We know that metabolism – the rate that we burn calories to keep our bodies running – changes as we age, but little is really known about the timeline of these changes.

Now, a new study looking at metabolism across the generations has come up with some rather surprising findings.

The researchers were able to pull in a huge amount of data from 6,421 people across 29 countries and with an age range of 8 days old to 95 years old. By using isotopes placed in drinking water and then tracked through urine, researchers worked out a daily energy expenditure figure for each participant.

Contrary to popular belief, pound-for-pound our metabolic rate peaks when we're infants. So, when we're teenagers, we're only burning calories at a slighter faster rate than when we're middle-aged.

The thickening waistlines associated with middle age might not all be down to a slow metabolic rate, in other words.

"There are lots of physiological changes that come with growing up and getting older," says evolutionary anthropologist Herman Pontzer, from 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."

As young people, our metabolisms seem to slow down by about 3 percent until our 20s, when they level off, the data showed – there's no real spurt over puberty. During our 20s through to our 50s, that's when our metabolic rate seems to be the most stable.

Once we hit our 60s, researchers found that our metabolisms seem to slow down by about 0.7 percent a year. By the time a person reaches their 90s, on average they need 26 percent fewer calories for energy per day than someone who's middle-aged – not just because of less muscle mass, but because their cells are slowing down.

But it's during the first 12 months of life that energy needs really shift. A 1-year-old burns calories around 50 percent faster for their body size than an adult. Even controlling for rapid increases in weight, energy use is "rocketing" in these early months, according to Pontzer.

"Something is happening inside a baby's cells to make them more active, and we don't know what those processes are yet," says Pontzer.

The use of the isotope water technique – known as the 'doubly labeled water' method – is important. It means the scientists were able to measure all the energy burned in a day, not just the essential calories required to stay alive, which other studies have focused on.

With the analysis method being so thorough, and the sample so large and broad, the study gives us a better idea of how metabolism changes separately from all the other ways that our bodies evolve as we get older. We still don't understand all of the changes, but at least we can now see them more clearly.

Another way that the findings could be useful is in tailoring health treatments to specific people and specific age ranges, taking shifts in metabolism into account. With more research, we should be able to learn more about exactly what is happening.

"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," says Pontzer.

"You really need a big data set like this to get at those questions."

The research has been published in Science.