A new study confirms a link between obesity and asthma.

A number of studies have shown an association between obesity and asthma, both of which have become much more common over the past three decades, Dr. Jun Ma of the Palo Alto Medical Research Institute in California note in the medical journal Allergy.

Ma and her team looked at about 4,500 men and women from the National Health and Nutrition Examination Survey for 2005-2006. About a third were overweight, and another third were obese.

Forty-one percent had some type of allergy, while 8 percent had asthma. The researchers wanted to tease out those rates because allergy and asthma are related in some people.

Twelve percent of the obese individuals had asthma, compared to six percent of the normal-weight study participants. And the likelihood of asthma rose as the body mass index - BMI, a relation of weight and height used to gauge obesity - increased and waist circumferences expanded.

The risk of asthma was more than tripled for the most obese individuals compared to normal weight people.

But the reason why the two might be related is still not clear. Some researchers have suggested the system-wide, low-grade inflammation that occurs with obesity may be a factor, while others have argued that resistance to the key blood-sugar-regulating hormone insulin - which rises with excess weight - is the reason for the link. That resistance often foretells the onset of diabetes.

Thirty-seven percent were either diabetic or had insulin resistance. The study did not find any evidence, however, that insulin resistance was responsible for the relationship, and allergy was not related to either weight or insulin resistance.

The findings don’t rule out the possibility that insulin may be a link between obesity and asthma, Ma told Reuters Health by e-mail. There are a host of other potential reasons for the association, which seems complex, she added.

SOURCE:  Allergy, published online May 7, 2010.