Blaming your bulge on bad genes just won't fly anymore.

Thanks to a study of Lancaster County's Old Order Amish, medical researchers now believe the fat gene can be outsmarted, but you have to get off your sofa to do it.

A study published in Monday's Archives of Internal Medicine focused on FTO, a common genetic variation that makes some people more likely to gain weight than those with a regular version of the gene.

But researchers found Amish who carried the genetic variation were no more likely to be overweight than those with the regular gene — as long as they got three to four hours of moderate activity every day.

"This is an important study because it's one of the first, if not the first, to show that genes can be outsmarted by physical activity," said the study's co-author, Dr. Soren Snitker of the University of Maryland. "Most important is the fact that among those who are the most active, there are no effects from those genes."

Scientists believe about 30 percent of white people of European ancestry, including the Amish, have the genetic variation that leads to a propensity for being overweight and obese.

For the study, subjects wore battery-operated monitoring devices that checked for physical activity for an entire week. But earlier studies that relied on subjects self-reporting physical activity showed similar results.

The Amish study took data from 704 people between 2003 and 2006. Blood samples determined which had the genetic variation linked to obesity. Among those with the variant, people who got about three or four hours of moderate physical activity a day weighed up to an average of 15 pounds less than the least active people.

Moderate physical activity includes gardening, housecleaning and brisk walking.

Snitker said the least active group are comparable to Americans who get little exercise beyond the normal activities of daily living.

Snitker said the Amish lifestyle and jobs that incorporate physical activity make it easier to fulfill the three to four hours needed to keep off excess weight. And some people who are overweight simply can't do the activity necessary to shed pounds.

"There are a lot of people who are overweight and have osteoarthritis in their knees and high blood pressure, so they can't do all this physical activity," Snitker said. "It's not that easy for people to be more physically active unless it's part of your occupation. If you think about the people who get more than a couple of hours of activity every day — and there are few people who do — they are Amish farmers, laborers, and Amish housewives who have 10 kids to take care of."

Snitker said the answer must come from a re-engineering of society — such as allowing stores to be built within walking distance of residential areas and making stairs as accessible as elevators in buildings.

Alice Yoder, Lancaster General's director of community health, agreed.

"We're not making that link between health and the environment," Yoder said. "Until we do that, it's going to be difficult for people to fit physical activity into their lives. We need to build communities with sidewalks and good lighting so people feel safe. It's one thing to say, 'This is what needs to be done to lose weight,' but the reality is our communities aren't built that way. The reality is our communities are built around our cars."

More research is needed to show if similar results would come in studies of other racial and ethnic groups. Yoder said she sees similar effects on Latinos who immigrate to the county.

"When they first come to Lancaster County, they're of a normal, healthy weight," she said. "When they adapt to the American style of eating and lack of physical activity, they become obese. But their family in Puerto Rico are not obese because they're eating more fruits and vegetables, they walk more and they're active more because of the nice weather. So that really speaks to this study, too."

E-mail: slindt@lnpnews.com