A new study suggests that children's metabolism temporarily slows during puberty, a pattern that might help explain the current teen obesity problem.
450 fewer calories at rest
The study found that kids' resting energy expenditure typically dropped during puberty. That refers to the number of calories the body burns at rest.
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.
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"Body mass is the strongest predictor of resting energy expenditure. So a fall in puberty, when growth is rapid, is unexpected," said lead researcher Dr Terence Wilkin, a professor of endocrinology and metabolism at the University of Exeter in England.
The reasons for the pattern aren't clear, but Wilkin speculated on an explanation: The human body may have evolved to conserve calories during the critical period of puberty, to help ensure adequate growth and development.
Free availability of calories
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, he said.
"The danger for any child at this age is the free availability of calories at a time when he has money in his pocket and increasing freedom from the home," Wilkin said.
Dr Scott Kahan is director of the National Centre for Weight and Wellness in Washington, D.C., and a spokesman for the Obesity Society.
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He agreed that the study results are surprising.
"As with any new finding, this needs to be verified in future studies," said Kahan, who wasn't involved in the research.
But if resting metabolism does indeed decline during puberty, he said, that would make it even more important for kids to stay active and have healthy diets during those years.
"We can't really do anything about resting energy expenditure," Kahan said. "But you can do something about physical activity, which also declines during puberty."
An aspect of human evolution?
The study findings are based on nearly 350 schoolchildren who were followed for more than a decade. Each year between the ages of 7 and 16, the children had their weight, body composition and resting metabolism measured.
On average, Wilkin's team found, kids were burning about one-quarter fewer calories at rest when they were 15 years old, compared to age 10. But by age 16, their resting metabolism started to rise again.
Whether that reflects some aspect of human evolution is not clear. But it's a "reasonable hypothesis", Kahan said.
"But what does this mean, practically?" he said. "I think the findings lend more weight to the importance of healthful eating and physical activity."
In this study, most kids started exercising less during puberty – which meant their voluntary, as well as involuntary calorie-burning dipped. However, Wilkin said, another analysis of the same group found that the effect of exercise on kids' risk of obesity was "essentially nil".
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He emphasized the role of diet.
During puberty, Wilkin noted, most kids are "ravenously" hungry – even though, if this study is correct, their calorie-burning declines.
"I'm not sure whether that's always the sign of a need to eat, or a primeval signal to ensure you do eat," Wilkin said.
Sound health habits
The problem, he added, is that kids today have easy access to calorie-laden foods that past generations did not.
Kahan advises parents to stock the kitchen with nutritious foods. "So when your kids are starving, that's what they reach for," he said.
He also emphasised the importance of starting sound health habits early: When kids go into puberty with healthy eating and exercise habits, they'll be more likely to maintain them.
But while nutrition and exercise are vital, Kahan said the new findings underscore another fact: Weight management is complex.
"If your child has weight issues, it's important to be nurturing and sensitive about it," Kahan said. "Remember that kids may be doing the best they can [with diet and exercise], but there are underlying physiological processes at work, too."
The study findings were published in the International Journal of Obesity.
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