During puberty, the number of calories girls and boys burn while at rest plummets.
According to the Centers for Disease Control and Prevention, around 20 percent of adolescents age 12-19 years are obese in the United States. Obese teenagers are more likely to have prediabetes, a condition in which blood glucose levels indicate a high risk for the development of diabetes.
Adolescents who are obese are also at a greater risk for bone and joint problems, sleep apnea, social and psychological problems, and they are more likely to have risk factors for cardiovascular diseases, such as high cholesterol or high blood pressure.
This finding contradicts expectations that calorie burn would rise with the growth spurt teenagers experience during puberty, and it may explain why youngsters become obese during this life stage.
Prof. Terry Wilkin, of Endocrinology and Metabolism at University of Exeter Medical School, and colleagues found that adolescents age 15 years burned 400-500 fewer calories - a decrease of around a quarter - while at rest per day than when they were 10 years old. However, calorie expenditure increased again by age 16.
Another key finding from the study was that teenagers exercise less throughout puberty, which also adds to excess calories that can contribute toward obesity. The reduction in physical activity is particularly significant in girls, decreasing by about one third from ages 7-16 years.
From age 10, calories burned per day declines
"Child obesity and associated diabetes are both among the greatest health challenges of our time," says Wilkin. "Our findings can explain why teenagers gain excess weight in puberty, and it could help target strategies accordingly."
Calories are expended either through physical activity or through processes that keep the person alive, such as thinking, keeping blood warm, and keeping the heart, liver, and kidneys working - processes that expend around 1,600 calories per day during adolescence.
The number of calories burned could be expected to increase with body size. However, in contrast, the team noted a decline in calories burned during puberty - from age 10 upward - which the researchers say is surprising given that puberty is a period of rapid growth and consequently would, in theory, use more calories.
They analyzed data from almost 350 U.K. school children involved in the EarlyBird Diabetes study - a 12-year project between 2000-2012 to determine which factors in childhood lead to diabetes in later life.
Children were assessed every 6 months between ages 5-16 years, and blood samples were taken to identify measures of metabolic health; body size and composition, metabolic rate, and levels of exercise were calculated. Of the total children, 279 were eligible for inclusion in the research.
Evolutionary trait may now contribute to rise in teen obesity
Previous research by Wilkin showed that children are more likely to gain weight during two stages of their development - once in infancy and again in puberty. While the peak in infancy could potentially be due to parental decisions of diet and lifestyle choices, the explanation for the second peak during adolescence has been unexplained until now.
Wilkin and team hypothesize that the weight gain observed in teenagers can be explained by the fall in calories they 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," says Wilkins.
"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."
Prof. Terry Wilkin
"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 have sufficient calories each day, the drop in spend means excess weight gain," he concludes.