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Most of us remember a time when we could eat whatever we wanted without gaining 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 at which our calorie burning potential peaks. But the researchers found that, pound for pound, infants had the highest metabolic rates of all.
Duke University Associate Professor Herman Pontzer has joined an international team of scientists to analyze the average calories burned by more than 6,600 people aged between one week and 95 years old during their daily lives in 29 countries around the world .
Focusing on puberty, menopause and other phases of life, Pontzer, co-author of the study, was surprised. “The odd thing is that the timing of our ‘metabolic life stages’ doesn’t seem to match these typical stages.”
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 to stay desire. But that’s only 50-70% of the calories we burn each day. It doesn’t account for the energy we spend on everything else – washing dishes, walking the dog, sweating at the gym, even just thinking or fussing.
To quantify the total daily energy expenditure, the researchers relied on the “double-labeled water” method. This is a urine test that involves making a person drink water in which the hydrogen and oxygen in the water molecules have been replaced by natural “heavy” forms, and then measure how quickly they are evacuated.
Scientists have used the technique – believed to be 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 were limited in size and scope due to cost. So, several laboratories decided to share their data and put their measurements together in a single database, to see if they could unravel truths that had not been revealed or that had only been suggested in previous work. .
Pooling and analyzing energy expenditure across the lifespan revealed a few surprises, including data showing babies have the highest metabolic rates of all.
In the first 12 months of an infant’s life, their energy needs increase, so by their first birthday, a one-year-old burns calories 50% faster for their body size than an adult.
And it’s not just because, in their first year, infants are busy tripling their birth weight. “Sure, they get taller, but even once you get that under control, their energy expenditure increases more than expected for their size and body composition,” said Pontzer, author of a book on the science of metabolism, Burn: How We Really Burn Calories, Lose Weight, and Stay Healthy.
An infant’s energy-intensive metabolism may partly explain why children who do not eat enough during this window of development are less likely to survive and grow into healthy adults.
“Something is going on inside a baby’s cells to make them more active, and we don’t yet know what those processes are,” Pontzer said.
After that initial surge in infancy, data shows that metabolism slows down by about 3% each year until we hit our 20s, when it stabilizes in a new normal.
Although adolescence is a time of growth spurts, researchers found no increase in daily calorie needs in adolescence after controlling for body size. “We really thought puberty would be different and it isn’t,” Pontzer said.
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30, 40 and 50 years old
Quarantine was another surprise. You may have been told that everything is downhill after 30 years when it comes to your weight. But while several factors could explain the thickening of the waistline that often appears during our first few years of labor, the results suggest that a changing metabolism is not one of them.
In fact, the researchers found that energy expenditure during these intervening decades – our 20s, 30s, 40s, and 50s – was the most stable. Even during pregnancy, a woman’s caloric needs were neither more nor less than expected given her extra bulk as the baby grows.
Data suggests that our metabolisms don’t really start to decline again until after age 60. The slowdown is gradual, only 0.7% per year. But a 90-year-old needs 26% fewer calories each day than a person in their 40s.
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The loss of muscle mass as we age may be partly to blame, say the researchers, because muscle burns more calories than fat. But that’s not all. “We controlled the muscle mass,” Pontzer said. “It’s because their cells are slowing down.”
The patterns held up even when different activity levels were taken into account, according to research, which was published Aug. 12 in the journal Science and funded by the National Science Foundation and the IAEA.
For a long time, resulting in changes in energy expenditure was difficult to analyze because aging goes hand in hand with so many other changes, Pontzer said. But research supports the idea that it’s more than age-related changes in lifestyle or body composition.
“All of this leads to the conclusion that tissue metabolism, the work that cells do, evolves over the course of life in ways that we have not fully appreciated before,” Pontzer said. “You really need a big data set like this to answer these questions. “
(Source: Duke University) – File photo by Emma Simpson
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