Body mass index and mortality from ischaemic heart disease in a lean population: 10 year prospective study of 220 000 adult men

Background Increased body mass index (BMI) is known to be related to ischaemic heart disease (IHD) in populations where many are overweight (BMI >= 25 kg/m(2)) or obese (BMI >= 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/m(2), 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/m(2) there was a positive association of BMI with risk, with each 2 kg/m(2) 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/m(2). 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/m(2)), but below that range the association may well be reversed.