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Metabolism does slow as we age, but not when most people might have expected. sanjeri/Getty Images
  • An international study has found that, after accounting for body size, energy expenditure peaks in infancy and then steadily declines until the age of about 20 years.
  • Contrary to the popular belief that metabolism slows in middle age, the research suggests that energy expenditure does not change until close to the age of 60 years, when it starts to fall again.
  • However, energy expenditure varies considerably among individuals, even after accounting for body size, sex, and age.
  • The life phases that the study reveals offer new perspectives on disease, drug activity, and healing, which all link closely to metabolic rate.

A large study that measured the total amount of energy that people expend as they go about their everyday lives has opposed the idea that metabolism slows in middle age.

The study showed that total energy expenditure, adjusted for body size, steadily declines from a peak in infancy until around 20 years of age and then remains stable until about 60. Only then does energy use begin to fall again.

The researchers were surprised to discover that, for their size, 1-year-olds burn calories 50% faster than adults.

“Of course they’re growing, but even once you control for that, their energy expenditures are rocketing up higher than you’d expect for their body size and composition,” says one of the researchers, Dr. Herman Pontzer, associate professor of evolutionary anthropology and global health at Duke University in Durham, NC.

There are several physiological turning points associated with growing older, says Dr. Pontzer, including puberty and menopause.

“What’s weird is that the timing of our ‘metabolic life stages’ doesn’t seem to match those typical milestones,” he says.

Another surprise was how little energy expenditure changes from early adulthood through middle age.

“Perhaps the most unexpected feature was the constancy of metabolic rate in both males and females between the ages of 20 and 60,” tweeted co-author Dr. John Speakman, of the University of Aberdeen in the United Kingdom.

“This suggests that if you are experiencing middle-age spread, it’s more likely to be because you are eating more rather than expending less,” he added.

The research appears in the journal Science.

The international team of researchers analyzed data on the total energy expenditure of 6,421 individuals ranging in age from 8 days to 95 years and living in 29 different countries.

In the past, research into energy expenditure has mostly focused on resting or basal metabolism, which is the number of calories burned just to keep the body ticking over.

Basal metabolism includes the energy that the body devotes to vital functions, such as breathing, digesting food, and pumping blood around the body.

However, this only accounts for 50–70% of all the calories that humans burn. For example, it does not include commonplace but energetic activities, such as walking, climbing stairs, jogging, or shopping for groceries.

The researchers behind the new study took a different approach and used the gold standard scientific technique for measuring total energy expenditure, known as “doubly labeled water.”

This method requires study participants to drink water that contains unusual isotopes (heavier versions) of hydrogen and oxygen atoms.

Researchers then analyze daily urine samples from each person to track the rates at which their body excretes each isotope.

The difference between the two elimination rates reveals how much carbon dioxide the person is producing, which, in turn, reflects the rate at which they are burning calories.

Since the 1980s, researchers have used doubly labeled water to monitor how many calories humans burn as they go about their daily activities. However, the high cost of the oxygen isotope has limited the scale of such studies.

The new study overcame this limitation by pooling the results from numerous studies around the world in a single database.

The authors write that the surprisingly high metabolic rate that they found in the tissues of infants might relate to their rapid growth and development.

In contrast, reduced energy expenditure in older people may reflect a decline in metabolism in their organs.

The scientists believe that the metabolic changes that they have identified will lead to further investigations of disease progression, drug activity, and healing, which are all intimately related to metabolic rate.

In addition, they note that their research identified considerable differences in energy expenditure among individuals, even after they accounted for body composition, sex, and age.

One of the limitations of the study was that it provided no information about possible contributory factors, such as diet and physical exercise.

The authors conclude:

“Elucidating the processes underlying metabolic changes across the life course and variation among individuals may help reveal the roles of metabolic variation in health and disease.”

In an accompanying comment article, two doctors argue that it may be no coincidence that tissue metabolism — which partly reflects the energy that the body devotes to maintenance and repair — begins to decline just as age-related diseases start to increase in frequency.

Timothy Rhoads and Rozalyn Anderson from the School of Medicine and Public Health at the University of Wisconsin-Madison point out that studies in animals show that metabolic changes are central to the aging process.

They write:

“The decline from age 60 is thought to reflect a change in tissue-specific metabolism, the energy expended on maintenance. It cannot be a coincidence that the increase in incidence of noncommunicable diseases and disorders begins in this same time frame.”

“Cellular activity certainly seems to be declining after [60 years of age], but it’s hard to say precisely which processes are changing,” Dr. Pontzer told Medical News Today.

“I agree that maintenance and repair might be declining, contributing to aging processes, but we’ll need more science to pin that down,” he added.