MONDAY, Sept. 8 (HealthDay News) -- Physical activity may reduce the risk of obesity in people with a genetic mutation that predisposes them to high body-mass index (BMI), says a U.S. study.

Recent research has shown a link between BMI and variants of the fat mass and obesity associated with the (FTO) gene. The mutations connected with obesity occur in about 30 percent of European populations and are associated with a 1.75-kilogram (3.9-lb.) increase in body weight, according to background information in the study.

While lifestyle factors such as diet and exercise are important factors in weight control, it's not exactly clear how they interact with genetics.

In this study, researchers analyzed DNA samples from 704 healthy Amish adults, average age 43.6, and also conducted a series of physiological tests on the participants, including recording their physical activity over a seven-day period.

Among the participants, 54 percent of men and 63.7 percent of women were overweight, and 10.1 percent of men and 30.5 percent of women were obese. The genetic analysis showed that 26 single-nucleotide polymorphisms (SNPs -- changes in a single base letter of DNA) in the FTO gene were associated with BMI.

Further investigation found that the two strongest SNPs were associated with BMI only in people with low physical activity scores. The SNPs had no effect on people with above-average physical activity scores.

The study was published in the Sept. 8 issue of the Archives of Internal Medicine.

"Activity levels in the 'high-activity' stratum were approximately 900 calories [860 calories for women and 980 calories for men] higher than in the 'low-activity' stratum, which, depending on body size, corresponds to about three to four hours of moderately intensive physical activity, such as brisk walking, housecleaning or gardening," the researchers wrote.

"In conclusion, we have replicated the associations of common SNPs in the FTO gene with increased BMI and risk to obesity in the Old Order Amish. Furthermore, we provide quantitative data to show that the weight increase resulting from the presence of these SNPs is much smaller and not statistically significant in subjects who are very physically active. This finding offers some clues to the mechanism by which FTO influences changes in BMI and may have important implications in targeting personalized lifestyle recommendations to prevent obesity in genetically susceptible individuals."

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