Adult-onset asthma was associated with several measures of large body size, particularly abdominal adiposity, in a large longitudinal study of women.
Even being modestly overweight at baseline increased the risk of developing asthma over time, said Julie Von Behren of the Northern California Cancer Center, Berkeley, and her associates.
Moreover, a large waist circumference, even among women of normal weight and body mass index (BMI), appeared to raise the risk for asthma.
Noting that “obesity has recently been identified as a risk factor for adult asthma, particularly in women,” Ms. Von Behren and her colleagues examined the issue using data from the California Teachers Study, an ongoing assessment of women teachers and school administrators that began in 1995.
The 88,304 subjects included in the analysis were either actively employed by or retired from the state school system. Anthropomorphic factors were assessed at baseline and in 1997, whereas measures related to asthma were assessed in 2000.
The overall prevalence of obesity at baseline was 13%, and the overall prevalence of current asthma was 7.6%. The prevalence of asthma at baseline was 10.9% among women with class I obesity, 13.4% among women with class II obesity, and 18.3% among extremely obese women.
Compared with subjects of normal weight, those who were overweight had an odds ratio of 1.4 for asthma. The odds ratio rose steadily as weight increased, to 3.3 for women with class III obesity.
In addition to BMI, weight gain since the age of 18 years, waist:height ratio, and waist circumference also were strongly associated with the development of asthma. Waist circumference, which more closely reflects visceral fat than does BMI, showed a particularly strong and independent association with asthma: Even women of normal BMI who had a waist measurement greater than 88 cm had a higher rate of asthma than did women with smaller waists.
(A waist circumference of 88 cm is the National Institutes of Health's cutoff for increased risk for diabetes, hypertension, and cardiovascular disease.)
Overweight and obesity also were associated with asthma severity. Compared with women of normal weight, those who were overweight had an odds ratio for severe asthma of 1.31, those who were obese had an odds ratio of 1.32, and those who were extremely obese had an odds ratio of 2.00, the investigators said (Thorax 2009 [doi:10.1136/thx.2009.114579]).
There are several possible mechanisms by which excess weight or abdominal adiposity may cause or exacerbate asthma, the investigators noted. Obesity can affect airways via its effects on atopy, lymphocyte ratios, immune responsiveness, and systemic inflammation. It is also associated with gastroesophageal reflux, a risk factor for asthma.
The heightened effect of obesity on asthma among women, compared with men, suggests that estrogen and other hormones also may play a role, possibly through the modulation of cytokine production. The study was sponsored by the National Cancer Institute. No conflicts of interest were reported.