Obesity isn't all about eating and inactivity, says an international group of researchers.

Just about everywhere you look, doctors are blaming America's obesity epidemic on two things: too much food -- especially widely marketed fast food and junk food -- and too little exercise, with too much time in front of the TV.

But we're paying too much attention to the "big two," argue David B. Allison, PhD, director of the University of Alabama at Birmingham clinical nutrition research center, and his colleagues.

"The importance of the big two is accepted as established, and other putative factors are not seriously explored," they argue. "The result may be well-intentioned but ill-founded proposals for reducing obesity rates."

To stimulate debate, Allison and colleagues suggest 10 other possible causes of obesity. Their article appears in this week's online edition of the International Journal of Obesity.

It's well accepted that reduced physical activity and fast food are linked to obesity. But the evidence that these are the main causes of obesity is "largely circumstantial," Allison and colleagues say.

Obesity researchers should broaden their horizon, they argue. So the researchers propose 10 other explanations for obesity, which are also supported by circumstantial evidence.

Even if some of these causes have only a small effect, Allison and his colleagues say, they may interact in ways that greatly magnify their individual effects.

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10 Causes of Obesity

The Alabama group puts forth these 10 "additional explanations" for obesity:

1. Sleep debt. Getting too little sleep can increase body weight. Today's Americans get less shut-eye than ever.

2. Pollution. Hormones control body weight. And many of today’s pollutants affect our hormones.

3. Air conditioning. You have to burn calories if your environment is too hot or too cold for comfort. But more people than ever live and work in temperature-controlled homes and offices.

4. Decreased smoking. Smoking reduces weight. Americans smoke much less than they used to.

5. Medicine. Many different drugs -- including contraceptives, steroid hormones, diabetes drugs, some antidepressants, and blood pressure drugs -- can cause weight gain. Use of these drugs is on the upswing.

6. Population age, ethnicity. Middle-aged people and Hispanic-Americans tend to be more obese than young European-Americans. Americans are getting older and more Hispanic.

7. Older moms. There's some evidence that the older a woman is when she gives birth, the higher her child's risk of obesity. American women are giving birth at older and older ages.

8. Ancestors' environment. Some influences may go back two generations. Environmental changes that made a grandparent obese may "through a fetally driven positive feedback loop" visit obesity on the grandchildren.

9. Obesity linked to fertility. There's some evidence obese people are more fertile than lean ones. If obesity has a genetic component, the percentage of obese people in the population should increase.

10. Unions of obese spouses. Obese women tend to marry obese men. If there are fewer thin people around -- and if obesity has a genetic component -- there will be still more obese people in the next generation.

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"We do not claim that all of the additional explanations definitively are contributors [to obesity] but only that they are as plausibly so as are the ‘big two’ and deserve more attention and study," Allison and colleagues conclude.

And the researchers’ list of 10 doesn't exhaust the possibilities. There may be even more explanations. Four examples of this include: a fat-inducing virus; increases in childhood depression; less consumption of dairy products; and hormones used in agriculture.

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SOURCE: Keith, S.W. International Journal of Obesity, advance online publication, June 27, 2006.