Body mass index and asthma incidence among USA adults.

The aim of this study was to examine the association between body mass index (BMI) and asthma incidence. Data from the baseline examination conducted during 1971-1975, and the first follow-up conducted during 1982-1984, of the National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study (a cohort study) was used. Asthma was self-reported or reported by proxies. BMI was calculated from measured height and weight obtained during the baseline examination. Among 9,456 participants aged 25-74 yrs who were free of asthma at baseline, 317 participants reported a diagnosis of asthma during the follow-up interview. Compared with participants with a BMI of 18.5-<25.0 kg.m(-2), the odds ratio (OR) for those with a BMI of > or =35 kg x m(-2) was 1.87 (95% confidence interval (CI) 1.12-3.13). ORs were similar for males and females. However, only 125 of the 298 participants who recalled a date of onset reported a diagnosis that occurred after their baseline examination. Among this group of participants, BMI was not significantly associated with asthma incidence (OR 1.52, 95% CI 0.62-3.77). In conclusion, although obese people reported more "incident" asthma during follow-up, it remains unclear whether this represents reactivation of previously diagnosed asthma or the onset of new cases, and whether these new cases actually represent true asthma or respiratory symptoms misdiagnosed as asthma.