In the study, teenagers had their blood tested for a variant of what scientists call the "fatso" gene. The teens wore monitoring devices for a week to measure their activity.
Exercising an hour or more a day made a big difference for the teens who were genetically predisposed to obesity.
Their waist measurements, body mass index scores and body fat were the same, on average, as the teenagers with regular genes.
The study appears in the April edition of Archives of Pediatrics and Adolescent Medicine.
The Associated Press