10 less publicized reasons for fat gain

1. Inadequate sleep -- average amounts of sleep have fallen among Americans, and many studies tie sleep deprivation to weight gain.

2. Increased consumption of endocrine disruptors, substances in some foods that may alter fats in the body.

3. Climate-controlled environments. Air conditioning and heating limit calories burned from sweating and shivering.

4. Decrease in tobacco use. Smoking is often linked to appetite suppression.

5. New prescription medicines that promote weight gain.

6. Changing demographics -- there are now more middle-aged and Hispanic Americans, groups that have higher obesity rates.

7. Women giving birth at an older age, which correlates with heavier children.

8. Genetic influences during pregnancy -- a so-called "fetally-driven positive feedback loop."

9. Natural selection -- heavier people tend to survive tough times better than skinnier humans.

10. Assortative mating, or "like mating with like" -- meaning fat people procreating with others of the same body type, gradually skewing the population toward the heavy end.

Source: International Journal of Obesity