The causes of the worldwide increase in asthma seen during the last decades remain largely unexplained, but lifestyle and diet are suggested to play important roles. In this follow up of a large-scale population sample in Sweden, we wanted to identify modifiable risk factors for the cumulative incidence over a 13-year follow-up period. In 1990, a self-administered questionnaire was completed by 12,560 individuals from three age groups (16, 30-39 and 60-69 years of age) in two counties of Sweden. In 2003, the eligible subjects (n = 11,282) were sent a new postal questionnaire. In total 8150 (response rate 73%) answered the questionnaire. The prevalence of asthma in 2003 had increased in all ages. In the young adults, the asthma prevalence rose from 11.3% in 1990 to 25.0% in 2003. Adult asthma onset was identified in 791 of the participants. Smoking [RR (95% CI) = 1.37 (1.12-1.68)], BMI [1.49 (1.25-1.77 per inter quartile range)], and nocturnal gastro-oesophageal reflux (GOR) [2.16 (1.72-2.72)] were significant independent risk factors for the cumulative incidence of asthma. The impact of risk factors differed between the age groups where BMI and GOR had a significantly higher impact in the middle aged and the elderly (p < 0.05). High consumption of fruit and fish was protective especially in the elderly [0.52 (0.35-0.77)]. No significant difference was found in the impact of risk factors between men and women. Weight loss, smoking cessation and a diet rich in fruit and fish may be of importance in preventing onset of adult asthma.
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