"I'm gaining weight because my metabolism slows down with age." Well… not necessarily!

Scientific work on energy expenditure across the lifespan has revealed something particularly interesting: when body size and composition are taken into account, an adult's metabolism does not progressively collapse from age 40, 50, or 60 onward, as is commonly believed.

So why do we so often feel that our metabolism slows down as the years go by?

Because what changes with age is not only our age itself. It is our body, our hormones, our physical activity, our sleep, our diet, and our lifestyle.

And the good news is that we can act on a large part of these factors.

1. Your muscle mass is a genuine metabolic asset

This is probably one of the most important elements. Over the years, if we do nothing to preserve it, we tend to gradually lose muscle mass.

Yet muscle is not simply there to allow us to lift weights or to move around. It is a metabolically active tissue.

Maintaining sufficient muscle mass helps preserve energy expenditure, insulin sensitivity, functional capacity, and, more broadly, better metabolic health.

This is why I consider maintaining muscle to be one of the great strategies for healthy ageing. After 40 or 50, muscle becomes an asset that must be protected. This involves, in particular, physical activity, strength training, and appropriate nutritional intake — especially protein.

2. Hormones matter too

Our hormonal environment changes with age. Testosterone, oestrogens, growth hormone, thyroid hormones, insulin, cortisol… all of them participate, directly or indirectly, in the regulation of body composition and metabolism.

This obviously does not mean that hormones should be systematically 'corrected' or that hormonal treatment should be prescribed for everyone.

On the other hand, when symptoms or clinical suspicion of a hormonal imbalance exist, understanding what is happening and investigating a possible abnormality can make medical sense. Metabolism never functions independently from the rest of our physiology.

3. Chronic inflammation and the mitochondria

There is also a phenomenon we refer to as inflammaging: a low-grade chronic inflammation that tends to increase with ageing.

This inflammation is influenced by many factors: excess visceral adipose tissue, sedentary behaviour, diet, sleep, chronic stress, certain pathological conditions… It can contribute to metabolic disruptions and is closely linked to mitochondrial health.

And mitochondria are essential. They are what allow our cells to convert nutrients into energy. A metabolically healthy organism is therefore not simply one that 'burns a lot of calories'. It is an organism capable of producing and using its energy efficiently.

4. As we age, we often move much less without realising it

This is a considerable factor, yet one that is very little known. There is what we call NEAT — Non-Exercise Activity Thermogenesis: the energy expenditure linked to all physical activities that do not correspond to a planned sports session.

Walking around the house. Getting up regularly. Cooking. Running errands. Taking the stairs. Working standing up. Gardening. Moving your legs or hands while talking. Taking a few steps during a phone call.

All these small activities seem insignificant when taken separately. Yet, accumulated over an entire day, the differences in NEAT between two individuals can represent several hundred kilocalories of daily energy expenditure.

Over the years, our lives sometimes gradually become more comfortable… but also much more sedentary. We drive more. We sit for longer. We move less spontaneously. And we then attribute this change entirely to our 'metabolism'.

5. Are you sleeping poorly? Your metabolism may suffer the consequences

Sleep is another element that is too often overlooked. Insufficient or poor-quality sleep can disrupt the mechanisms that control appetite, insulin sensitivity, glucose regulation and certain hormonal responses.

And when we are tired, another phenomenon comes into play: we also tend to move less and to seek out more energy-dense foods.

Sleep therefore forms an integral part of a metabolic strategy. Trying to optimise your metabolism while neglecting your sleep is contradictory.

6. Does every calorie require the same amount of energy to be metabolised?

No. This is what is known as the thermic effect of food. Our body itself expends energy to digest, absorb and metabolise what we eat. And this effect is not the same for all macronutrients.

Proteins in particular have a thermic effect that is noticeably higher than that of carbohydrates and fats. They also have another fundamental advantage as we age: they contribute to maintaining muscle mass when combined with appropriate physical activity.

This does not mean that carbohydrates or fats should be eliminated. It means that we need to look at the overall quality of our diet, rather than thinking purely in terms of calories.

A diet dominated by ultra-processed products, low in fibre and very energy-dense, will not produce the same effects on satiety, body composition and metabolic health as a diet rich in appropriate proteins, vegetables, fibre, good sources of fats and minimally processed foods.

So, is it really age that makes us gain weight?

Not on its own. What often accompanies age can promote weight gain: less muscle, less daily movement, greater sedentary behaviour, hormonal changes, sometimes poorer sleep, a diet that is no longer suited to our needs, an increase in visceral fat mass, and sometimes a less favourable inflammatory and metabolic environment.

This is why two people of the same age can present with completely different metabolic situations.

And this is probably the most important message I want to share with you: ageing does not mean being condemned to gain weight. Rather than trying to 'hack' your metabolism with miracle solutions, you need to create the conditions that allow it to function properly.

If this kind of explanation helps you better understand your health, share this article with someone who still tells you: 'It's normal that I'm gaining weight… it's just age.'