Background: Increased body mass index (BMI) is known to be related to ischaemic heart disease (IHD) in populations where many are overweight (BMI>or=25 kg/m2) or obese (BMI>or=30). Substantial uncertainty remains, however, about the relationship between BMI and IHD in populations with lower BMI levels.
Methods: We examined the data from a population-based, prospective cohort study of 222,000 Chinese men aged 40-79. Relative and absolute risks of death from IHD by baseline BMI were calculated, standardized for age, smoking, and other potential confounding factors.
Results: The mean baseline BMI was 21.7 kg/m2, and 1942 IHD deaths were recorded during 10 years of follow-up (6.5% of all such deaths). Among men without prior vascular diseases at baseline, there was a J-shaped association between BMI and IHD mortality. Above 20 kg/m2 there was a positive association of BMI with risk, with each 2 kg/m2 higher in usual BMI associated with 12% (95% CI 6-19%, 2P=0.0001) higher IHD mortality. Below this BMI range, however, the association appeared to be reversed, with risk ratios of 1.00, 1.09, and 1.15, respectively, for men with BMI 20-21.9, 18-19.9, and <18 kg/m2. The excess IHD risk observed at low BMI levels persisted after restricting analysis to never smokers or excluding the first 3 years of follow-up, and became about twice as great after allowing for blood pressure.
Conclusions: Lower BMI is associated with lower IHD risk among people in the so-called normal range of BMI values (20-25 kg/m2), but below that range the association may well be reversed.