Body mass index as predictor for asthma: a cohort study of 118,723 males and females.

The objective of the present study was to quantify the relationship between body mass index (BMI; in kilogrammes per metre squared) and asthma in middle-aged males and females, and to evaluate change in BMI as a risk factor for asthma. Asthma incidence was estimated from data on redeemed prescriptions of anti-asthmatic drugs during the period 2004-2007, retrieved from the nationwide Norwegian Prescription Database. BMI was measured during health surveys in 1994-1999 in >100,000 individuals born during 1952-1959. Change in BMI was based on self-report. Relative risks were estimated using Poisson regression. The relative risk associated with a 3-unit increase in BMI ranged from 1.14 (95% confidence interval 1.10-1.18) in current smokers to 1.27 (1.22-1.32) in never-smokers after adjusting for confounders. The relative risk associated with a 3-unit increase in BMI was 1.21 (1.16-1.26) after adjusting for confounders, including sex, smoking and BMI. Asthma incidence, as measured by anti-asthmatic drug use, was positively related to both BMI and change in BMI. For BMI, the association was stronger for never-smokers than for ex-smokers and current smokers.