Obesity is a risk factor for adult-onset asthma, but the contribution of excess abdominal fat to this relation is unclear. Investigators for the California Teachers Study administered questionnaires to a cohort of 133,480 female school teachers and administrators. Information was obtained about current asthma-related symptoms as well as height and weight at age 18 and at study entry. Participants measured and self-reported their waist circumference.

Of 88,300 eligible participants, 13% were obese (BMI >30.0 kg/m²). Prevalence of asthma was 7.6% overall, 11% in obese class I women (BMI, 30.0–34.9), 13% in obese class II women (BMI, 35.0–39.9), and 18% in extremely obese women (BMI >40.0). Overweight women (BMI, 25.0–29.9) had 40% higher risk for asthma than normal-weight women (BMI, 18.0–24.9), whereas extremely obese women had more than threefold higher risk (odds ratio, 3.3). Analysis adjusted for smoking status, age, race, and ethnicity showed that women who reported waist measurements >88.0 cm (35.2 inches) compared with those who had waist circumferences 88.0 cm had higher risk for asthma regardless of whether they were normal weight (OR, 1.4), overweight (OR, 1.7), or obese (OR, 2.4).

Comment: These authors did not control for potential confounders such as other comorbid conditions, family histories of asthma, or environmental exposure to allergens. Nonetheless, their findings show that excess weight alone is not a key determinant of asthma risk; where fat is deposited is also crucial. Visceral fat differs metabolically from other body fat stores in its association with inflammatory and prothrombotic states (e.g., insulin resistance, cardiovascular disease). Thus, although higher BMI generally was associated with excess risk for asthma — an inflammatory condition of the airways — women who had large waist measurements were at even greater risk. With 66% of the U.S. population now overweight or obese, these results suggest a reason for the rising prevalence of asthma: the expanding American waistline.

— Wendy S. Biggs, MD

Published in Journal Watch Women's Health September 24, 2009