Some people inherit a tendency to pack on pounds, but those in the Amish religious sect who spend hours each day farming, walking, house cleaning and gardening have a way to block the gene's effects, researchers have found.

Physical activity is generally recommended for everyone to stay healthy, but the new findings suggest exercise is particularly important for those with the gene variant.

"These findings emphasize the important role of physical activity in public health efforts to combat obesity, particularly in genetically susceptible people," researchers wrote in Monday's issue of Archives of Internal Medicine.

Study co-author Dr. Soren Snitker of the University of Maryland and his colleagues studied 704 Amish men and women. Blood samples were taken to determine who had a variation in the FTO gene linked to obesity.

Among those with the variant, those who got about three or four hours of moderate physical activity a day weighed up to about 15 pounds less on average than the least active people, the researchers found.

Gene may affect protein function

The least active group of Amish got as much exercise as other people who only burn calories performing activities of daily modern life.

Researchers don't know how the FTO gene affects weight, but it is thought to be involved in the function of a protein that may affect how the body regulates food intake.

"It's only if you're not active that the gene hurts," Snitker said. "If you are active, then either way it doesn't matter whether you have the gene or not."

And people shouldn't interpret the findings to mean "I don't have this gene variant so I don't need to be physically active," said Dr. Joel Hirschhorn, a genetics and obesity researcher at Children's Hospital Boston.

Exercise may change how other obesity gene variants are expressed, he noted.

Monitoring devices worn for week

Unlike previous studies that relied on self reports of exercise levels, Snitker's team used a more reliable measure: battery-operated monitoring devices worn around-the-clock for a week, said lead author Evadnie Rampersaud of the University of Miami.

More research is needed to see if the results hold true in other racial and ethnic groups, said Amy Zlot, a genetics researcher with Oregon's Department of Human Services, who was not involved in the study.

It's thought that about 30 per cent of white people of European ancestry have the gene variant.

It is too soon to recommend routine genetic testing for the public, experts said. The FTO blood test is only available for research purposes so far.

The farm-oriented Amish eschew cars and other modern conveniences, and take care of many children.

People living a typical modern life would benefit from more physical activity, Snitker said: watching less TV in favour of a brisk walk, using stairs instead of elevators and taking up structured exercises like swimming.

With files from the Associated Press