Metabolism Does Not Slow Down in Mid-Life as is Commonly Believed, Says Study

Most of us remember a time when we could eat whatever we wanted and not gain weight. But a new study suggests that your metabolism – the rate at which you burn calories – actually begins its inevitable decline much later than we all assume.

Additionally, we tend to think of our teens and 20s as the age when our calorie-burning potential peaks. But the researchers found that, pound for pound, the babies had the highest metabolic rates of all.

Duke University Associate Professor Herman Pontzer joined an international team of scientists to analyze the average calories burned by more than 6,600 people between the ages of one week and 95 years as they went about their daily lives in 29 countries across the world. all the world.

Pontzer, the study co-author, focused on puberty, menopause, and other phases of life, and was shocked. “The strange thing is that the timing of our ‘metabolic stages of life’ doesn’t seem to coincide with those typical milestones.”

Previously, most large-scale studies measured how much energy the body uses to perform basic vital functions such as breathing, digesting, pumping blood; in other words, the calories you need just to stay alive. But that amounts to only 50% to 70% of the calories we burn each day. It doesn’t take into account the energy we spend on everything else: washing the dishes, walking the dog, sweating at the gym, even just thinking or fidgeting.

To arrive at a number for total daily energy expenditure, the researchers relied on the “double-labeled water” method. It is a urine test that involves a person drinking water in which the hydrogen and oxygen in the water molecules have been replaced by naturally occurring “heavy” forms, and then measuring how quickly they are removed.

Scientists have used the technique, considered the gold standard for measuring daily energy expenditure during normal daily life, outside the laboratory, to measure energy expenditure in humans since the 1980s, but studies have been limited in size and scope due to cost. So several labs decided to share their data and collect their measurements in a single database, to see if they could unravel truths that were not revealed or only hinted at in previous work.

The combination and analysis of energy expenditures over a lifetime revealed some surprises, including data showing that babies have the highest metabolic rates of all.

During the first 12 months of a baby’s life, their energy needs skyrocket, so that by their first birthday, a one-year-old burns calories 50% faster for their body size than an adult.

And that’s not just because, in their first year, babies are busy tripling their birth weight. “Of course they are growing, but even once you control that, their energy expenditures skyrocket more than you would expect for their body size and composition,” said Pontzer, author of a book on the science of metabolism. . Burn: How We Really Burn Calories, Lose Weight, and Stay Healthy.

A baby’s gas-guzzling metabolism may partly explain why children who don’t eat enough during this developmental window are less likely to survive and become healthy adults.

“Something is happening inside a baby’s cells to make them more active, and we don’t yet know what those processes are,” Pontzer said.

After this initial increase in childhood, data shows that metabolism slows by about 3% each year until we reach our 20s, when it stabilizes in a new normal.

Despite the fact that adolescence is a time of growth spurt, the researchers did not see any increase in daily caloric needs in adolescence after taking body size into account. “We really thought puberty would be different and it isn’t,” Pontzer said.

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30, 40 and 50

Middle age was another surprise. You may have been told that everything is downhill after 30 when it comes to your weight. But while several factors could explain the thickening of the waist that often arises during our best years of work, the findings suggest that a changing metabolism is not one of them.

In fact, the researchers found that energy expenditures during these intervening decades – our 20s, 30s, 40s, and 50s – were the most stable. Even during pregnancy, a woman’s caloric needs were neither higher nor lower than expected given her added bulk as the baby grows.

The data suggests that our metabolism doesn’t really start to slow down again until after age 60. The slowdown is gradual, only 0.7% per year. But a person in their 90s needs 26% fewer calories a day than a middle-aged person.

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Losing muscle mass as we age may be part of the culprit, researchers say, as muscle burns more calories than fat. But it is not the whole picture. “We control muscle mass,” Pontzer said. “It’s because their cells are slowing down.”

The patterns held true even when different levels of activity were taken into account, according to the research, which was published Aug. 12 in the newspaper Science and funded by the National Science Foundation and the IAEA.

For a long time, what drives changes in energy expenditure has been difficult to analyze because aging goes hand in hand with so many other changes, Pontzer said. But research supports the idea that it’s about more than age-related changes in lifestyle or body composition.

“All of this points to the conclusion that tissue metabolism, the work that cells do, is changing throughout life in ways that we had not fully appreciated before,” said Pontzer. “You really need a big data set like this to answer those questions.”

(Source: Duke university) – File photo of Emma Simpson

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