Teenagers may be said to eat their parents out of house and home, but research suggests their daily energy expenditure isn’t much greater than that of adults.
An international team of researchers has tracked the total daily energy expenditure of more than 6,000 people aged from eight days to 95 years, turning many tropes about metabolism on their heads.
“This study reveals that there are lots of misconceptions about how metabolic rate changes with age,” said Prof John Speakman, co-author of the research, of the University of Aberdeen.
Writing in the journal Science, Speakman and colleagues report how they used data from what are known as doubly labelled water studies to calculate the energy expenditure of 6,421 people across 29 countries, with data from additional studies involving women who were pregnant or had recently given birth.
Doubly labelled water studies involve giving participants water that contains heavy forms, or isotopes, of hydrogen and oxygen and then tracking their presence in participants’ urine over time. The heavy oxygen is washed out faster than the hydrogen: that difference is related to the rate of carbon dioxide produced and hence gives an estimate of energy expenditure.
The team found total daily energy expenditure and “basal expenditure”, the energy needed to carry out fundamental metabolic functions such as breathing, rose with body size. After taking this into account, they found infants up to one month old used around the same total amount of energy a day as adults, and that this rose rapidly to about 50% above adult values at one year of age.
The researchers added that after this time, total daily energy expenditure declines slowly through childhood and adolescence, at a rate of about –2.8% a year, to reach adult levels by about 20 years of age. It then plateaus until about the age of 60, whereupon it begins to decline once more. Total energy expenditure in adults even remains stable during pregnancy.
Similar – although not identical – trends were seen for basal energy expenditure while modelling work added further insights, “indicating that variation in physical activity and tissue-specific metabolism contribute to total expenditure and its components across the life span”, the researchers wrote.
Speakman said the results suggest that growth isn’t the only reason children use more energy than adults. “It seems to be a combination of growth, greater physical activity and higher levels of cellular metabolism. They just seem to be doing lots more of everything,” he said.
The study also suggests that, contrary to popular belief, teenagers may not need constant access to the fridge. “Their expenditure is slightly higher than in the adult years but not very much,” said Speakman. “If they need to eat more – it’s marginal.”
Speakman said the huge rise in energy expenditure of young infants, and the constancy of the metabolic rate through adulthood from ages 20 to 60, was a surprise.
“Previously there was a suggestion that metabolism might slow in your 30s and that was then thought to [cause] susceptibility to middle age spread,” he said. “We found no evidence to support that. So if you are piling the weight [on] and your waistline is expanding during your 30s and 40s, it’s probably because you are eating more food, then expending less energy.”
Gareth Lavery, professor of molecular metabolism at the University of Birmingham and who was not involved in the work, described the study as unique and comprehensive.
“These intriguing results suggest we may need to take more care to tailor health advice and disease management in the context of an individual’s age and metabolic rate,” he said.
“The next step in leveraging this information is to apply what we now know about metabolic variation and energy expenditure in humans to shape nutritional and public health strategies that preserve the quality of our later years.”