By Jura Koncius
Washington Post Staff Writer
Thursday, July 20, 2006; H05

Forget that bacon cheeseburger you ate last night. Is your air conditioning making you fat?

This horrifying, yet delicious, notion was put forward in a scientific study, released last month, on possible contributing causes of obesity. Among the top factors -- besides the "Big Two" of overeating and lack of exercise -- are overweight parents, older women giving birth, medications such as certain antidepressants or antihistamines, inadequate sleep, quitting smoking, and -- most intriguingly -- keeping the temperature too cool and comfortable.

According to the report, published in the International Journal of Obesity ( http://www.nature.com/ijo ), research shows that when humans are overheated, their food intake tends to go down. "Restaurateurs report that when their air conditioning goes out, they lose business. People don't want to eat a lot in the heat," says David Allison, professor of biostatistics at the University of Alabama at Birmingham, who led the study team.

He points out that since 1980, when residential air conditioning started becoming more common, the rates of obesity in this country have steadily risen. "On a 100-degree day back then, with no air conditioning, would you want to go to an all-you-can-eat buffet and gorge yourself?"

What's more, perspiring burns calories. "You need to expend energy to sweat to keep your body cool," Allison says. "That could lead to slightly lower body weight, all other things being equal."

But before you push the thermostat up to 78 and wait for your waistline to start shrinking, consider this: Would you use your treadmill more or less in a hot and steamy house?