Putting on weight as a teenager is largely due to a dramatic drop in calories boys and girls burn off, according to a study.
Scientists say teenagers burn up to 500 fewer calories a day during puberty than younger children, which could help explain why obesity is increasing.
The 12-year study unexpectedly found that when children reach puberty, girls and boys experience a rapid drop in the number of calories they burn, at a time when the number would be expected to rise with the growth spurt.
The research by Professor Terry Wilkin, of the University of Exeter, found that 15-year-olds use 400 to 500 fewer calories while at rest per day than when they were 10 - a fall of around a quarter.
By the age of 16, calorie expenditure begins to climb again.
For comparison, a McDonald's Big Mac burger contains 508 calories and it would take an hour of Zumba to burn 500 calories through exercise.
The study also found that teenagers exercise less during puberty, adding to the calorie excess that underlies obesity.
This drop is particularly stark in girls, whose activity level drops by around a third between the ages of seven and 16.
The findings, which come after the Government launched a strategy last month to tackle the dramatic rise in childhood obesity, may help explain why many youngsters become overweight in puberty.
Obesity can lead to major health problems in later life, including heart disease, cancer and diabetes, and costs the NHS more than £4 billion a year.
"Child obesity and associated diabetes are both among the greatest health challenges of our time," Prof Wilkin said.
"Our findings can explain why teenagers gain excess weight in puberty, and it could help target strategies accordingly."
Prof Wilkin said calories are burnt off in two ways - voluntary spend through physical activity and the much larger involuntary spend, simply to stay alive.
Thinking, keeping blood warm, and keeping the heart, liver and kidneys working together use up to 1,600 calories per day in adolescence.
This involuntary spend might be expected to rise with body size, and among the children studied, the calorie expenditure rose as expected from the age of five.
But researchers were surprised to see the children experience a sudden drop in calorie expenditure during the onset of puberty. This was particularly surprising as it is a period of rapid growth, and growth uses a lot of calories.
Between 2000 and 2012 the researchers analysed data from nearly 350 schoolchildren in Plymouth, of whom 279 had data eligible for the study.
Burning calories uses a fixed amount of oxygen, so the children rested in a sealed canopy and their oxygen consumption was measured over a period of time, to enable researchers to calculate their calorie use from the amount of oxygen consumed.
Prof Wilkin said: "When we looked for an explanation for the rising obesity in adolescence, we were surprised to find a dramatic and unexpected drop in the number of calories burned while at rest during puberty.
"We can only speculate as to why, but it could be a result of an evolutionary trait to save calories for growth that may now contribute to a dangerous rise in adolescent obesity in cultures where food is in abundance.
"It could be that we have evolved to preserve calories to ensure we have enough to support changes in the body during puberty, but now they have sufficient calories each day, the drop in spend means excess weight gain."
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "Teenagers consume three times the recommendation for sugar so it's not surprising that over one in three are overweight or obese.
"This study further highlights the importance of teenagers being regularly active and cutting back on fatty, sugary food and drink to help maintain a healthy weight."
The study, Evidence for energy conservation during pubertal growth, is published in the International Journal of Obesity.