Metabolism changes with age, just not when you think it does

Researchers have measured life's metabolic highs and lows from birth to old age, and have discovered the rate at which we burn calories actually peaks much earlier, and starts to decline much later, than previously thought.

An international team of scientists, including Professor John Speakman from the University of Aberdeen, analyzed the average calories burned by more than 6,600 people ranging from one week old to age 95 as they went about their daily lives in 29 countries worldwide. The study, is published today August 13th 2021 in the journal Science. 

It is widely believed that our teenage years and our 20s are the ages when calorie-burning potential hits its peak. However, the researchers found that, pound for pound, infants had the highest metabolic rates of all. 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.  

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 we need just to stay alive. That amounts to 50-70% of the total calories we burn each day because it doesn’t take into account the energy we spend doing everything else: from washing the dishes, walking the dog, breaking a sweat at the gym, to digesting food and even just fidgeting.  

To come up with a number for total daily energy expenditure, the researchers relied on the “doubly labeled water” method - a urine test that involves having a person drink water in which the hydrogen and oxygen in the water molecules have been replaced with naturally occurring “heavy” forms, and then measuring how quickly they’re flushed out.

Scientists have used the technique, which is considered the gold standard for measuring daily energy expenditure during normal daily life outside of the lab, to measure energy expenditure in humans since the 1980s, but studies have been limited in size and scope due to cost. So multiple laboratories decided to share their data and gather their measurements in a single database, to see if they could tease out truths that weren’t revealed or were only hinted at in previous work. 

 It was being able to pool and analyse these data that the team were able to reveal these surprising insights.  

Professor John Speakman, co-lead corresponding author, said: “Even when you take into account an infant’s body weight and composition, their expenditures are much higher than predicted. This is probably in part because they also transition from being mostly sedentary during the first weeks of life, to being really active a year or so later. Plus their resting metabolic rates go up enormously at the same time. Potentially related somehow to their phenomenal growth rate. 

“An infant’s ‘gas-guzzling’ 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. 

“After this initial surge in infancy, the data show that metabolism slows by about 3% each year until we reach our 20s, when it levels off into a new normal.” 

Despite the teen years being a time of growth spurts, the researchers didn’t see any uptick in daily calorie needs in adolescence after they took body size into account.  

The other lead corresponding author, Professor Pontzer from Duke University added:  “We really thought puberty would be different and it’s not. Midlife was another surprise. Perhaps you’ve been told that it’s all ‘downhill after 30’ when it comes to your weight. But while several factors could explain the thickening waistlines that often emerge during our prime working years, the findings suggest that a changing metabolism isn’t one of them.” 

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 as the baby grows.  

The data suggest that our metabolisms don’t really start to decline again until after age 60. The slowdown is gradual, only 0.7% a year. But a person in their 90s needs 26% fewer calories each day than someone in midlife. 

Professor Pontzer continued: “Lost muscle mass as we get older may be partly to blame since muscle burns more calories than fat but it’s not the whole picture. We controlled for muscle mass so it seem it’s because our cellular metabolism is also slowing down. 

“We found 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, but the research lends support to the idea that it’s more than age-related changes in lifestyle or body composition.” 

Professor Speakman added: “By combining data across many component studies this paper, for the first time, reveals what is happening to our metabolism over the whole lifecourse and the contributions to those changes in resting and activity metabolism.  Compiling and curating the database was a lot of work but now it is starting to bear fruit and we are seeing these surprising results it makes it all worthwhile."

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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