Body mass index and mortality: the influence of physical activity and smoking

To study the association between body mass index (BMI) and mortality, and to evaluate the effect of physical activity during leisure time and smoking on this association in a general male population.

During 1974-1978, all men aged 35-49 yr living in three Norwegian counties were invited to a cardiovascular screening, and 87.1% attended and had their weight and height measured. Men with recognized cardiovascular diseases, diabetes mellitus, or cancer at screening were excluded. The cohort (N = 22,304) was followed for an average of 16.3 yr with respect to total and cause-specific mortality.

During follow-up, 1909 men died. We found a J-shaped association between BMI and total mortality, and the form of association was similar for death from cardiovascular diseases. Although not statistically significant, a J-shaped association was also suggested in never-smokers. Irrespective of BMI level, ex- and never-smokers had lower mortality than current smokers. Obese smoking men had a relative risk of dying of 2.01 (95% CI: 1.29-3.11) compared with obese never-smokers, and a relative risk of 4.55 (95% CI: 3.34-6.20) compared with normal weight never-smokers (BMI 22-24.9 kg x m(-2)). Within each category of physical activity during leisure time, obese men had a similar increased relative risk of death compared with normal-weight individuals. However, the U- to J-shaped association between BMI and mortality seemed to disappear by increasing level of physical activity, but this finding was not significant.

This study suggests a J-shaped association between BMI and total mortality, also when stratified on smoking habits and physical activity. The suggested linear trend in the most physical active men needs to be reassessed.