In the battle over Americas expanding waistline, much of the emphasis is on diet and exercise. A new study suggests other potential causes, including too much air conditioning, should be studied as well.

The report, published in the International Journal of Obesity, said researchers found no link between obesity and the proximity of children to fast food restaurants. On the other hand, researchers cite biology to link the growing problem of obesity and the prevalence of air conditioning over the last three decades.

Mammals, they say, use up more energy to maintain their bodies at an acceptable temperature.

The more time humans spend in a climate-controlled environment, the less their bodies have to work. They also suggest people tend to eat less when theyre hot.

"If you lived in Alabama in the summer of 1960, without air conditioning in a car or restaurant, you probably wouldnt want to go to an all-you-can-eat buffet," said David Allison, a researcher at the University of Alabama at Birmingham.

Allison said other factors could also contribute to weight gain, such as over-medication. He said the widespread use of anti-psychotics, anti-depressants, mood stabilizers, contraceptives and antihistamines, all can cause users to put on pounds.

The researchers urged their colleagues to focus on some of the unexamined causes of obesity, and not so much on fast food and lack of exercise.