Teenagers can pile on excess weight more easily because the body goes into “energy saving mode” during adolescence, research suggests.

The 12-year study found that 15-year-olds use 400 to 500 fewer calories while at rest each day than when they were 10 years old - a drop in resting metabolism of around a quarter.

Obesity rates are already known to rise significantly in early adolescence, but this had typically been put down to children losing interest in sport or adopting unhealthy eating habits as they gain greater access to junk food.

The latest work suggests an alternative explanation - that the body suddenly starts burning calories more slowly at puberty, making it easier for a person to rapidly bulk up if they eat slightly more than their body needs.

Prof Terry Wilkin, who led the work at the University of Exeter, said: “This makes teenagers very vulnerable to weight increase.”

The study focused on resting energy expenditure - the calories that we use up to keep the body ticking over (maintaining body temperature, thinking, breathing and so on). This makes up about 75% of our total energy expenditure, with the rest being used up by actually moving around and exerting ourselves.

Between the ages of five and 10, this involuntary spend was seen to rise with body size, as might be expected. But the scientists were surprised to see a sudden drop in calorie expenditure during puberty, from the age of 10 onwards.

By the age of 15, children were using up around 500 fewer calories each day while at rest - the equivalent to a McDonalds Big Mac or an hour-long exercise class. The trend appeared to reverse by the age of 16 when metabolism was seen to start to increase again.

The study, published on Wednesday in the International Journal of Obesity, relied on data gathered from nearly 350 school children based in Plymouth, UK who were assessed every six months.

Burning calories uses up a fixed amount of oxygen. So the scientists were able to measure resting energy use by placing the children in a sealed canopy, which Wilkin describes as similar to a “Buzz Lightyear hood”, where their oxygen consumption was measured over a period of time.

Wilkin believes the drop in metabolism during puberty makes sense from an evolutionary perspective, as food has tended to be scarce for most of human history. If fewer calories are required for the body’s basic functions, energy can be re-directed towards growing. This suggests that while teenagers still need to eat more during puberty, because they’re getting taller and developing so quickly, they don’t need as many extra calories as you might expect.

Having a lower baseline energy use would also make teenagers prone putting on weight much more quickly than a younger child (or possibly adult) would do.

“This is probably a throwback to a time when man’s availability of nutrition was limited and the extra requirement of calories to get through puberty was substantial,” said Wilkin.

“Contemporary children are trapped with this low energy expenditure during a period of four or five years and they are eating as if there was no tomorrow because that’s the way teenagers eat nowadays.”

Russell Viner, a professor of adolescent health at University College London , said: “Our bodies aren’t particularly adapted for a world where there’s food everywhere and this study, if it’s replicated, suggests teenagers bodies are particularly ill-suited.”

Ken Ong, a paediatrician and epidemiologist at the University of Cambridge, said: “Adolescence is a well-known period of life when people do put on excess weight,” but added that the latest result was “quite a surprising finding” as changes in a person’s basic metabolism had not previously been considered a factor in weight gain at this age.

The latest figures, for 2014/15, show that 19.1% of children in Year 6 (aged 10-11) were obese. A separate study of more than 370,000 children in England published this year shows that this rises to 25% of girls and 24% of boys between the ages of 11 and 15.

“There’s been a lot of focus on childhood obesity and adults, but adolescents have been really understudied,” said Viner. “This [Wilkin] study is a reminder that the teenage period is incredibly important.”

The scientists have not yet established what causes the sudden drop in resting metabolism at puberty. Some previous research has attributed shifts in metabolism to changes in fat and muscle mass during adolescence. But Wilkin said the explanation “doesn’t hold water” because similar changes in metabolism were seen in girls and boys, while changes in body composition are different between sexes.

The study also tracked hormone levels, including testosterone and oestrogen and leptin, which is known to be involved in appetite and none of these seemed to be strongly linked. “At the moment we don’t know how the body is achieving this,” said Wilkin.