Why are we so fat? The list gets longer!

If you were beginning to suspect that the list of things which could make you fat seems to be growing longer on an almost daily basis - you were right!

New research is now saying that obesity in the U.S. cannot just be blamed on diet and lack of exercise.

In a review of data on possible causes of obesity, the contention expressed is that many aspects of modern living from lack of sleep - to exposure to environmental chemicals - to living with air conditioning - may be contributing factors in the obesity stakes in America.

The researchers say that obesity research and prevention strategies need to look beyond food industry practices such as portion sizes and added sugar and reduced physical activity, because a range of other factors are just as plausible.

Dr. David B. Allison director of the Clinical Nutrition Research Center at the University of Alabama, says that diet and exercise are undoubtedly important but the evidence linking obesity to food industry marketing and lack of gym class is circumstantial.

Allison, a co-author of the report says there is other evidence which suggests that a chronic lack of sleep boosts appetite and eating, and studies show that U.S. adults have gone from sleeping for an average of 9 hours to about 7 hours.

Industrial chemicals that act as endocrine disruptors may also contribute to an increase in body fat as they alter hormonal activity when they get into the body and people have been increasingly exposed to such chemicals through the food chain in recent years.

Air conditioning too may be a factor as the body burns calories when forced to regulate its own temperature and people tend to eat less in hot, humid weather.

Allison and colleagues have come up with a list of 10 potential obesity risk factors which include increased rates of older mothers, whose children may be more prone to excess weight gain; a range of medications, such as antidepressants, which can promote weight gain; and a decrease in smoking rates, because people often gain weight when they quit and the absence of nicotine, an appetite suppressant encourages snacking.

Genetic factors, such as the heritability of a person's body mass index (BMI), could also be a factor as studies show that BMI has a heritability of about 65 percent.

According to Allison he is not suggesting people should stop taking their prescriptions, keep smoking or turn off the air con, and he says diet and exercise remain key factors in obesity.

Allison believes researchers and policymakers should be "open-minded" about the potential contributors to the obesity problem and not assume that the answer lies in simple solutions such as banning fizzy drinks from schools or installing sidewalks to encourage walking.

Some experts question the claims and say as people stay thin in all different climates, it is unlikely air conditioning plays much of a role and they are also concerned about linking obesity to genetics.

The review is published in the International Journal of Obesity.