Many of us might say that we remember a time when we could eat anything we wanted and not gain weight. Metabolism and total energy expenditure do, in fact, change throughout our lives, and the results of a new study has documented a series of distinct, related changes in unprecedented detail. And the findings suggest that metabolism—the rate at which our bodies burn calories—actually peaks much earlier in life, and starts its inevitable decline later than we might guess. The study data—drawn from a large cohort of humans spanning from birth to old age—shed new light on human development and aging and could help to shape targeted nutrition and health strategies across the human lifespan.

“There are lots of physiological changes that come with growing up and getting older,” said study co-author Herman Pontzer, PhD, associate professor of evolutionary anthropology at Duke University. “Think puberty, menopause, other phases of life. What’s weird is that the timing of our ‘metabolic life stages’ doesn’t seem to match those typical milestones.” Pontzer and an international team of scientists report on their findings in Science, in a paper titled, “Daily energy expenditure through the human life course.”

All of life’s essential tasks, from development and reproduction, to maintenance and movement, require energy, the authors wrote. “Total daily energy expenditure (total expenditure; megajoules per day) is thus central to understanding both daily nutritional requirements and the body’s investment among activities.” However, scientists actually know very little about total expenditure in humans, or how it changes over the course of a life span.

Previously, most large-scale studies have measured how much energy the body uses to perform basic vital functions such as breathing, digesting, pumping blood, and other essential activities. These are effectively the calories you need just to stay alive. But that amounts to only 50%–70% of the calories that we burn each day. It doesn’t take into account the energy we spend doing everything else: washing the dishes, walking the dog, breaking a sweat at the gym, even just thinking or fidgeting.

For their newly reported study Pontzer and an international team of scientists analyzed the average calories burned by more than 6,600 people, male and female, ranging from one week old to age 95 years, and in 29 countries, as they went about their daily lives. To come up with a number for total daily energy expenditure, the researchers relied on the “doubly labeled water” method. For this urine test the individual first drinks water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring “heavy” forms, and the evaluation involves measuring how quickly they’re flushed out.

The technique is considered the gold standard for measuring daily energy expenditure during normal daily life, outside of the lab, and scientists have used the method to measure energy expenditure in humans since the 1980s. However, to date, studies have been limited in size and scope. “Doubly labeled water studies provide measurements of total expenditure in free-living subjects but have been limited in sample size (n < 600 subjects), geographic and socioeconomic diversity, and/or age.”

For the newly reported study, multiple labs decided to share their data and gather their measurements in a single database, to see if they could tease out new insights that either hadn’t previously been revealed or were only hinted at in previous work.

Pooling and analyzing energy expenditures across the entire lifespan revealed some surprises. Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak. But the researchers found that, pound for pound, infants had the highest metabolic rates of all. They found that energy needs shoot up during the first 12 months of life, such that by their first birthday, a one-year-old burns calories 50% faster for their body size than an adult. “For subjects between 9 and 15 months of age, adjusted total and basal expenditures were nearly ~50% elevated compared with that of adults,” the authors commented.

And that’s not just because, in their first year, infants are busy tripling their birth weight. “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,” said Pontzer.

An infant’s super-fast metabolism may partly explain why children who don’t get enough to eat during this developmental window are less likely to survive and grow up to be healthy adults. This early period of metabolic acceleration corresponds to a critical period in early development in which growth often falters in nutritionally stressed populations,” the investigators stated. “Increasing energy demands could be a contributing factor.” Pontzer commented, “Something is happening inside a baby’s cells to make them more active, and we don’t know what those processes are yet.”

The study data also showed that, after this initial surge in infancy, metabolism slows by about 3% each year until we reach our 20s, when it then levels off to a new normal. “After rapid acceleration in total and basal expenditure during the first year, adjusted expenditures progressively decline thereafter, reaching adult levels at ~20 years of age,” the investigators wrote. And despite the teen years being a time of growth spurts, the researchers didn’t see any uptick in daily calorie needs during adolescence, after they had taken body size into account. “We really thought puberty would be different and it’s not,” Pontzer said.

Midlife was another surprise. We may have been told that when it comes to weight, its downhill after 30. But while several factors could explain the thickening waistlines that often start to develop at this stage in life, the new study findings suggest that a changing metabolism isn’t one of them. “Adult expenditures, adjusted for body size and composition, are remarkably stable, even during pregnancy and postpartum,” the team noted.

In fact, the researchers discovered that energy expenditures during these middle decades— our 20s, 30s, 40s and 50s—were the most stable. Even during pregnancy, a woman’s calorie needs were no more or less than expected given her added bulk as the baby grows.

The data suggested that our metabolisms don’t really start to decline again until after age 60. “Total and basal expenditure and fat-free mass were all stable from ages 20 to 60 years,” the investigators wrote. “At ~60 years of age, total and basal expenditure begin to decline, along with fat-free mass and fat mass,” the team noted. The slowdown from 60 years onwards is also gradual, only 0.7% a year, so, a person in their 90s needs about 26% fewer calories each day than someone in midlife.

Lost muscle mass as we get older may be partly to blame, the researchers say, since muscle burns more calories than fat. But it’s not the whole picture. “We controlled for muscle mass,” Pontzer said. “It’s because their cells are slowing down.” The patterns held even when differing activity levels were taken into account.

For a long time, what drives shifts in energy expenditure has been difficult to parse because aging goes hand in hand with so many other changes, Pontzer noted. But the new research lends support to the idea that it’s more than age-related changes in lifestyle or body composition. “All of this points to the conclusion that tissue metabolism, the work that the cells are doing, is changing over the course of the lifespan in ways we haven’t fully appreciated before,” Pontzer said. “You really need a big data set like this to get at those questions.”

The authors further pointed out that metabolic models of life history have previously commonly assumed continuity in tissue-specific metabolism over the life course, with metabolic rates increasing in a “stable, power-law manner.” However, the team noted, “Measures of humans here challenge this view, with deviations from the power-law relationships for total and basal expenditure in childhood and old age. These changes present a potential target for investigating the kinetics of disease, drug activity, and healing, processes that are intimately related to metabolic rate … 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.”

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