A new study in Science suggests that your metabolism, the rate at which you burn calories, peaks earlier and declines later than you think.
Four Pennington Biomedical researchers were part of an international team that studied over 6,600 people’s daily calorie burn rates.
Participants ranged in age from one week to 95 years and came from 29 different countries.
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The majority of previous large-scale studies quantified the amount of energy consumed by the body for basic vital functions such as breathing, digesting, and pumping blood — the calories required to maintain life. However, basic functions account for between 50 percent and 70 percent of the calories we burn each day.
They exclude the energy expended on other activities such as dishwashing, walking the dog, working up a sweat at the gym, or even just thinking or fidgeting.
The researchers used the “doubly labelled water” method to determine the total daily energy expenditure. It is a urine test that involves having a subject drink water that has had the hydrogen and oxygen in the water molecules replaced with naturally occurring “heavy” forms, and then measuring how quickly they are flushed out.
Since the 1980s, scientists have used the technique – widely regarded as the gold standard for estimating daily energy expenditure in normal daily life outside the laboratory – to estimate human energy expenditure. However, previous studies were cost-constrained in terms of size and scope. To circumvent this constraint, multiple laboratories pooled their data in a single database to see if they could unearth truths that had been concealed or only hinted at in previous studies.
Pooling and analyzing energy expenditures over the course of a person’s life uncovered some unexpected findings.
Energy needs shoot up during the first 12 months of life. By their first birthdays, babies burn calories 50 percent faster for their body size than adults.
And that’s not just because infants are busy tripling their birth weight in their first year.
“The babies grow rapidly, which accounts for much of the effect. However, after you control for this, their energy expenditures tend to be higher than what you would expect for their body size,” Dr. Martin said.
An infant’s explosive metabolism may help explain why children who don’t get enough to eat during this developmental stage are less likely to survive and grow up to be healthy adults.
After the initial surge in infancy, a person’s metabolism slows by about 3 percent each year until our 20s, when it levels off into a new normal.
Surprisingly, the growth spurts of adolescence didn’t generate an increase in daily calorie needs after researchers took body size into account.
Another surprise? People’s metabolisms were most stable from their 20s through their 50s. Calorie needs during pregnancy grew no more than expected.
The findings suggest that other factors lie behind the so-called “middle-age spread.”
The data suggest that our metabolisms don’t really start to decline again until after age 60. The slowdown is gradual, only 0.7 percent a year. But a person in their 90s needs 26 percent fewer calories each day than someone in midlife.
Lost muscle mass as we get older may be partly to blame, the researchers say, since muscle burns more calories than fat. But it’s not the whole picture.
The patterns held even when differing activity levels were taken into account.
Aging goes hand in hand with so many other physiological changes that it has been difficult to parse what drives the shifts in energy expenditure. But the new research supports the idea that it’s more than age-related changes in lifestyle or body composition.
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