Metabolic

Metabolism changes with age – but exactly when is pretty surprising

Researchers have accurately measured the metabolic ups and downs of life from birth to old age, and the results may surprise you.

Most of us remember a time when we could eat anything we wanted without gaining weight. But a new study suggests that your metabolism – the rate at which you burn calories – actually peaks much earlier in life and begins its inevitable decline later than you might suspect.

The results were recently published in the journal Science.

“There are many physiological changes that come with growing up and getting older,” said study co-author Herman Pontzer, associate professor of evolutionary anthropology at Duke University. “Think about puberty, menopause, other phases of life. The strange thing is that the timing of our ‘metabolic phases of life’ doesn’t align with these typical milestones. “

Pontzer and an international team of scientists analyzed the average calorie consumption of more than 6,600 people aged between one week and 95 years in 29 countries around the world.

In the past, most large-scale studies measured how much energy the body used to perform basic vital functions such as breathing, digestion, blood pumps – the calories you need to stay alive. But that only makes up 50 to 70% of the calories we burn every day. It doesn’t take into account the energy we spend on everything else: washing dishes, walking the dog, working up a sweat in the gym, even just thinking or fidgeting.

In order to determine a number for the total daily energy consumption, the researchers resorted to the method of “doubly labeled water”. It’s a urine test where a person drinks water that has replaced the hydrogen and oxygen in the water molecules with naturally occurring “heavy” forms, and then measures how quickly they are flushed out.

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

The bundling and analysis of the energy consumption over the entire service life produced some surprises. Some people consider their teens and 20 to be the age at which their calorie burning potential peaks. But the researchers found that pound for pound, infants had the highest metabolic rates of all.

Energy requirements skyrocket in the first 12 months of life, so a one-year-old burns calories for his height 50% faster than an adult on his first birthday.

And not just because infants are busy triple their birth weight in their first year of life. “Of course they grow, but even when you have this under control, their energy expenditure is higher than you would expect based on their height and composition,” said Pontzer, author of Burn on the Science of Metabolism.

An infant’s gasoline-guzzling metabolism may partly explain why children who do not get enough to eat during this development window are less likely to survive and grow into healthy adults.

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

After this initial spike in infancy, the data shows that metabolism slows by about 3% each year until we reach our 20th birthday, when it settles down to a new normal.

Although the teenage years were a period of growth spurts, the researchers did not see an increase in daily caloric needs in adolescence after taking height into account. “We really thought puberty was different and it’s not,” said Pontzer.

Midlife was another surprise. You may have been told that after 30 it goes downhill when it comes to your weight. But while several factors could explain the thicker waists that often occur in our prime working years, the results suggest that changing metabolism is not one of them.

In fact, the researchers found that energy use was most stable during those middle decades – our 20s, 30s, 40s, and 50s. Even during pregnancy, a woman’s caloric needs were no more or less than expected, as she increases in mass as the baby grows.

The data suggest that our metabolism does not really start to decrease again until after the age of 60. The slowdown is gradual, only 0.7% per year. But someone in their 90s needs 26% fewer calories each day than someone in their mid-life.

Loss of muscle mass with age could be partly responsible, the researchers say, since muscle burns more calories than fat. But it’s not the whole picture. “We checked the muscle mass,” said Pontzer. “That’s because your cells are slowing down.”

The patterns held even when different activity levels were taken into account.

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

“All of this indicates that tissue metabolism, the work that cells do, changes over the course of life in ways that we have not yet fully understood,” said Pontzer. “You really need a big data set like this to answer these questions.”

Reference: “Daily energy consumption in the course of human life” by Herman Pontzer, Yosuke Yamada, Hiroyuki Sagayama, Philip N. Ainslie, Lene F. Andersen, Liam J. Anderson, Lenore Arab, Issaad Baddou, Kweku Bedu-Addo, Ellen E Blaak , Stephane Blanc, Alberto G. Bonomi, Carlijn VC Bouten, Pascal Bovet, Maciej S. Buchowski, Nancy F. Butte, Stefan G. Camps, Graeme L. Close, Jamie A. Cooper, Richard Cooper, Sai Krupa Das, Lara R . Dugas, Ulf Ekelund, Sonja Entringer, Terrence Forrester, Barry W. Fudge, Annelies H Goris, Michael Gurven, Catherine Hambly, Asmaa El Hamdouchi, Marjije B. Hoos, Sumei Hu, Noorjehan Joonas, Annemiek M. Joosen, Peter Katzmarzyk, Kitty P. Kempen, Misaka Kimura, William E. Kraus, Robert F. Kushner, Estelle V. Lambert, William R. Leonard, Nader Lessan, Corby Martin, Anine C. Medin, Erwin P. Meijer, James C. Morehen, James P. Morton, Marian L. Neuhouser, Teresa A. Nicklas, Robert M. Ojiambo, Kirsi H. Pietiläinen, Yannis P. Pitsiladis, Jacob Plange-Rhule (vers torben), G uy Plasqui, Ross L. Prentice, Roberto A. Rabinovich, Susan B. Racette, David A. Raichlen, Eric Ravussin, Rebecca M. Reynolds, Susan B. Roberts, Albertine J. Schuit, Anders M. Sjödin, Eric Stice, Samuel S. Urlacher, Giulio Valenti, Ludo M. Van Etten, Edgar A. Van Mil, Jonathan CK Wells, George Wilson, Brian M. Wood, Jack Yanovski, Tsukasa Yoshida, Xueying Zhang, Alexia J. Murphy-Alford , Cornelia Loechl, Amy H. Luke, Jennifer Rood, Dale A. Schoeller, Klaas R. Westerterp, William W. Wong, John R. Speakman and IAEA DLW Database Consortium, August 13, 2021, Science.
DOI: 10.1126 / science.abe5017

This research was supported by the United States National Science Foundation (BCS-1824466), the International Atomic Energy Agency, Taiyo Nippon Sanso, and SERCON.

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