New study by the University of Exeter Medical School in the United Kingdom finds that the rise in obesity rates during adolescence may be due to a substantial fall of calories burned during the rapid growth phase of puberty.
It is possible that modern teenagers are trapped by a trait which evolved thousands of years ago to help them through puberty, but which now leaves them vulnerable to obesity.
According to the Centers for Disease Control and Prevention, around 20 percent of adolescent’s age 12-19 years are obese in the world. 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.
A new 12-year study found that 15-year-olds use 400 to 500 fewer calories while at rest each day than when they were 10 years old, a drop in resting metabolism of around a quarter. 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.
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.
Burning calories uses up a fixed amount of oxygen. So the scientists were able to measure resting energy use by placing the children in a sealed canopy, which Wilkin describes as similar to a “Buzz Lightyear hood”, where their oxygen consumption was measured over a period of time.
Wilkin believes the drop in metabolism during puberty makes sense from an evolutionary perspective, as food has tended to be scarce for most of human history. If fewer calories are required for the body’s basic functions, energy can be re-directed towards growing. This suggests that while teenagers still need to eat more during puberty, because they’re getting taller and developing so quickly, they don’t need as many extra calories as you might expect.
The worst outcome is that adolescents and their families take these findings to mean that they can do nothing about teenage obesity. The best is that a new explanation for teenage obesity leads to better understanding, and an avoidance of the foods that are the cause.