Does metabolism slow with age? Not when you think

The short answer

• 20–60

  Ages when daily energy use held steady, after allowing for body size

• −0.7%

  Fall in energy use each year after about age 60

• 6,421

  People from 29 countries, aged 8 days to 95 years, in the biggest study

• −499kcal

  A day below expected for The Biggest Loser contestants, 6 years after an extreme diet

Key findingYour metabolism doesn’t slow down at 30 or 40. Allowing for body size, energy use held steady from 20 to about 60, then fell slowly. Eating less lowers it too: mostly because a smaller body needs less energy, and sometimes a little more than that.

What is metabolism?

Metabolism is all the energy (calories) your body uses. A large part of it keeps you alive at rest: your heart, brain, liver and other organs. That part is your BMR, or resting metabolism. In adults it is about 45–70% of the total.

The rest goes on moving, exercise and digesting food. All of it together is your TDEE: all the calories you burn in a day.

Bigger bodies use more energy, mostly because they have more lean tissue, such as muscle and organs. So to compare people fairly, scientists have to allow for body size. Otherwise a small 70-year-old would look “slow” just for being small.

Does metabolism slow with age?

The best answer comes from a 2021 study in the journal Science. A large international team pooled data on 6,421 people from 29 countries, aged 8 days to 95 years. They used a method called doubly labelled water. It measures how much energy people burn in their normal daily lives, outside a lab.

Body size explained most of the differences between people: 83% of them. After allowing for it, the researchers found four stages of life.

• Babies: about the adult level at birth, rising to about 50% above it by age 1.
• Children and teenagers: a slow fall of about 3% a year, reaching the adult level at about 20.
• Adults aged 20 to 60: steady, even during pregnancy.
• Over 60: a slow fall of about 0.7% a year.

At what age does your metabolism slow down?

At about 60, not 30 or 40. The turning point in the study came at about 63, with a likely range of 60 to 66. After that, energy use for body size fell by about 0.7% a year. People in their 90s burned about 26% less than middle-aged adults of the same size.

Resting energy on its own may start to dip a little earlier, perhaps in the late 40s. But the researchers had few resting measurements between ages 45 and 65, so they couldn’t be sure.

Men and women didn’t differ once body size and makeup were allowed for. Men do burn more on average, but they also tend to be bigger, with more lean mass.

Why is my metabolism slow?

If you feel your metabolism is slow, these studies point to a few common reasons:

• Body size. A smaller body, with less muscle, burns less. Size explained 83% of the differences in the 2021 study.
• People differ. Even at the same size, sex and age, energy use varied by 20% or more from person to person.
• Recent weight loss. A smaller body needs less, and energy use can dip a little further for a while. More on that below.
• Age over 60. Energy use falls slowly from then on.

Watches don’t settle the question either: their calorie counts can be well off, as our guide on fitness tracker calories shows. And if you feel very tired or cold all the time, or your weight changes a lot without a clear reason, see a doctor.

What is metabolic adaptation?

When you eat less and lose weight, your body burns less energy. Most of this is simple: a smaller body needs less fuel. Metabolic adaptation is any extra drop on top of that, when energy use falls lower than your new size predicts.

The best-known example comes from The Biggest Loser, a TV weight-loss contest. Researchers followed 14 contestants with severe obesity. In 30 weeks of an intense diet and exercise programme, they lost 58 kg on average. Six years later, they had regained 41 kg of it.

Their resting energy fell from 2,607 to 1,996 kcal a day during the show. Six years later it was 1,903 kcal, even though most of the weight was back. That was about 500 kcal a day less than expected for their body size and age.

Two findings surprised the researchers. People whose metabolism slowed most by the end of the show didn’t regain more weight. And those who kept the most weight off six years later had the biggest slowdown. The authors think the body pushes back in proportion to how hard you keep holding your weight down.

But this was an extreme group: huge weight loss, very fast, with hard exercise. A 2022 systematic review, a careful review of all the studies on one question, looked at ordinary weight loss. It covered 33 studies with 2,528 adults. Most studies found some extra drop. But the better-designed studies found a small drop, or no clear drop at all. And it tended to fade once people’s weight had been steady for a while.

Is 3,500 calories really a pound?

An old rule says you must cut 3,500 kcal to lose a pound of body weight. In metric, that is about 7,700 kcal per kg. So cutting 500 kcal a day should take off about half a kilo, or one pound, every week.

A 2008 paper by Hall, a scientist at the US National Institutes of Health, showed the first problem. The rule assumes everything you lose is fat. But you also lose some lean tissue, which holds far less energy. How much depends on how much fat you have. The rule roughly fits people with more than about 30 kg of body fat, but it is too high for leaner people.

Why weight loss slows down

The rule’s bigger problem is time. As you lose weight, your smaller body burns less energy, so the same diet leaves a smaller and smaller gap. A 2011 paper in The Lancet showed this with a computer model of the body, checked against real diet studies.

Take a 100 kg man who eats 480 kcal a day less. The old rule predicts he loses 22 kg in the first year, and more every year after. The model predicts about half that in year one. His weight then levels off near 75 kg. He reaches half of his total loss in about a year, and 95% of it in about three years.

Hall’s team turned this into a simple rule of thumb for an average adult with overweight. Every lasting cut of about 24 kcal a day takes about 1 kg off your weight in the end. Half of that change comes in about a year, and nearly all of it within about three years.

So a lasting cut of 250 kcal a day would take off about 10 to 11 kg in total (our calculation). About half of it would go in the first year. The old rule would promise 12 kg in the first year alone, and 24 kg by the end of the second.

The same paper offers another reason why diets often stall at 6 to 8 months. Metabolism alone can’t explain that stall in the model. It fits much better if people slowly drift back towards their old eating.

Claims and evidence

• ClaimYour metabolism slows down in your 30s.

  EvidenceIn 6,421 people, energy use for body size held steady from 20 to about 60. It only began to fall after that, by about 0.7% a year.

• ClaimWomen have a slower metabolism than men.

  EvidenceOnce body size and makeup were allowed for, men and women burned the same. Men do burn more on average, but they also tend to be bigger, with more lean mass.

• ClaimDieting damages your metabolism for good.

  EvidenceThe Biggest Loser group still burned about 500 kcal a day less than expected after 6 years. But that was an extreme case. In a review of 33 studies, better-designed ones found a small extra drop or none, and it faded once weight was steady.

• ClaimCutting 500 kcal a day takes off half a kilo every week, forever.

  EvidenceYour energy use falls as you shrink, so weight loss slows and levels off. For a 100 kg man, the 3,500-calorie rule predicted about twice the first year’s loss that a body model did.

What this means for you

What the research can’t tell you yet

• Change in one person. The 2021 study measured each person once. It shows how age groups differ, not exactly how your own energy use changes over the years.
• Your own number. People of the same size, sex and age differed by 20% or more.
• Ordinary dieting. The Biggest Loser study had only 14 people on an extreme programme. The review of ordinary weight loss found methods that varied a lot between studies.
• Models are averages. Hall’s rule of thumb is for an average adult with overweight. It assumes you keep to the diet and don’t change how much you move.

How Bodyvault uses this research

Bodyvault’s starting calorie estimate uses the Mifflin-St Jeor formula, which takes your age, sex, height and weight into account. With Bodyvault Pro, the adaptive mode then learns your real maintenance calories from your weigh-ins and logged food over four weeks. It updates your target once a week. So if your energy use drifts after weight loss, your target can follow it instead of sticking to a fixed formula.

General information about fitness and nutrition, not medical advice. If you have a health condition, are pregnant or take medication, talk to your doctor before changing your diet or training.