More evidence today that our genes aren't always our destiny — with an inconvenient caveat for couch potatoes: Physically active people who carry gene mutations linked to obesity are no more likely to be overweight than those without the variants — as long as they exercise at least three hours a day.

Scientists monitored the physical activity of 704 Amish men and women for a week with accelerometers, devices worn on the hip that keep tabs on movement. Those who carried two copies of the FTO gene variant, which previously has been shown to increase the risk of obesity, had a 27 percent risk of being fat, compared to a 16 percent risk among those who had no mutations on the gene. But heavy labor such as brisk walking, house-cleaning and gardening canceled out the gene's weight-gaining effect.

The findings are published in today's Archives of Internal Medicine.

"It's likely that physical activity will blunt the effect of this gene as well as other genes," said study co-author Soren Snitker, an assistant professor of medicine at the University of Maryland Medical Center in Baltimore. The FTO gene, he added, originally may not have had any association with weight, especially when most of us lived the physically active lifestyle of today's Amish farmers. But the gene's "obese-ogenic" effect may be turned on by high-fat diets, inactivity or other environmental factors, Snitker said.

(Among the general U.S. population, the risk of obesity is 20 percent for those without FTO variants. It's 26 percent among people with one variant, and 31 percent in those who carry two, Snitker said. Among the Amish, who scientists studied because of their similar genetics and lifestyle, carrying one FTO variant added an extra 3.9 pounds; having two tacked on 7.7 pounds.)

But there's also an opposite effect for people without the variants, Snitker said — one you've probably observed among the effortlessly slender.

"If they're inactive, they don’t become much heavier," Snitker said. "They just have to look at an exercise bike and the pounds start falling off — they're slim already."

A separate study in the journal Acta Paediatrica found that babies with the highest levels of pesticides in their umbilical cords were twice as likely to be obese as other children, the British Independent is reporting.

(Image from iStockphoto)