While we are all aware that our metabolism changes with age, the results of a study published today in the journal Science reveal unexpected trends in the timing of metabolic changes.
“There are lots of physiological changes that come with growing up and getting older,” said study co-author Professor Herman Pontzer 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.”
Pontzer and his team of scientists combined the results from metabolic studies in many different laboratories all over the world. In all instances, metabolic rates were measured using doubly labeled water, a type of drinking water that is marked with heavy forms (isotopes) of hydrogen and oxygen.
The rates of elimination of these substances are used to measure metabolic rate, or the speed at which we burn calories. In total, the average calories burned were measured for more than 6,600 people from 29 different countries. The ages of the participants varied from one week to 95 years old.
The results from this very large sample presented a few surprises. Contrary to the expectation that people in their teens and early 20s would burn calories fastest, it was infants that had the highest metabolic rates. Their energy needs increase substantially during the first year of life.
By the time they reach a year of age, infants burn calories 50 percent faster than an adult, once body size is taken into consideration. This is partly because they are increasing in size.
“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 Professor Pontzer. “Something is happening inside a baby’s cells to make them more active, and we don’t know what those processes are yet.”
Another surprise in the findings was that our energy expenditure during our 20s, 30s, 40s and 50s remains pretty stable. It is only after we turn 60 that our metabolic rates begin to decrease, and then the decrease is relatively slow. This may be due to the fact that we lose muscle mass as we age, and muscle tissue burns more calories than fat. But this is not the whole explanation. “We controlled for muscle mass,” said Professor Pontzer. “It’s because their cells are slowing down.”
The researchers attribute the change in metabolic rate to more than just changes in lifestyle and body condition as we age.
“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,” said Professor Pontzer. “You really need a big data set like this to get at those questions.”