TUESDAY, Nov. 1 (HealthDay News) -- Folks genetically predisposed to obesity can reduce their odds of piling on the pounds by staying physically active, a new study suggests.

A large international group of researchers found that the so-called "fat mass and obesity associated" (FTO) gene, which is known to increase the risk of obesity, has a 27 percent weaker effect on physically active adults compared to inactive ones. Their conclusion comes from a meta-analysis of 45 prior studies analyzing data from more than 218,000 participants.

"I think it is important to highlight that you don't have to run a marathon or necessarily join the gym, but walking the dog, cycling to work, taking the stairs . . . about one hour [of activity] a day, five days a week, will have the effect we saw in our study," said study author Ruth Loos, group leader of the Genetic Aetiology of Obesity Programme at Addenbrooke's Hospital in Cambridge, England. "We hope that studies like ours convince people that even when genetically susceptible, a healthy lifestyle will help in the prevention of weight gain."

The study is published online Nov. 1 in the journal PLoS Medicine.

Two-thirds of Americans are either overweight or obese, along with nearly one-fifth of children up to age 19, according to the U.S. Centers for Disease Control and Prevention. Obesity is known to increase the risk for heart disease, diabetes, stroke and certain forms of cancer.

Loos said that prior research by her team on 20,000 adults also had shown that physical activity reduced the effect of the FTO gene on obesity risk, but subsequent studies hadn't always led to consistent results.

"What did surprise us was that the [exercise] effect was more pronounced in North Americans than in Europeans," she said. "We speculate that this might be due to the fact the Europeans are 'generally' less obese and more physically active than North Americans, and that there is a greater range of BMI [body mass index] and physical activity in North Americans such that the effect can be larger."

Some of those predisposed to being overweight may feel there's little point in resisting nature's pull, Loos noted. In fact, she added, a recent study on the effects of genetic testing showed that people who were informed they had a higher-than-average genetic susceptibility for obesity increased their dietary fat intake over the following three months, suggesting that the genetic information might have given them a sense of no control.

But this fatalism is misplaced, she said, though more research is necessary to understand the impact of genes and environment on weight.

Dr. Robert Berkowitz, senior medical director of the Weight and Eating Disorders Program at the University of Pennsylvania, said the American obesity epidemic of the last three decades hasn't resulted from changed genes, but changed habits.

"It's good to see that physical activity can really help people despite having the [obesity] gene," Berkowitz added. "It really is a genes-environment interaction. Most of us are faced with sedentary jobs, so we're not as active as we used to be even 30 or 40 years ago. I think it all makes it difficult for a person coping with a weight problem."

More information

The U.S. National Library of Medicine has more about obesity.