London, Sep 9 (IANS): When young teenagers reach puberty, both girls and boys tend to experience a rapid drop in the number of calories that they burn, finds a new study.

The research, that also explains the dramatic rise in childhood obesity in puberty, found that 15-year-olds use 400 to 500 fewer calories while at rest per day compared to when they were 10-years-old -- a fall of around a quarter.

This was particularly surprising as it is a period of rapid growth, and growth uses lots of calories, the researchers said.

However, by the age of 16, their calorie expenditure begins to climb once again.

In addition, the study also found that teenagers exercise less during puberty, adding to the calorie excess that underlies obesity.

This exercise drop is particularly stark in girls, whose activity level drops by around a third between the ages of seven and 16.

"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," said Terry Wilkin, Professor at the University of Exeter in Britain.

Childhood obesity is one of the most serious global public health challenges for the 21st century, according to the World Health Organization (WHO).

"Child obesity and associated diabetes are both among the greatest health challenges of our time. Our findings can explain why puberty why teenagers gain excess weight in puberty, and it could help target strategies accordingly," Wilkin added, in the paper published The International Journal of Obesity.

During the 12-year-long study, between 2000 and 2012, the research team analysed data gathered from nearly 350 school children in Britain.

The children were assessed every six months between the ages of five and 16, during which blood samples were given to assess metabolic health and measurements of size, body composition, metabolic rate and physical activity taken. Of this set, 279 children gave data that made them eligible for the latest study.

Burning calories uses up a fixed amount of oxygen.

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.