Does Metabolism Slow with Age?

In 2021, an international team examined whether metabolism slows with age by analyzing energy expenditure in 6,421 people from 29 countries. The youngest participant was eight days old and the oldest was 95.

The researchers used data collected with the doubly labeled water method—one of the most reliable ways to measure a person’s total energy expenditure under real-world, everyday conditions.

The scale of the study allowed the researchers to map how energy expenditure changes across almost the entire human lifespan.

The result was far more interesting than a simple “yes” or “no.”

Metabolism Does Not Follow a Straight Line

When we talk about “metabolism,” we often mean how much energy the body expends.

But total daily energy expenditure has several components: the energy needed to maintain the body’s basic functions, the energy used in physical activity, and the energy spent processing food.

Body size and composition also matter enormously. A larger body and more fat-free mass usually mean higher absolute energy expenditure.

So if we want to know whether age itself changes metabolism, simply comparing the calories expended by a younger and an older person is not enough.

We must account for differences in body size and composition.

That is exactly what the researchers did.

The First Year Is Extraordinary

Newborns do not begin life with the highest energy expenditure relative to their body size.

During the first year, however, energy expenditure rises sharply.

At around age one, daily energy expenditure adjusted for body composition is roughly 50% higher than adult levels.

This is a period of exceptionally rapid growth and development.

After that, the trend reverses.

Throughout childhood and adolescence, adjusted energy expenditure gradually declines, reaching typical adult levels at around age 20.

Between Ages 20 and 60

This is where the result becomes particularly interesting.

Once the researchers accounted for differences in fat-free mass and fat mass, total daily energy expenditure remained remarkably stable between roughly ages 20 and 60.

There was no clear metabolic crash at 30.

Nor at 40.

Nor an inevitable, gradual slowdown with each decade of middle age.

Even pregnancy did not emerge as a distinct period of elevated adjusted energy expenditure—the increase in absolute energy needs corresponded to changes in body mass and composition.

This does not mean the body is the same at 25 and 55.

It means something much more specific:

after accounting for body composition, the study found no large age-related decline in total daily energy expenditure during this period.

Then Why Does the Body Change?

Here it is easy to make the opposite mistake.

If metabolism is relatively stable between 20 and 60, that does not mean body weight, body composition, and energy needs cannot change.

They can.

Muscle mass may change with age.

Physical activity may change.

We may spend more time sitting.

Diet, sleep, work, and the surrounding environment may change.

Absolute energy expenditure, however, depends strongly on body composition and how much we move.

A person may therefore expend less energy at 50 than at 25 without this necessarily reflecting an automatic age-related metabolic crash.

That distinction matters.

Fat-Free Mass Matters

One of the strongest associations in the study was between energy expenditure and fat-free mass.

This encompasses not only muscle but all non-fat tissues—including organs, which have different and often high energy demands.

The popular idea that “more muscle = faster metabolism” contains some truth, but it is an oversimplification.

Human energy expenditure reflects the combined activity of many organs and tissues, physical activity, the processing of food, and other physiological processes.

Muscle mass is part of the picture.

It is not the whole picture.

The Picture Changes After 60

That stability does not last throughout life.

After about age 60, adjusted total energy expenditure begins to decline gradually.

In the analysis, the decline was about 0.7% per year, even after changes in body composition were taken into account.

Among people aged 90 and over, adjusted daily energy expenditure was about 26% lower than in middle-aged adults.

This suggests that later life involves more than a loss of body mass alone.

Changes in organs and tissues, cellular physiology, body composition, and physical activity are all likely to contribute, but the precise role of each remains under investigation. Contemporary reviews also emphasize the roles of body composition, organs and tissues, and physical activity in changes in energy expenditure across the lifespan.

“Slow Metabolism” Is Not One Thing

That is another reason this subject can be confusing.

When someone says, “My metabolism has slowed down,” they may mean many different things.

They may mean that they expend less energy on physical activity.

That their body composition has changed.

That their total daily energy needs are lower.

Or simply that their body weight is changing in a way they have not seen before.

These are not interchangeable.

“Metabolism” is therefore a useful word, but it is sometimes too broad to serve as an explanation on its own.

Age Matters—But Not Quite as We Expect

The 2021 study does not show that age is irrelevant to metabolism.

It shows almost the opposite: energy expenditure follows distinct stages across the human lifespan.

The first year is a period of exceptionally high energy needs relative to body size.

This is followed by a gradual decline through childhood and adolescence.

Between roughly ages 20 and 60, adjusted total daily energy expenditure remains relatively stable.