Teens who exercise for at least an hour per day have much better control of their body weight - even if they are genetically 'predisposed' to obesity.

The findings come from a report in the April issue of Archives of Pediatrics & Adolescent Medicine, one of the JAMA/Archives journals.

The paper claims that there is "compelling evidence" that human obesity is a "multifactorial disorder" where both genes and lifestyle choices such as exercise and diet are important contributors.

The study was lead by Jonatan R. Ruiz, Ph.D., of the Karolinska Institutet in Huddinge, Sweden, and it monitored 752 adolescents who were part of a cross-sectional study in 10 European countries between October 2006 and December 2007.

A total of 37 per cent (257 teenagers) showed no copies of the obesity mutation, while 47 per cent (354) had one copy and 16 per cent (123) had two copies. The mutation was associated with a higher body mass index (BMI), higher body fat percentage and a larger waist circumference.

Teens who exercised the recommended hour or more a day showed a much lower recurrence of the gene mutation.

The report stated: "These findings have important public health implications and indicate that meeting the physical activity recommendations may offset the genetic predisposition to obesity associated with the FTO polymorphism in adolescents.

"Indeed, adolescents meeting the daily physical activity recommendations may overcome the effect of this gene on obesity-related traits."