portal vein, which delivers blood to the liver. This flood of fatty acids impairs liver function, increases hepatic glucose production, and contributes to fatty liver disease. They also secrete hormones that directly interfere with insulin signalling.

The result is that visceral fat actively promotes insulin resistance. It's not just passive storage. It's an endocrine organ that's working against you.

This explains why waist circumference is a better predictor of metabolic health than body weight alone. Two people can weigh the same, but the one carrying more visceral fat will have significantly worse insulin sensitivity, higher inflammatory markers, and greater cardiovascular risk.

The combination of low muscle mass and high visceral fat is particularly dangerous. You've lost the tissue that clears glucose efficiently and gained tissue that actively impairs glucose regulation. The metabolic deck is stacked against you in both directions.

The Muscle-Fat Ratio Problem

This brings us to body composition, the ratio of muscle to fat that determines so much of your metabolic health.

You can be at a "healthy" weight according to BMI and still have poor body composition. This is sometimes called being "skinny fat" or, in clinical terms, sarcopenic obesity. Normal weight, but low muscle mass and high body fat percentage. From the outside, you might look fine. From a metabolic standpoint, you're in trouble.

As people age without resistance training, they typically lose muscle and gain fat simultaneously. Weight might stay stable, so they assume nothing has changed. But the composition of that weight has shifted dramatically. Muscle has been replaced by fat. Metabolic capacity has declined. Insulin sensitivity has worsened. All while the number on the scale stayed the same.

This is why weight loss alone is not the answer. If you lose weight through calorie restriction without resistance training, you lose both fat and muscle. Your weight drops, but your body composition may not improve. You end up lighter but with a similar or even worse ratio of fat to muscle. The metabolic problems persist.

The goal is not just to lose weight. The goal is to improve body composition by building or preserving muscle while reducing fat. This shifts the ratio in the right direction and addresses insulin resistance at its source.

How Resistance Training Improves Insulin Sensitivity

Resistance training improves insulin sensitivity through multiple mechanisms, both acute and chronic. Training is the single most powerful factor we have to improve tissue-specific insulin sensitivity.

Acute effects: A single bout of resistance training increases glucose uptake into muscle cells for 24-48 hours afterwards. This happens partly through insulin-dependent mechanisms and partly through insulin-independent mechanisms. Muscle contractions activate GLUT4 transporters, the proteins that move glucose into cells, even without insulin present. Your muscles become glucose sinks that pull sugar from your blood regardless of how well your insulin is working.

Chronic effects: Over time, resistance training increases muscle mass, which expands your glucose storage capacity. It also improves the insulin signalling cascade within existing muscle cells, making them more responsive to insulin's signal. More receptors, better signal transduction, more efficient glucose uptake.

Glycogen depletion and the partitioning effect: This is where it gets interesting. Resistance training depletes muscle glycogen stores. When muscle glycogen is depleted, insulin sensitivity in that tissue improves dramatically. The muscle is now primed to absorb glucose to refill those stores.

Here's why this matters for body composition: insulin sensitivity can be different in different tissues. The ideal scenario is high insulin sensitivity in your muscles and low insulin sensitivity in your fat cells. When your muscles are insulin sensitive, they absorb glucose efficiently. When your fat cells are less insulin sensitive, they don't store calories as easily.

When you train with weights, you deplete muscle glycogen and dramatically increase muscle insulin sensitivity. If your fat cells remain relatively insulin resistant (which they tend to be in people carrying excess body fat), you create a partitioning effect. Calories preferentially go to muscle rather than fat. Your body is essentially redirecting nutrients away from fat storage and toward muscle tissue.

This is why beginners who carry excess body fat often experience something remarkable: they lose fat and gain muscle simultaneously. Their fat cells don't want more calories (insulin resistant), but their trained muscles are hungry for glucose (insulin sensitive after training). The body shifts calories from one compartment to the other.

High-Rep, Low-Rest Training for Glycogen Depletion

If depleting muscle glycogen improves insulin sensitivity, then training in a way that maximises glycogen depletion should maximise that benefit. This is where high-rep, low-rest training comes in.

Muscle glycogen is the primary fuel source for moderate to high-intensity resistance training. The amount of glycogen you deplete depends on several factors: the number of sets, the number of reps per set, the rest periods between sets, and the total time under tension.

Higher rep ranges (15-20 reps per set) deplete more glycogen per set than lower rep ranges. Shorter rest periods (30-60 seconds) prevent full glycogen replenishment between sets, leading to progressive depletion throughout the workout. Full-body training depletes glycogen across more muscle groups than split routines.

A typical glycogen depletion protocol might involve 4-6 sets of 15-20 reps per exercise, with rest periods of 30-60 seconds, covering all major muscle groups in a single session.

The goal is time under tension and total work volume rather than maximal load.

This type of training creates significant metabolic stress. You'll feel it. The burn is intense because hydrogen ions accumulate faster than your body can clear them. Your heart rate stays elevated because the short rest periods don't allow full cardiovascular recovery. And as glycogen depletes, the working muscles have less readily available fuel.

The "Crash": Metabolic Stress vs. Low Blood Sugar This type of training creates immense metabolic stress. You might experience dizziness, nausea, the shakes, or a cold sweat. It feels exactly like a sugar crash, but for most people, it isn't true clinical hypoglycaemia.

It is usually a combination of two things:

1. Metabolic Acidosis: You are generating lactate and hydrogen ions faster than your untrained system can clear them. This rapid rise in acidity triggers a "panic" signal in the brain, causing nausea.

2. The Relative Drop: If you have insulin resistance, your baseline blood sugar is likely high. As your muscles suck up glucose, your levels drop rapidly toward normal. Your brain senses this rate of change and sounds the alarm, releasing adrenaline even though your levels are actually safe.

This is normal and not dangerous for most people, but it can be unpleasant. If you experience these symptoms during training, it means the protocol is working. You're depleting glycogen effectively. Stop the set, rest, and let your blood sugar stabilise before continuing. Having a small amount of fast-acting carbohydrate available (juice, glucose tablets, fruit) can help if symptoms are severe.

The symptoms typically resolve within minutes of stopping exercise. If you consistently feel terrible during these workouts, you may need to eat more carbohydrates in the hours before training, or reduce the volume until your body adapts.

Who should use this approach: Glycogen depletion training is one of the fastest ways to improve muscle insulin sensitivity. If you're insulin resistant and want to see rapid improvements in how your body handles carbohydrates, this style of training delivers results quickly.

It's particularly effective when timed before a higher-carbohydrate meal. Deplete glycogen, then eat. Those carbohydrates are far more likely to be stored as muscle glycogen rather than contributing to fat storage. Your muscles are primed and hungry for glucose.

People following cyclical dieting approaches, where periods of lower carbohydrate intake alternate with higher carbohydrate refeeds, can use glycogen depletion before the refeed to maximise glycogen storage and the anabolic response.

For general health and body composition, incorporating one or two glycogen depletion sessions per week alongside your regular strength training can accelerate improvements in insulin sensitivity beyond what strength training alone provides.

Who needs to be careful? There is a major difference between "feeling" low and "being" low. If you are medicated for diabetes (Insulin, Sulfonylureas, etc.): You are at risk of true, dangerous hypoglycaemia because your medication forces glucose down regardless of your activity level.

You must monitor your blood sugar closely and consult your GP before attempting this intensity. You may need to adjust your dosage. If you are unmedicated: Your liver acts as a safety buffer, preventing you from dropping to dangerous levels. The symptoms you feel are discomfort, not danger. Push through the discomfort intelligently, but respect your limits as you build tolerance.

Mitochondrial function: Resistance training improves mitochondrial density and function in muscle cells. More mitochondria means greater capacity to oxidise fuel. Better mitochondrial function means more efficient energy production and less metabolic waste accumulation.

The research consistently shows that resistance training improves insulin sensitivity independent of weight loss. You don't have to lose fat to see benefits. Building muscle improves glucose regulation even if body weight stays the same. A 2013 meta-analysis in Diabetes Care found that resistance training reduced HbA1c (a marker of long-term blood sugar control) by 0.5% on average, a clinically meaningful improvement.

Combining resistance training with fat loss produces additive effects. You're expanding glucose disposal capacity while simultaneously reducing the tissue that impairs insulin signalling. The metabolic benefits compound.

How Cardiovascular Training Contributes

Cardiovascular training also improves insulin sensitivity, though through somewhat different mechanisms.

Endurance exercise increases mitochondrial density in muscle cells, improving their capacity to oxidise both glucose and fatty acids for fuel. It improves capillary density, enhancing nutrient delivery to muscle tissue. It reduces intramuscular fat, the fat stored within muscle cells that impairs insulin signalling.

Higher-intensity cardiovascular work has additional benefits. High-intensity interval training has been shown to improve insulin sensitivity more than moderate continuous training in several studies. The metabolic stress of high-intensity work triggers adaptations that lower-intensity work does not.

Cardiovascular training also helps with fat loss by slightly increasing energy burnt, which indirectly improves insulin sensitivity by reducing the visceral fat that promotes resistance.

The optimal approach combines both resistance and cardiovascular training. Resistance training builds the muscle that disposes of glucose. Cardiovascular training improves the oxidative capacity of that muscle and helps reduce the fat that impairs insulin signalling. Neither alone is as effective as both together QUICK WIN

The 10-Minute Rule: A Tactical Intervention You don't always need to sweat to manage your blood sugar. There is a "hack" that utilises your muscle's glucose-disposal machinery without the intensity of a gym session: the post-meal walk.

When you sit sedentary after a meal, glucose pools in your bloodstream, requiring a large insulin spike to clear it. However, low-intensity movement activates your muscles just enough to open the glucose gates (GLUT4 transporters) mechanically, bypassing the need for a massive insulin surge.

A 10-minute brisk walk immediately after eating can reduce the post-meal blood sugar peak by up to 22%. Helping with mechanical glucose clearance. Make this a non-negotiable habit, especially after your largest meal of the day.

What About Nutrition?

Training creates the conditions for improved insulin sensitivity. Nutrition determines whether you capitalise on those conditions or undermine them.

Protein intake is critical. Muscle is built from amino acids. Without adequate protein, you cannot build or even maintain muscle mass effectively. For adults over 40, protein needs are higher than often recommended, roughly 1.6 to 2.2 grams per kilogram of bodyweight daily. Most people don't eat enough. Prioritising protein at every meal supports muscle protein synthesis and helps preserve the tissue that regulates your blood sugar. Sleep and Stress: The Invisible Handbrake You cannot out-train a sleep deficit. Sleep deprivation mimics insulin resistance. Just four nights of sleep restriction (4-5 hours per night) causes healthy cells to become significantly less sensitive to insulin.

Furthermore, chronic stress drives high levels of cortisol. Cortisol specifically encourages fat storage in the visceral area the exact deep abdominal fat that drives metabolic dysfunction. If you are training hard and eating well but the waistline isn't moving, look at your sleep hygiene and stress levels. Recovery is where the insulin sensitivity is actually restored.

Carbohydrate timing matters. Eating carbohydrates after resistance training takes advantage of enhanced insulin sensitivity and depleted glycogen stores. The same carbohydrates eaten at rest, when muscle glycogen is full, are more likely to be converted to fat. You don't have to eliminate carbohydrates, but timing them around training sessions optimises their metabolic fate.

Total calorie balance still applies. If you're carrying excess body fat, particularly visceral fat, reducing it will improve insulin sensitivity. This requires eating fewer calories than you expend over time. But the composition of your diet matters for what kind of weight you lose. Adequate protein and resistance training ensure you lose fat while preserving muscle.

Processed foods create problems. Highly processed foods are typically low in protein, high in refined carbohydrates, and engineered to be hyperpalatable. They spike blood sugar rapidly, provoke large insulin responses, and don't trigger satiety effectively. Reducing processed food intake reduces the metabolic load your body has to handle.

Fibre helps. Dietary fibre slows glucose absorption, reducing the spike in blood sugar after meals. It feeds beneficial gut bacteria, which produce short-chain fatty acids that improve insulin sensitivity. It increases satiety, helping with calorie control. Most people don't eat enough. Vegetables, legumes, and whole grains are the primary sources.

None of this requires extreme measures. You don't need to go keto or eliminate food groups or count every calorie. Eating mostly whole foods, prioritising protein, timing carbohydrates sensibly, and maintaining a modest calorie deficit if fat loss is needed will address the nutritional side of insulin sensitivity. One Supplement to Consider: While real food comes first, Magnesium is worth your attention it is responsible for over 300 actions in the body. It is a critical mineral for insulin signalling, and many people with insulin resistance are chronically deficient because high blood sugar causes the kidneys to excrete magnesium at a higher rate. A high-quality Magnesium Glycinate or Citrate supplement can support your body's ability to handle glucose.

The Compounding Problem of Inaction

Here's what happens if you do nothing.

Muscle mass continues to decline at 3-8% per decade. Each year, your glucose disposal capacity shrinks slightly. Insulin resistance worsens incrementally. If calorie intake stays the same or increases, visceral fat accumulates gradually.

These changes are slow enough that you don't notice them day to day. You don't wake up one morning suddenly insulin resistant. It happens over years, the cumulative effect of body composition shifting in the wrong direction.

By the time symptoms become obvious, the dysfunction is advanced. Energy crashes after meals. Stubborn fat that's harder to shift than it used to be. Blood tests that show elevated fasting glucose or HbA1c creeping toward the diabetic range.

Type 2 diabetes is not inevitable, but it is the default outcome for a population that loses muscle mass, gains visceral fat, and doesn't train. The UK has nearly 5 million people diagnosed with diabetes, and 90% of cases are type 2. Another 13.6 million are at elevated risk. These numbers are projected to rise.

The downstream consequences of type 2 diabetes are severe. Cardiovascular disease. Kidney damage. Nerve damage. Vision loss. Increased risk of dementia. Reduced life expectancy by up to 10 years.

None of this is inevitable. All of it is addressable. But addressing it requires action, not just on diet, but on the body composition that's driving the problem.

What You Should Do

If you're over 40 and not resistance training, your insulin sensitivity is declining. That's not a judgement. It's a physiological reality. The question is what you're going to do about it.

Start resistance training. Two to three sessions per week. Compound movements that work large muscle groups. Squat, deadlift, press, row. Progressive overload over time. This is the most powerful intervention for improving insulin sensitivity and the one most people neglect.

Add cardiovascular work. Include some higher-intensity efforts each week, not just steady-state walking. Intervals on a bike, rower, or steep hill. Hard enough that you can't hold a conversation. This improves oxidative capacity and contributes to fat loss.

Prioritise protein. Aim for 1.6 to 2.2 grams per kilogram of bodyweight. Distribute it across your meals. Don't save it all for dinner. Protein at breakfast and lunch supports muscle protein synthesis throughout the day.

Time your carbohydrates. Eat the majority of your starchy carbohydrates around your training sessions, particularly afterwards. This is when your muscles are most receptive to glucose uptake.

Reduce processed foods. You don't need to be perfect. But shifting the balance toward whole foods, meat, fish, eggs, vegetables, fruits, legumes, whole grains, will reduce the metabolic stress your body has to handle.

If you're carrying excess fat, create a moderate deficit. Losing visceral fat will improve insulin sensitivity. But don't crash diet. Severe restriction costs muscle mass, which makes the underlying problem worse. A modest deficit combined with resistance training and adequate protein allows fat loss while preserving muscle.

The Choice

You can continue as you are. Muscle mass will decline. Body fat will accumulate. Insulin sensitivity will worsen. The trajectory leads somewhere you don't want to go.

Or you can intervene now, while intervention is still straightforward. Build muscle. Lose fat. Improve the body composition that determines your metabolic health. The same training that keeps you strong enough to live independently also keeps your blood sugar under control.

This is not about aesthetics. It's about whether your body can handle the food you eat without slowly poisoning itself. It's about whether you spend your later decades healthy and functional or managing a chronic disease and its complications.

The mechanisms are understood. The interventions work. The only variable is whether you do them.

Work With Me

I train adults over 40 who want to build muscle, improve body composition, and take control of their metabolic health. If this article made you realise something needs to change, drop me a message and lets see if training can help nudge the needle.