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  • Research from Duke University has found new information about when this process actually starts and when we can actually blame age for an expanding waistline.
  • Infants actually have the highest metabolism according to the new research.
  • Metabolism rates don’t significantly decline until a person reaches their late 50s to 60s.

Many of us think of metabolism as peaking during teenage years when people can “eat anything” without gaining weight and then seeing a slowdown in the metabolic system in midlife in the 30s and 40s as it becomes difficult to keep weight off.

But now, new research finds these long-held ideas about metabolism and age might be incorrect.

But research from Duke University has found new information about when this process actually starts and when we can actually blame age for an expanding waistline.

“We wanted to understand how our bodies change over the lifespan,” study coauthor Herman Pontzer, PhD, associate professor of evolutionary anthropology at Duke University, told Healthline.

Pontzer and an international team of scientists analyzed the average calories burned by more than 6,600 people ranging from 1 week old to 95 years old as they went about their daily lives in 29 countries worldwide.

Previous large-scale studies measured only how much energy we use to maintain vital functions such as breathing, digesting, pumping blood — the calories we burn just staying alive, also called our basal metabolic rate (BMR).

According to Jacqueline A. Vernarelli, PhD, director of research education and consultation for the College of Health Professionals at Sacred Heart University in Connecticut, our BMR is our “cost of living.” It’s the energy we use just to maintain basic physiologic processes at rest.

“If you just lay in bed all day, without moving or eating, how much energy, how many calories would the cellular processes in your body use? That’s your basal metabolic rate or BMR,” she explained. “BMR is the biggest component of the number of calories we burn each day, accounting for about two-thirds of our daily energy expenditure.”

However, this doesn’t represent all the calories we burn being active every day, from walking up a flight of stairs to tapping a keyboard or even just thinking — our “daily energy expenditure.”

Pontzer said daily energy expenditure represents the work our bodies do all day, every day. “By measuring total daily energy expenditures, we can understand how busy our cells and our organs are,” he said.

He noted that this has big implications for understanding human biology, including disease progression, our nutritional requirements, and more.

Researchers used the gold standard for determining our metabolic rate during our daily lives.

They did this by using the “doubly labeled water” method to calculate total daily energy expenditure.

“Basically, stable radioisotopes (deuterium and oxygen-18) are dosed via drinking water,” explained Dr. Minisha Sood, an endocrinologist at Lenox Hill Hospital in New York. “And it is possible to track their elimination from the body by collection urine samples over 7 to 14 days.”

She said equations are then used to calculate total energy expenditure (TEE).

“This study had a large enough sample, across the full age range, to pull out the effects of age and development after accounting for the effects of body size,” said Pontzer. “The results provide the first clear understanding of the work our bodies do every day, over the course of our lives.”

Previous studies that have looked at metabolic changes have been limited in size and scope due to cost, so multiple labs shared their data in a single database to see if researchers could discover new facts or confirm information only hinted at in previous work.

Previous research concluded puberty is the time when our BMR really revs up, but this study found that infants have the highest metabolic rates of all.

Pontzer and his team discovered that energy needs “shoot up” during the first 12 months of life, and by their first birthday, a child burns calories 50 percent faster for their body size than an adult.

This isn’t just because infants are busy tripling their birth weight during their first year of life.

“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,” Pontzer said in a statement.

After surging in infancy, the data shows our metabolism slowing by roughly 3 percent each year until we hit our 20s when it levels off into a new standard.

Researchers didn’t find any increase in teens’ daily calorie needs even considering adolescent growth spurts after considering body size.

“I wasn’t surprised that age affected energy expenditures, but I was surprised at the size, the effect, and the timing of changes,” said Pontzer. “We didn’t expect that newborns would look like tiny adults in terms of their metabolic rates or that metabolic rates would peak at 1 year [of age] and then slowly decline to 20 years.

“We thought we would see an effect of puberty, or menopause, on metabolism,” he continued. “But we didn’t find evidence for either.”

While we all think of middle age as when you can just look at food and gain weight, Pontzer’s findings say otherwise.

Although several factors could explain the expanding waistlines associated with later adulthood, data suggest that a slowing metabolism isn’t one of them.

Researchers discovered that energy expenditures from our 20s to our 50s were the most stable. Even during pregnancy, a woman’s calorie needs were no more or less than expected, given the added weight of a growing baby.

The data suggest that our metabolisms don’t significantly decline again until after age 60. The slowdown is a little less than 1 percent a year, but once you reach your 90s, you’ll need roughly 25 percent fewer calories every day than someone in midlife.

Sarcopenia, or loss of muscle due to age, could be partly to blame since muscle burns more calories than fat. But researchers controlled for muscle mass to conclude that it was also because our cells are actually slowing down.

“This might surprise some people who expect and/or observe a decline in ‘metabolism’ with increased difficulty avoiding weight gain starting typically in their 30s or 40s with a progression of this problem in their 50s and 60s,” said Sood. She also emphasized the importance of maintaining muscle mass and engaging in regular physical activity to help avoid the gradual weight gain of aging.

“While we are just learning about factors that influence our BMR at the cellular level, there are several things we do already know about weight balance,” explained Vernarelli. “The less muscle mass we have, the less metabolically active tissue in our body, which in turn decreases the amount of calories we burn each day.”

She emphasized that there’s no magic bullet to prevent age-related weight gain — just healthy habits.

“The take-home here is that our metabolism doesn’t come to a screeching halt when we hit 30, and maintaining a healthy weight is the result of a combination of healthy behaviors,” said Vernarelli. “Including engaging in regular physical activity, maintaining muscle mass, and limiting consumption of calorically dense foods.”

Recent research finds that our metabolism doesn’t slow down as much as we thought with age, and teens aren’t the calorie-burning marvels we thought.

Study findings suggest that we burn calories the fastest in our first year of life, steady down to our “normal” rate by our 20s and stay that way into our 50s. Also, our calorie needs decline only by about 25 percent by our 90s.

Experts say that the best way to maintain a healthy weight throughout life is eating nutritious meals and regular physical activity that helps build muscle.