An 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.
Research led by the University of Exeter Medical School, found unexpectedly that when boys and girls reach puberty, they experience a rapid drop in the number of calories they burn -- at a time when the number would be expected to rise with the growth spurt.
Metabolism drops starting at age 10
The research found that 15-year-olds burn 400 to 500 fewer calories while at rest per day compared to when they were 10-years-old – a drop of 25 per cent.
But by the age of 16, their calorie expenditure begins to climb again.
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 may help explain why may youth 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.
We spend calories in two ways -- through physical activity and through our resting metabolism. (Resting metabolic rate is the energy required to keep your body functioning including your lungs breathing and your heart beating while at rest.)
Resting metabolism might be expected to rise with body size, and among the children studied, the calorie expenditure rose as expected from the age of five onwards. However, researchers were surprised to see the children studied experience a sudden drop in calorie expenditure during puberty, from the age of 10 onwards. This was particularly surprising as it is a period of rapid growth, and growth uses lots of calories.
During the 12-year-long study, conducted between 2000 and 2012, the research team analyzed data gathered from nearly 279 school children in the UK. 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 were taken.
Children susceptible to weight gain in infancy and puberty
The study builds on earlier research that found children are particularly susceptible to weight gain at two stages: once in infancy, likely attributable to diet and lifestyle choices made by the child's parents, and again in puberty. The new research suggests that weight gain in puberty may be explained by the drop calories burned at rest during puberty.
The researchers speculate that this sudden decline in resting metabolism 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 and calories are in abundance.
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