WASHINGTON DC, United States--Physical exercise can reduce a genetic predisposition to obesity by an average of 40 percent, a new study showed Tuesday.

The research challenges the notion that an inherited propensity to obesity is impossible to overcome and boosts the case for the benefit of more exercise for anyone looking to shed some weight.

The study, published in this week's Public Library of American Science Medicine journal, is based on examination of 20,430 people living in Norwich, Britain.

It found most of the study's participants inherited between 10 and 13 genetic variants known to increase the risk of obesity, with some inheriting as many as 17 variants, and others as few as six.

Using modelling techniques, the researchers found that each genetic variant was linked to an increase in body mass index (BMI)--a measure of body size based on both height and weight.

"Each additional genetic variant in the score was associated with an increase in BMI equivalent to 445 grams (1.6 ounces) in body weight for a person 1.70 meters (5.7 feet)," the study said.

For physically active individuals, the increase was just 379 grams (13 ounces).

That was "36 percent lower than in physically inactive individuals in whom the increase was 592 grams (21 ounces) per variant," the research found.

Each additional variant also raised the individual's chances of obesity 1.1-fold, but in physically active individuals, "the increased odds per variant for obesity risk were 40 percent lower."

Ruth Loos of Britain's Medical Research Council Epidemiology Unit, who carried out the research, said the study "challenged deterministic view of the genetic predisposition to obesity."

"Our findings further emphasize the importance of physical activity in the prevention of obesity," Loos said.