How metabolism works and what you can actually change

What is metabolism?

Metabolism is all the processes your body uses to turn food into energy and into the material it rebuilds itself from. When people say they have a “slow” or “fast” metabolism, they usually mean how much energy (how many calories) their body uses in a day.

That energy goes three main ways:

• Basic functions, meaning what you use at rest: your heart beats, your brain works, you breathe, you keep your temperature steady. For most people, this is the largest share.
• Digestion: digesting and using food also takes energy. Protein takes more than fat or carbohydrates.
• Movement: from sport to walking around the house, climbing stairs or small movements during the day. This is the part that varies most from person to person.

Does metabolism slow down after 30 or 40?

Many people blame extra kilos on a metabolism that “slows down” after 30 or 40. A study published in 2021 in Science analysed energy use in over 6,000 people, from newborns to 95-year-olds, using a very precise method. The result surprised many:

• in one-year-olds, metabolism relative to body size is about 50% higher than in adults;
• it falls slowly until around 20;
• it stays practically stable between 20 and 60, even during pregnancy;
• it only declines after 60.

That means the kilos gained between 30 and 50 usually come from something else: less movement, more food, less sleep, stress, changes in routine. And those are things you can change.

Why do some people burn more energy than others?

Bigger bodies use more energy, because they have more cells to maintain. Muscle mass matters most: muscle uses energy even at rest, more than fat tissue does.

Other factors:

• how much you move during the day, not just during exercise: walking, standing, housework;
• genetics, to a lesser extent than people think;
• some conditions, such as a thyroid that works too slowly or too fast;
• some medicines, which can change appetite or weight.

If you want to see how your movement, sleep and meals connect, your free MentorSano account keeps your daily journal, and the daily lesson briefly explains topics like this one, tied to what you're working on.

How can you influence your metabolism?

What you do	What happens
Strength exercises twice a week	you keep and build muscle, which uses energy even at rest
More movement during the day	it's the part of energy use you can increase most easily
Protein at every meal	it fills you up better and helps you keep muscle
Enough sleep	short sleep increases hunger and cravings for calorie-dense food
No very strict diets	when food drops suddenly, the body loses muscle too and adapts by using less energy

What doesn't work?

Many products promise to “boost your metabolism”: teas, pills, “fat-burning” foods. Their effect, where there is one, is very small compared with daily movement or keeping your muscle. Coffee and green tea slightly raise energy use for a while, but not enough to change your weight without other changes.

Likewise, eating many times a day to “keep your metabolism going” has no solid evidence behind it. What and how much you eat in total matters more than how often.

Why is it useful to understand metabolism?

When you know where your energy goes, you can choose better where to put your effort. Instead of looking for a miracle product, you can focus on the things that show results over time: moving more during the day, keeping your muscle, sleeping enough and eating meals that fill you up. They're ordinary things, but research shows they're what lasting change rests on.