Teens worldwide aren't getting enough exercise -- especially girls

The World Health Organization recommends adolescents spend at least one hour a day exercising enough to basically break a sweat — a brisk walk or a more intense pick-up soccer game. Lot's of research suggests this level of activity is crucial for supporting growing bodies and minds. But new global data suggests only a small minority of kids and teens get that much.

In fact, one in four report not hitting 60 minutes of moderate-to-vigorous exercise any day of the week, according to a new study in The Lancet Global Health. Girls, as well as adolescents from middle- and lower-income countries reported even lower levels of exercise. "The findings are disappointing, because one of the major results is that the trend over the last 15 years is flat," says Fiona Bull, head of the physical activity unit at the World Health Organization, who co-authored the study.

"These data are a call for us to relook at what we're doing. Business as usual will not change these trends." Changing habits in those teen years could have a lifelong impact, says Solveig Cunningham, a global health researcher at Emory University. "As an adolescent, if your baseline is zero days of physical activity, that's going to be your baseline as you go into adulthood." So how do researchers know whether adolescents are — or aren't — working up a sweat?

Researchers would love to be able to strap smartwatches to kids to get detailed data, but that would be costly. The next best strategy is to ask a basic question — how many days during the past week were you active for at least 60 minutes? The report draws on data from surveys posting some version of this question to over 3 million adolescents from 140 countries over the past 15 years.

Only about 14% of kids worldwide reported getting an hour a day of exercise. Globally, the average adolescent reported less than 3 such days a week, the study found. Kids from low- and middle-income countries tended to be less active than their counterparts from wealthier Western countries, the study found.

There's no one explanation for this pattern, but a few likely causes stand out. "Physical activity is an issue of opportunity," says Andrea Ramirez Varela, an epidemiologist at the University of Texas Health Science Center in Houston. In many lower income countries, she says that the environment — where they live, go to school and play — often restricts opportunities to be active.