Nutrition
James Swift · 1 Feb 2026 · 16 min read
Insulin resistance is multifactorial, but skeletal muscle is one of the most powerful, modifiable levers you have over it. What the evidence actually supports, and what it doesn't.
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. Insulin resistance is genuinely multifactorial — genetics, visceral and liver fat, physical inactivity, sleep, stress, certain medications and medical conditions can all play a part. But one contributor gets far less attention than it deserves: how much muscle you're actually carrying.
You do not need to be overweight to develop insulin resistance. Being under-muscled can be enough on its own for some people, even at a stable bodyweight — your muscles are a major site for clearing glucose from the blood, and when that tissue shrinks, there's less capacity to absorb what you eat. For others, weight gain is the bigger driver, particularly visceral fat around the organs. Both routes point toward the same practical answer: building and keeping muscle helps, regardless of which one applies to you.
Skeletal muscle is one of the most powerful, modifiable levers you have over your metabolic health. It is not the only cause of insulin resistance, and resistance training is not the only fix — but it is the lever most people never pull, and it's where the rest of this article focuses.
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.
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. Elevated insulin makes it harder to draw on stored fat for fuel, on top of — not instead of — the calorie balance that determines fat gain in the first place. Once you're carrying excess body fat and are insulin resistant, that combination makes fat loss feel harder than the numbers alone would suggest.
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.
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 roughly 70-90% of glucose uptake (4). 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 (1).
Resistance training reliably improves glycaemic control and insulin sensitivity, and greater muscle mass is consistently associated with better metabolic outcomes. What the evidence does not yet establish is that muscle growth itself is the sole mechanism producing that improvement. A 2024 systematic review and meta-analysis found that resistance training improved glucose homeostasis even though the accompanying gains in fat-free mass did not consistently explain the size of that improvement (2), and a separate mechanistic review reached the same conclusion: the two changes tend to happen together without the evidence yet showing that one is directly driving the other (3). Skeletal muscle is a major site of insulin-stimulated glucose uptake regardless, which is reason enough to build and keep it.
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.
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.
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.
The mechanism behind resistance training's benefit for insulin sensitivity is well established: muscle contractions pull glucose into working muscle through insulin-independent pathways, and over time resistance training also improves how well the insulin-signalling machinery inside muscle cells works. None of that requires a specialised protocol.
What the evidence actually supports is straightforward, and it matches current clinical guidance: regular resistance training, covering the major muscle groups, with the load progressed over time and performed at an intensity you can sustain and recover from. The American Diabetes Association's 2026 Standards of Care recommend 2-3 resistance sessions a week on non-consecutive days for adults with diabetes, alongside 150 minutes or more of moderate-to-vigorous aerobic activity most weeks (5). Consistency across weeks and months matters far more than chasing the most metabolically stressful version of a workout.
Resistance training improves insulin sensitivity independent of weight loss — you don't have to lose fat to see benefits, and building or maintaining muscle improves glucose regulation even if body weight stays the same. Meta-analyses of resistance training in people with type 2 diabetes consistently find a meaningful reduction in HbA1c, a marker of long-term blood sugar control (6).
Combining resistance training with fat loss produces additive effects: you expand your glucose-disposal capacity while reducing the visceral fat that impairs insulin signalling. The two work together rather than substituting for each other.
If you become dizzy, shaky, faint, confused, nauseated, or develop a cold sweat during training, stop the exercise. Do not assume the cause. If you monitor your blood glucose, follow your established diabetes or hypoglycaemia plan. If you take insulin or a sulfonylurea, talk to the clinician managing your diabetes about how exercise interacts with your medication before you change your training — physical activity can affect hypoglycaemia risk, and that guidance needs to come from them, individualised to you, not from an article.
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.
Training creates the conditions for improved insulin sensitivity. Nutrition determines whether you capitalise on those conditions or undermine them.
Protein intake matters. Muscle is built from amino acids, and without adequate protein you cannot build or maintain muscle mass effectively. The appropriate amount varies with body size, age, training, total energy intake and health status rather than having one target that fits everyone — active lifters often use higher intakes in a sports-nutrition context, but that's a different question from the treatment target for insulin resistance. People with kidney disease or another condition affecting dietary protein should follow the target agreed with their clinician or dietitian. Prioritising protein at every meal, within whatever range is right for you, 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.
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 over 4.7 million people diagnosed with diabetes, around 90% of cases are type 2, and a further 13.6 million people are at elevated risk (7). 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.
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. The right amount depends on your body size, age, training and health status — there's no single number that fits everyone, and if you have kidney disease or another condition affecting dietary protein, follow the target your clinician or dietitian has set. Distribute what you do eat across your meals rather than saving it all for dinner; protein earlier in the day supports muscle protein synthesis throughout.
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.
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.
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.
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