Women with extra fat around their waists are more likely to develop asthma, even if they aren't overweight, a new study finds.
The California Teachers Study of more than 88,000 women found the same association between obesity and increased incidence of asthma that has been seen in other research, according to the Aug. 25 online report in the journal Thorax.
But it also found a 37 percent increased incidence of asthma among women with a waist circumference of 88 centimeters -- about 35 inches -- even if they were of normal weight.
That finding was an offshoot of a study originally intended to look at factors related to breast cancer in women, said study author Julie Von Behren, a research associate at the Northern California Cancer Center. But the researchers also got a lot of other information about the participants, including waistline measurements and asthma risk factors, such as smoking exposure.
"We had a lot of detailed information, also on body weight at age 18 and later," Von Behren said.
Using the standard designations of "overweight" for a woman with a body-mass index (BMI) of 30 or higher and "extreme obesity" for a body-mass index (BMI) of 40 or higher, the study found a doubled incidence of asthma among the obese women and a more than tripled incidence among the extremely obese.
While the study was not designed to determine why the location of body fat could play a role in development of asthma, "waist size can be an indicator of the type of body fat," Von Behren explained. "Abdominal fat is visceral fat, which is more biologically active. It has been linked to diabetes and heart disease."
Fat around the waist "could be acting in some inflammatory way," she said.
That is a plausible, though unproven, explanation, said Dr. Alejandro Arroliga, a pulmonologist and chairman of medicine at the Scott & White Memorial Hospital and Clinic in Temple, Texas.
"We know that obesity can cause an inflammatory state," Arroliga said. "Markers of inflammation are increased in obesity."
Other studies have documented the overall association between obesity and asthma, he said. "This is one of the biggest, with more than 88,000 women. It's huge," Arroliga said.
While one conventional explanation is that body fat puts a squeeze on airways, some previous studies have pointed toward the composition of body fat as a possible element in asthma risk, he said.
"But it is still unclear why there is this association," Arroliga said. "The biological explanation lags behind the epidemiological evidence."
Whatever the reason, the association with asthma provides just another reason not to put on extra weight, Von Behren said.