Muscle, Metabolism & Ageing Well: Why Muscle Is Central to Metabolic Health, How It Controls Blood Sugar and Energy Use, and Why It Matters More With Age

If there is one body tissue that quietly determines how well you age, it is muscle.

Not because you need to look a certain way. Not because you need to train like an athlete. Not because everyone should be chasing more size. Muscle matters because it is one of the most powerful metabolic organs you have. It is where a huge proportion of your daily energy use takes place. It is one of the main places glucose can be stored and used safely. It produces signalling molecules that influence inflammation and insulin sensitivity. It stabilises joints, protects bone, and preserves independence. It is also one of the strongest predictors of whether you remain capable, mobile, and resilient as the years pass.

Most people have been taught to think of metabolism in terms of calories. Calories in, calories out, and everything else is secondary. But metabolism is not just energy accounting. Metabolism is how your body handles fuel and distributes it. It is how efficiently glucose is cleared from the bloodstream. It is how easily you can switch between using carbs and fat. It is how stable your energy feels between meals. It is how much of your energy intake goes toward building and repairing tissue versus being stored as visceral fat and driving inflammation.

Muscle sits right at the centre of all of that. And the reason this matters so much with age is simple. Without deliberate effort, most people lose muscle over time. And when muscle declines, metabolic health usually declines with it.

To understand why, we need to look at what muscle actually does inside the body.

 

Muscle Is Not Just For Movement. It’s a Major Glucose Disposal System

One of the core roles of skeletal muscle is movement. It contracts to create force. But metabolically, skeletal muscle is also one of the largest sinks for glucose in the human body. When you eat carbohydrate, glucose rises in the bloodstream. Insulin is released. Insulin tells tissues to take glucose out of the blood. Muscle is one of the biggest tissues responding to that signal.

Muscle can take glucose and burn it immediately for energy. It can also store glucose as glycogen, which is essentially carbohydrate storage inside muscle fibres. This glycogen can later be used during activity. This means muscle acts like a sponge for glucose. The more muscle you have, and the more metabolically active it is, the more capacity you have to clear glucose after meals and store it safely.

This matters because excess glucose in the bloodstream is not benign. Over time, elevated blood glucose increases oxidative stress, drives glycation, and damages blood vessels and tissues. A body that can clear glucose efficiently is a body that reduces that damage automatically.

Now, here is the crucial part. Muscle glucose uptake is influenced not only by insulin, but by contraction. When muscles contract during activity, they can pull glucose into cells through mechanisms that do not rely on insulin in the same way. That means exercise gives you two powerful benefits at once. It improves immediate glucose clearance, and it improves insulin sensitivity over time.

So muscle is not just something that “uses calories.” It is a direct controller of blood sugar physiology.

 

Muscle Is One of the Biggest Determinants of Insulin Sensitivity

Insulin sensitivity is essentially how well your cells respond to insulin’s signal. If insulin sensitivity is high, a small amount of insulin does the job. If insulin sensitivity is low, the body has to produce more insulin to achieve the same effect. This higher insulin environment has consequences. It makes fat burning harder, because insulin suppresses fat breakdown. It increases fat storage signalling. It increases inflammation. It can change lipid handling in the liver and raise triglycerides. It creates the metabolic pattern that underpins much of modern chronic disease.

Muscle is one of the biggest drivers of insulin sensitivity because it is such a major glucose disposal site. If muscle mass is low, the body has fewer places to put glucose. That means glucose stays elevated for longer after meals. The pancreas produces more insulin to push it down. Over time, cells adapt by becoming less responsive. Insulin resistance develops.

If muscle mass is higher, the opposite occurs. There is more capacity to clear glucose. Less insulin is required. The insulin signal becomes more effective. Blood sugar stability improves. And the whole metabolic environment becomes calmer.

This is one reason why “metabolic health” is not just about what you eat. It is about how much muscle you have and how well that muscle is being used.

 

Muscle Is A Reservoir of Metabolic Flexibility

Metabolic flexibility is the ability to switch between fuels, using carbohydrate when it is available and using fat when it is not. Muscle is central to that flexibility because it can store glycogen and it can oxidise fatty acids efficiently when trained to do so. A well-conditioned muscle is like a versatile engine. It can run on different fuels depending on demand.

When muscle is inactive for long periods, its metabolic machinery becomes less flexible. Mitochondrial density can decline. Fat oxidation capacity reduces. Glucose transport becomes less efficient. The body becomes more reliant on frequent carbohydrate intake because the ability to access stored fuel is impaired.

When muscle is used regularly, particularly through a combination of resistance training and aerobic movement, mitochondrial function improves, glucose handling improves, and fat oxidation improves. The body becomes better at running on stored energy between meals and better at handling carbohydrate when it is consumed.

So muscle is not only about strength. It is about fuel versatility, which is a major contributor to stable energy day-to-day.

 

Muscle Is A Metabolic Endocrine Organ

Muscle does not just respond to hormones. It produces signals too.

When muscle contracts, it releases molecules called myokines. These act like chemical messengers that communicate with other tissues. Some myokines have anti-inflammatory effects. Some influence insulin sensitivity. Some interact with fat tissue and liver metabolism. Some influence brain health. This is one of the reasons exercise has effects far beyond “burning calories.”

In a sedentary state, you don’t get that myokine signalling. Muscle becomes quieter metabolically. In a physically active state, muscle becomes an active endocrine organ, shaping the body’s inflammatory and metabolic environment in beneficial ways.

This is part of why movement is so powerful. It is not simply energy expenditure. It is whole-body signalling.

 

Why Muscle Loss Happens With Age and Why It Matters So Much

Muscle loss with age is so common that it has its own clinical name: sarcopenia. It can begin earlier than most people realise, often from midlife onward, particularly if activity levels drop, protein intake is inadequate, sleep is poor, and chronic stress is high.

Sarcopenia matters because it changes the trajectory of ageing. It increases frailty risk. It increases fall risk. It increases fracture risk. It reduces mobility. It reduces independence. It also worsens metabolic health.

The metabolic consequences of muscle loss are often underappreciated. With less muscle, blood sugar control worsens. Insulin resistance becomes more likely. Visceral fat tends to increase. Inflammation tends to rise. Energy becomes less stable. Recovery from illness slows. The ability to tolerate physiological stress decreases.

This is one reason why people can “suddenly” gain weight in their 40s, 50s, and 60s even when they are eating similarly. It is not that the body randomly starts storing fat. It is that muscle has quietly declined, energy expenditure has declined, glucose disposal capacity has declined, and insulin has had to work harder. The metabolic environment changes. Fat storage becomes more likely, particularly around the abdomen.

This is also why the scale can be misleading. Someone can maintain the same body weight while losing muscle and gaining fat, a process sometimes called sarcopenic obesity when it becomes pronounced. The weight might not change dramatically, but metabolic health worsens because the proportion of metabolically protective tissue has declined.

 

Muscle Protects Bone, Joints, and Long-Term Mobility

Ageing well is not only about disease risk. It is about function. Can you get up from the floor. Can you climb stairs. Can you carry shopping. Can you travel. Can you remain independent. Can you avoid being injured by minor falls.

Muscle strength and power are central to this.

Muscle protects joints by stabilising them. It reduces the mechanical load that ends up on joint surfaces. It supports healthy movement patterns. It reduces pain risk. Muscle also supports bone health. Bone responds to mechanical load. When muscles pull on bones during movement and strength training, it signals bones to maintain density. Without that mechanical signalling, bone is more likely to lose density over time.

This is why strength training is not a cosmetic activity. It is a musculoskeletal preservation strategy. And it becomes more important with age, not less.

 

How Nutrition Supports Muscle and Metabolic Health

If muscle is so central, the next question is how to preserve it and support it properly. Nutrition plays a major role here, not just through protein but through the entire metabolic environment.

Protein is the obvious starting point because muscle is built from amino acids. But it is not just about “getting enough protein.” It is about supporting muscle protein synthesis, which is the process of building and repairing muscle tissue. Muscle protein synthesis responds to dietary protein, but it also responds to resistance training and overall energy balance.

As people age, muscle becomes less responsive to protein, a phenomenon often called anabolic resistance. This means older adults may need a higher protein intake, and more strategically distributed protein intake, to stimulate muscle repair and growth compared to younger individuals. It also means strength training becomes even more important, because exercise sensitises muscle to protein and makes dietary amino acids more likely to be used for repair rather than oxidised for energy.

The quality of protein matters too. Proteins rich in essential amino acids, particularly leucine, are especially effective at stimulating muscle protein synthesis. This can come from animal sources such as eggs, dairy, fish, and meat, or from plant sources when they are combined properly, such as legumes with grains, tofu and tempeh, and higher-protein plant foods such as lentils. The goal is not ideological perfection. The goal is amino acid sufficiency.

Energy intake also matters because severe caloric restriction makes it harder to preserve muscle, particularly if protein intake is not high and resistance training is not present. This is one reason crash dieting can backfire metabolically. Weight can drop, but muscle is lost along with fat, and metabolic health worsens in the long term.

Micronutrients matter as well. Vitamin D plays a role in muscle function and strength. Magnesium is essential for muscle contraction and energy production. B vitamins support mitochondrial function and energy metabolism. Omega-3 fats appear to support muscle protein synthesis signalling and may help preserve muscle function in older adults, partly through anti-inflammatory effects. Creatine, while often thought of as a “gym supplement,” is one of the most studied compounds for supporting muscle power and may be particularly useful with age because it supports high-energy phosphate availability in muscle.

Inflammation is also relevant. Chronic inflammation increases muscle breakdown and reduces muscle protein synthesis. It shifts the body toward catabolism, meaning breakdown rather than repair. This is why anti-inflammatory dietary patterns can indirectly preserve muscle by keeping the internal environment more anabolic.

Blood sugar stability matters too. Large glucose swings and insulin resistance are linked to poorer muscle quality over time, and metabolic dysfunction tends to accelerate muscle loss. So the work of improving metabolic health and the work of preserving muscle are not separate projects. They are the same project expressed through different tissues.

 

Exercise: The Non-Negotiable Signal for Muscle Preservation

Nutrition supports muscle. But exercise is the signal that tells the body muscle is needed.

Without that signal, the body has no reason to maintain expensive tissue. Muscle is metabolically costly. It requires energy and amino acids. In times of low demand, the body will preferentially reduce it. Resistance training is the clearest signal that muscle must be preserved. It creates mechanical tension that stimulates muscle protein synthesis and strengthens connective tissue, tendons, and bone.

Aerobic exercise supports muscle in a different way. It improves mitochondrial function, improves fat oxidation, supports insulin sensitivity, and improves cardiovascular delivery of oxygen and nutrients. It also supports recovery and reduces inflammation. For ageing well, a combination of resistance training and aerobic movement is ideal, because it supports both muscular strength and metabolic flexibility.

But the most important point is that “exercise” does not have to be extreme. The body responds to consistent, progressive signalling. You can build and preserve muscle with intelligent, appropriate resistance work at any age, provided it is scaled to the individual.

 

The Bigger Picture: Muscle Is the Foundation of Healthspan

Lifespan is how long you live. Healthspan is how long you live well.

Muscle is one of the most important foundations of healthspan because it supports the core functions that make life enjoyable and independent. It stabilises metabolic health by buffering blood sugar and improving insulin sensitivity. It reduces visceral fat accumulation by improving fuel handling. It produces beneficial signalling molecules during movement. It protects joints and bone. It improves resilience during illness and recovery. It reduces frailty risk.

In many ways, muscle is the body’s metabolic savings account. It gives you capacity. It gives you resilience. It gives you flexibility. And as you get older, those qualities matter more than almost anything else.

If you want to age well, you do not need to chase perfection. You need to protect muscle deliberately. You need to eat in a way that supplies the building blocks and maintains metabolic stability. You need to move in a way that tells your body muscle is still required. And you need to do it consistently enough that the body adapts.

Because the goal is not just to live longer. It is to live longer with strength, stability, and the freedom to move through life without the body becoming the limiting factor.