Acceleration in obesity among young teenagers could be explained by a 12-year-long study which found that the number of calories they burn while at rest drops suddenly in puberty.
The research found that when children reach puberty they experience a rapid drop in the number of calories they burn despite the growth spurt that take place. This applied for both girls and boys.
The research by Professor Terence Wilkin, of the University of Exeter Medical School, 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. But by the age of 16, their calorie expenditure begins to climb once again. For comparison, a McDonalds Big Mac 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 exercise drop is particularly stark in girls, whose activity level drops by around a third between the ages of seven and 16.
The new findings, which come after the government launched a strategy to tackle the dramatic rise in childhood obesity, may help to explain why many youngsters become obese in puberty. The World Health Organization (WHO) regards childhood obesity as one of the most serious global public health challenges for the 21st century. The National Child Measurement Programme (NCMP), which measures the height and weight of around one million school children in England every year, found that a third of 10-11 year olds were overweight or obese.
Professor Wilkin 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.”
We spend calories in two ways, voluntarily through physical activity and the much larger involuntary spend, simply to stay alive. Thinking, keeping warm, and keeping the heart, liver and kidneys working together use up to 1,600 calories per day in adolescence.
During the 12-year-long study, between 2000 and 2012, the research team analysed data gathered from nearly 350 school children in the Earlybird study, based in Plymouth. The children were assessed every six months between the ages of five and 16, when blood samples were taken to assess metabolic health and measurements of size, body composition, metabolic rate and physical activity were taken. Of this set, 279 children gave data that made them eligible for the latest study.
The study builds on previous research conducted by Professor Wilkin which showed that children are particularly susceptible to weight gain at two stages of life, the first 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. The new research suggests it may be due to a drop in the number of calories young teenagers burn while at rest during puberty.
Professor 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 we they have sufficient calories each day, the drop in spend means excess weight gain”
The study 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. Type 2 diabetes, so-called ‘adult’ diabetes, is by far the commonest form, and it is of concern that teenagers and even younger children are now affected by it. 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.
The research involved collaboration with Plymouth University and was funded by the Bright Futures Trust, Fountain Foundation, BUPA Foundation, and the EarlyBird Diabetes Trust. The study is published in the Nature journal The International journal of Obesity.