Teenage Obesity Link to Slowdown in Calorie Burn During Puberty

09 Sep 2016 --- Teenage weight gain is down to a significant reduction in the amount of calories adolescents burn while they rest, according to a 12-year study. 

The acceleration in obesity among young teenagers could be explained by the The University of Exeter Medical School research which found the number of calories teenagers burn while at rest drops suddenly in puberty.

The findings, led by Professor Terence Wilkin, reveal that 15 year olds burn up to 500 fewer calories a day compared to when they were aged 10 - a drop of around 25 percent. 

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

“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,” he says. 

The study also found 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.

The findings, published in The International Journal of Obesity, follow the UK government’s childhood obesity strategy and could help explain why young people can become obese during their pubescent years. Childhood obesity is one of the most serious global public health challenges of this century, according to the World Health Organization (WHO).

The National Child Measurement Program (NCMP), which measures the height and weight of approximately one million school children in England every year, found that a third of 10-11 year olds were overweight or obese.

People spend calories in two ways: voluntary spend through physical activity and the much larger involuntary spend, simply to stay alive - thinking, keeping the 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 schoolchildren studied, the calorie expenditure rose as expected from the age of five onwards. However, researchers were surprised when the children experience a sudden drop in calorie expenditure from the age of 10 upwards. Especially at a period of rapid growth which uses a significant amount of calories. 

The research was carried out between 2000 and 2012 and analyzed data from almost 350 kids from Plymouth, England. They were assessed every six month between age 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.

Energy-Saving Mode 

Professor Wilkin’s previous research showed that children are particularly susceptible to weight gain at two stages – once in infancy, probably attributable to diet and lifestyle choices made by the child’s parents, and again in puberty. This second peak was previously unexplained but now the new research could shed more light, suggesting it may be explained by a drop in the number of calories young teenagers burn while at rest during puberty.

“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,” he adds.

“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 we they have sufficient calories each day, the drop in spend means excess weight gain.”

The study involved collaboration with Plymouth University and was funded by the Bright Futures Trust, Fountain Foundation, BUPA Foundation, and the EarlyBird Diabetes Trust.

It was set up to try to establish why so many young people are at risk of developing diabetes. Some 2.3 million people in the UK know they have diabetes. A further 750,000 have diabetes but don't yet know it. By the time they are diagnosed, half will already have complications. 

It has been predicted that, unless present trends are slowed, one in five of children born in 2000 will develop diabetes in their lifetime – largely because of obesity.

by Gaynor Selby