A new study suggests that children's metabolism temporarily slows during puberty — a pattern that might help explain the current teen obesity problem. The study found that kids' resting energy expenditure, which refers to the number of calories the body burns at rest, typically dropped during puberty.
On average, the researchers found, 15-year-olds used about 450 fewer calories at rest each day, compared to when they were 10 years old. The shift is surprising, experts said, since larger bodies usually burn more calories at rest — to fuel brain activity, the cardiovascular system and the other bodily processes that keep us alive. Body mass is the strongest predictor of resting energy expenditure.
So a fall in puberty, when growth is rapid, is unexpected, they said. The reasons for the pattern are not clear, but doctors speculate an explanation: The human body may have evolved to conserve calories during the critical period of puberty, to help ensure adequate growth and development. During the long span of human history when food was scarce, that would have made sense. But now, when abundance is often the issue, that drop in calorie-burning during puberty may feed excess weight gain.
If resting metabolism does indeed decline during puberty that would make it even more important for kids to stay active and have healthy diets during those years. While we cannot really do anything about resting energy expenditure, we can do something about physical activity, which also declines during puberty. In this study, most kids started exercising less during puberty — which meant their voluntary, as well as involuntary calorie-burning dipped. However, another analysis of the same group found that the effect of exercise on kids' risk of obesity was essentially nil, thereby emphasizing the role of diet.
During puberty, most kids are ravenously hungry, even though, as the study indicates, their calorie-burning declines. The problem is that kids today have easy access to calorie-laden foods that past generations did not.