The rate at which humans burn calories peaks much earlier and declines much later than previously thought, a study has indicated.
Researchers analysed calories expended by more than 6600 people aged between one week and 95 years old across 29 countries as they went about their daily lives for the study.
Results show that, pound-for-pound, infants have the highest metabolic rates, with a one-year-old using up calories 50% faster for their body size than adults.
It sheds new light on the widely held belief that teenage years and a person’s 20s are when calorie-burning hits its peak, say researchers.
Findings also indicated that, after remaining relatively stable through your 20s to your 50s, it is not until age 60 and beyond that metabolic rates start to decline gradually.
The study, Daily energy expenditure through the human life course, published in the journal Science, was the result of work by an international team of scientists including Professor John Speakman, from the University of Aberdeen.
Professor Speakman, a senior author on the paper, 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 growth spurts during teenage years, researchers said there was no uptick in daily calorie needs in adolescence after controlling for body size.
Energy expenditure during the 20s, 30s, 40s and 50s was the most stable, with metabolism not declining again until after age 60, the paper said.
The slowdown is gradual at just 0.7% a year, but someone in their 90s needs 26% fewer calories per day than someone in midlife, it added.
Professor Herman Pontzer, of Duke University in North Carolina in the US, said: “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.”
Most prior large-scale studies in this area have been limited in size due to cost, so several laboratories pooled their data into a single database for analysis.
To come up with a number for total daily energy expenditure, the scientists used a method called “doubly labelled water”, a complex urine test considered to give the most accurate results.
Prior studies in this area mainly measured the basic vital functions such as breathing, digesting food and pumping blood – which amounts to the calories needed simply to stay alive and is estimated to be around 50-70% of total caloric expenditure, said researchers.
But it often did not take into account the energy spent doing everything else: from washing the dishes, walking the dog, working out at the gym and even just fidgeting, they added.
Prof Speakman said: “By combining data across many component studies, this paper for the first time reveals what is happening to our metabolism over the whole life-course 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.”
The research was supported by the United States National Science Foundation, the International Atomic Energy Agency, Japanese multinational gas producer Taiyo Nippon Sanso and the company Sercon.