A study in the International Journal of Obesity found energy used at rest was 25% lower in 15-year-olds compared with when they were 10 - a fall of 500 calories a day.
This is equivalent to cutting out an hour's strenuous exercise every day.
The study said the findings were unexpected but could help explain why a lot of teenagers put on excess weight.
Most of the energy people burn is just to keep their bodies ticking over - using the brain, heart, liver and kidneys - rather than through physical activity.
In a decade-long study following 279 children, researchers from the University of Exeter Medical School were surprised to find the number of calories the children burned at rest fell sharply during puberty.
This is a period of rapid growth, which usually uses up lots of calories, but it was only around the age of 16 that the number of calories burned at rest started to rise again.
During puberty there was also a noticeable drop in physical exercise, particularly among girls, which could add to weight gain, the researchers said.
About a third of school children aged 10-11 in England are overweight or obese.
Prof Terry Wilkin, from the University of Exeter Medical School, said: "Child obesity and associated diabetes are both among the greatest health challenges of our time.
"Our findings can explain why teenagers gain excess weight in puberty, and it could help target strategies accordingly."
Prof Simon Capewell, vice-president of the Faculty of Public Health, said the findings showed there was an even greater need to improve children's diets and protect them from junk food and sugary drinks.
"Adolescents sitting around using their smartphones and iPads are being bombarded with junk food marketing - while using even fewer calories than we previously thought.
"Last month's government strategy on childhood obesity confirmed the duty on sugary drinks, but was otherwise a disastrous missed opportunity.
"We need much tougher regulation around the marketing of junk food to children - particularly on TV and online."

The regulator said the bid did not comply with the laws of having its principal place of business in the Chinese territory.
The city's other carriers had also objected, it said.
Qantas said it would reconsider basing the low-cost carrier in Hong Kong.
The carrier is a three-way joint venture between Qantas Airways, China Eastern Airlines and Hong Kong investment firm Shun Tak Holdings.
"Jetstar Hong Kong cannot make its decisions independently from that of the two foreign shareholders," said the Air Transport Licensing Authority when handing down its decision.
It also said the city's other carriers, including Cathay Pacific and Dragon Airlines, had objected to Jetstar's application.
Qantas chief executive Alan Joyce said the decision was disappointing for shareholders as well as "for the travellers that Jetstar Hong Kong planned to serve".
"It's the travelling public who have lost out, because the message from this decision is that Hong Kong appears closed to fresh aviation investment even when it is majority locally-owned and controlled," he said in a statement on Friday.
He added that Hong Kong was going in the opposite direction of other aviation markets in Asia that were "opening up".
"Given the importance of aviation to global commerce, shutting the door to new competition can only serve the vested interests already installed in that market."
The airline, along with its partners, said it had not yet decided on whether to appeal the decision.
The joint venture was valued at $10m Australian dollars ($7.7m; Â£4.9m), according to Qantas.
Shares of Qantas were down 1.4% in Sydney in reaction to the news.
A sharp drop in calories burned off while resting during adolescence could be contributing to the rise in obesity.
[NEXT_CONCEPT]
Hong Kong authorities have rejected an application by Qantas-backed Australian budget carrier Jetstar to set up a local airline, after nearly two years of deliberation.