updated 6/27/2006 12:01:27 AM ET 2006-06-27T04:01:27

Why are so many people fat? Scientists have come up with some novel excuses, including air conditioning, lack of sleep, fewer smokers, and more sex among obese people, which can produce chubby kids.

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Twinkies aren't the only things weighing America down, these researchers contend in a report published Tuesday in the International Journal of Obesity.

"I think it's very creative," said Dr. Robert Kushner, medical director of the weight management program at Northwestern Memorial Hospital in Chicago, who had no role in the report. "We are facing an epidemic with no tipping point in the near future. At this point, there are no silly ideas."

However, some critics say the authors' "Top Ten" list of alternative explanations reads more like material for a David Letterman routine than a scientific study.

"I'd put this in the category of `calorie distracters' — `Let's just do anything to get people to stop worrying about having to eat less and move more,'" said Marion Nestle, a nutrition professor at New York University and frequent food industry critic. "‘And let's not say a word to food companies about misleading and manipulative marketing practices, especially those directed toward children.'"

David Allison, a University of Alabama biostatistician, invited 19 other scientists in the United States, Canada and Italy, to work on the report.

Top 10
They looked at more than 100 studies on potential contributors to obesity besides diet and exercise, and concluded there was at least some support for 10:

1. Inadequate sleep. (Average sleep amounts have fallen, and many studies tie sleep deprivation to weight gain.)

2. Endocrine disruptors, which are substances in some foods that may alter fats in the body.

3. Nice temperatures. (Air conditioning and heating limit calories burned from sweating and shivering.)

4. Fewer people smoking. (Less appetite supression.)

5. Medicines that cause weight gain.

6. Population changes. (More middle-agers and Hispanics, who have higher obesity rates.)

7. Older birth moms. (That correlates with heavier children).

8. Genetic influences during pregnancy.

9. Darwinian natural selection. (Fat people outsurvive skinny ones).

10. Assortative mating, or "like mating with like," as Allison puts it. Translation: fat people procreating with others of the same body type, gradually skewing the population toward the heavy end.

Don't light up
Not that people necessarily should try to alter these factors, Allison said. For example, "we would never recommend that people start smoking to reduce their body weight."

The same for medications that can lead to weight gain, though doctors may want to consider alternatives if a patient piles on pounds, said Dr. Louis Aronne, a Weill-Cornell Medical School nutrition expert who is past president of the Obesity Society, the leading group of researchers in the field.

Allison said no food or beverage makers funded any part of the report, though he and some collaborators consult for such companies.

The point is, there is more to obesity than diet and exercise, he said. "These are 10 reasonable hypotheses, and as scientists, we should be open-minded," Allison said.

Copyright 2006 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.


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