What is metabolism, really?

Your metabolism is constantly at work

While you are reading this, billions of chemical reactions are taking place inside your body. Your heart is contracting, your brain is consuming energy, your liver is transforming molecules, your kidneys are filtering your blood, your muscles are maintaining your posture, your cells are producing proteins, your DNA is being maintained and repaired, your immune system is monitoring your body, and your skin is renewing itself. All of this requires energy and raw materials. This vast ensemble of chemical reactions is your metabolism — and it is happening around the clock, even while you sleep.

Picture your body as a vast city

This is probably the simplest image. A city has power plants, factories, roads, warehouses, recycling systems, communication networks, and waste-treatment facilities. Your body works in a remarkably similar way: food supplies some of the raw materials, the digestive system processes them, the blood transports them, hormones issue instructions, the liver sorts, transforms, and stores, the muscles consume large amounts of energy, and inside our cells the mitochondria help convert that energy into a usable form. All of this must remain coordinated.

Metabolism has two major directions

There are two words worth remembering: catabolism and anabolism. They may sound very medical, but they are actually extremely simple.

Catabolism: breaking down to recover

Your body takes large molecules and breaks them down into smaller ones. Food supplies us in particular with carbohydrates, fats, and proteins, which are digested and converted into usable molecules such as glucose, fatty acids, and amino acids. These molecules can then be used to produce energy or to serve as raw materials. That is catabolism: dismantling, recovering, transforming.

Anabolism: rebuilding

The other direction is exactly the opposite: your body uses energy and small molecules to build something more complex — constructing muscle, producing certain hormones, renewing a cell membrane, synthesising proteins, renewing the skin, or repairing tissue. That is anabolism. And your health depends at all times on the balance between the two.

But where does the energy come from?

We eat food, but your cells obviously cannot use a piece of salmon or an apple directly: everything must be transformed. Digestion releases nutrients, those nutrients circulate in the blood, enter various metabolic pathways, and a portion of their energy is ultimately converted into one fundamental molecule: ATP.

ATP: the small energy currency of your cells

Imagine that your body has an energy currency called ATP (adenosine triphosphate). Your cells use it constantly: to contract a muscle, to transport certain molecules across a membrane, to manufacture new molecules, and to carry out many cellular functions. Yet we have relatively little ATP immediately available, which means we must continually regenerate it. And this is where structures that I find particularly fascinating come into play: the mitochondria.

The mitochondria: our energy powerhouses?

They are often described as the 'powerhouses' of the cell. The image is simplified, but useful. The mitochondria use energy derived from nutrients — glucose, fatty acids, and, under certain circumstances, some amino acids — to help produce ATP. Through several complex metabolic reactions, their products can feed into the Krebs cycle and then the mitochondrial respiratory chain. You don't need to memorise these names; simply remember that a significant portion of the energy contained in what you eat is progressively converted into a form of energy your cells can actually use.

Why metabolism does not mean 'burning calories'

Because energy is not used solely for movement. Even lying completely still in your bed, you consume an enormous amount of energy. Why? Because the brain, heart, breathing, liver, kidneys, body temperature, cellular ion gradients, tissue renewal, and thousands of other processes all need to be maintained. This is what we refer to, among other things, as basal metabolism.

What is basal metabolism?

It is the minimum energy required for your body to function under very standardised resting conditions, and it generally accounts for the largest share of our daily energy expenditure. Here is something many people are unaware of: your body may expend more energy simply keeping you alive than it does during your workout.

So what determines our energy expenditure?

Several factors. First, there is resting metabolism. Then there is voluntary physical activity: walking, running, cycling, strength training, playing tennis. But there is also something fascinating, which I have already touched on: NEAT (Non-Exercise Activity Thermogenesis) — that is, all the energy expended outside of sleep, eating, and structured exercise: walking around one's office, climbing stairs, cooking, doing housework, standing up, fidgeting, gardening, running errands. All these small movements, accumulated over the course of a day, can represent a considerable difference between two individuals.

And eating also consumes energy

It may seem paradoxical, but digesting, absorbing and converting food requires energy: this is the thermic effect of food. And not all macronutrients carry the same metabolic cost — proteins, in particular, have a proportionally higher thermic effect than carbohydrates and fats. This is one of the reasons why the composition of what we eat matters, and not just the calorie count printed on a label.

Why can two people have different energy expenditures?

Because no two of us are identical machines. Energy expenditure depends, among other things, on height, weight, body composition, lean mass, age, sex, physical activity, NEAT, and hormonal and physiological factors. Genetics also plays a role.

Muscle is particularly interesting

Muscle is a metabolically active tissue. That said, be wary of an idea that is often overstated on the internet: gaining one kilogram of muscle does not suddenly turn your body into a calorie-burning furnace at rest — the direct effect on basal metabolism does exist, but remains relatively modest. On the other hand, maintaining good muscle mass is extraordinarily valuable for many other reasons: muscle contributes to mobility, strength, glucose storage and utilisation, insulin sensitivity, bone health indirectly through mechanical loading, independence as we age, and the capacity to remain physically active. It is ultimately this whole picture that becomes metabolically very significant.

Hormones are also conductors of the orchestra

Our metabolism is continuously regulated by hormonal signals: insulin, glucagon, thyroid hormones, cortisol, sex hormones and many others. Take insulin: after a meal, the rise in blood glucose stimulates its secretion; insulin signals to various tissues that nutrients are available and plays a part in their use and storage. Between meals or during prolonged exercise, the hormonal context shifts and the body is better able to draw on its reserves. Our metabolism is therefore dynamic: it adapts constantly.

And the thyroid?

The thyroid does indeed play a major role in metabolic regulation; its hormones influence energy expenditure and a great many of the body's functions. This is why true hypothyroidism can be accompanied by fatigue, feeling cold, a general slowing down, changes in bowel habits and sometimes weight gain. However, not every instance of weight gain means that 'the thyroid is not working properly'. This is precisely why a medical work-up is worthwhile when suggestive symptoms are present.

And as we age, does our metabolism collapse?

No — and this is a misconception I particularly enjoy correcting. We readily attribute our weight gain to age: 'it's normal, I'm 50, my metabolism doesn't work the way it used to.' The reality is more nuanced. A major study published in Science showed, notably, that after adjusting for height and body composition, daily energy expenditure remains remarkably stable throughout much of adulthood, before declining later with ageing. In other words, it is not simply your birthday that, year after year, gradually switches off your metabolism.

What often changes with age is also our lifestyle: we walk less, we lose muscle, we sit more, we exercise less, our sleep can deteriorate, our diet changes, and certain hormonal shifts appear. And gradually, our energy balance and body composition change.

This is excellent news.

Because a significant proportion of these factors can be modified. We cannot stop time, but we can act on our muscle mass, our physical activity, our diet, our sleep, our metabolic health, and our excessive exposure to sedentary behaviour. And that is far more interesting than searching for a supplement supposedly designed to 'boost metabolism'.

Can we really 'hack our metabolism'?

I prefer the word optimise. Your metabolism is not a computer to be hacked, and there is no magic button that doubles energy expenditure. What we can do, however, is create the best possible conditions for our physiology to function properly: moving regularly, doing strength training, maintaining cardiovascular activity, consuming adequate protein according to individual needs, favouring minimally processed and nutrient-dense foods, sleeping enough, avoiding a chronic calorie surplus, and managing a hormonal condition when one genuinely exists. That is what true metabolic optimisation looks like.

Losing weight does not always mean improving your metabolism.

This is another fundamental point. Imagine two people who each weigh exactly 80 kg. The first has significant muscle mass, good cardiovascular fitness, and little visceral fat; the second has less muscle and more visceral abdominal fat. The number on the scale is identical, yet their metabolic situation can be very different. That is why, in metabolic medicine, I do not focus solely on weight: I focus on body composition.

And above all, on visceral fat.

Not all body fat carries the same metabolic significance. Visceral fat, located deep around the abdominal organs, is particularly associated with insulin resistance, certain lipid abnormalities, metabolic fatty liver disease, low-grade inflammation, and an increased cardiometabolic risk. This is why someone may lose relatively few kilograms yet substantially improve their metabolic health if they reduce visceral fat, increase muscle mass, improve insulin sensitivity, and boost physical capacity.

What having a 'good metabolism' really means

It is not being able to eat anything without gaining weight. A healthy metabolism is an organism capable of properly utilizing nutrients, efficiently producing the energy it needs, switching between energy storage and mobilization when required, maintaining well-regulated blood sugar, preserving its tissues, adapting to physical activity, and sustaining its internal balance. This balance has a name: homeostasis.

Homeostasis: probably the true goal

Your body is constantly trying to keep certain variables within ranges compatible with life: temperature, blood sugar, pH, pressure, electrolytes, energy availability, and thousands of other parameters. You eat, it adapts; you fast overnight, it adapts; you run, it adapts; you are cold, it adapts; you sleep, it shifts its functioning again. This capacity for adaptation lies at the heart of human physiology.

The next time someone tells you 'my metabolism is slow'…

Ask yourself instead: which part of metabolism are we talking about? Energy expenditure? Thyroid function? Muscle mass? Insulin sensitivity? Physical activity? Body composition? Lipid metabolism? Because metabolism is not an organ, nor a hormone, and certainly not a switch you can set to 'fast' or 'slow'. It is an immense biological network operating continuously in virtually every one of your cells.

Key takeaways

Your metabolism encompasses all the chemical reactions that allow your body to live: it transforms nutrients, produces energy, builds and repairs tissues, stores certain reserves and mobilizes others, manufactures molecules, eliminates and recycles, and maintains your internal balance. The mitochondria play a major role in energy production; the muscles, liver, and brain all take part; hormones regulate it; your diet supplies its raw materials, and your physical activity continuously reshapes its demands.

I am Dr Florian Vallecillo. And if I had to leave you with just one idea, it would be this: your metabolism is not simply what makes you gain or lose weight — it is what keeps you alive. When we talk about improving our metabolic health, the goal is therefore not only to burn more calories, but to help our body better produce, use, store and distribute its energy while preserving its capacity to adapt. And that is precisely why understanding your metabolism is one of the first steps toward better understanding your health.