After reaching a certain age lots of us find ourselves confronted with an ever-expanding muffin top every time we look in the mirror. This is commonly put down to our metabolism – the rate at which our bodies burn calories – slowing down as we enter middle age and beyond.
But a new study led by researchers from Duke University in the States suggests this isn’t the case. In fact, our metabolisms peak much earlier in our lives and start to decline much later, they say.
“There are lots of 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 puberty, menopause, other phases of life. What's weird is that the timing of our ‘metabolic life stages’ doesn't seem to match those typical milestones.”
The team analysed the average daily calories burned by more than 6,600 people ranging from one-week-olds to 95-year-olds from 29 countries using the so-called ‘doubly labelled water’ method. This involves having a person drink water in which the hydrogen and oxygen have been replaced with naturally occurring “heavy” forms, and then measuring how quickly they pee it out.
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They found that, pound for pound, infants had the highest metabolic rates of all, with energy needs skyrocketing during the first few months of life. At their metabolic peak at one year old, infants burn calories 50 per cent faster for their body size than an adult.
“Of course, they're growing, but even once you control for that, their energy expenditures are rocketing up higher than you'd expect for their body size and composition,” said Pontzer.
“Something is happening inside a baby’s cells to make them more active, and we don't know what those processes are yet.”
After this initial surge, our metabolisms then slow down by about 3 per cent a year until we reach our 20s, before levelling off and remaining stable throughout our 30s, 40s and 50s. Only after age 60 do they start slowing down, but even then only at a rate of less than one per cent a year.
“All of this points to the conclusion that tissue metabolism, the work that the cells are doing, is changing over the course of the lifespan in ways we haven’t fully appreciated before,” Pontzer said. “You really need a big data set like this to get at those questions.”