Objective: To assess the relationship between body mass index (BMI) and ischaemic heart disease (IHD) mortality, especially in populations with low mean BMI levels.
Methods: We examined the data from a population-based, prospective cohort study of 220 000 Chinese men aged 40 - 79, who were enrolled in 1990 - 1991, and followed up ever since to 1/1/2006. Relative risks of the deaths from IHD by the baseline BMI were calculated, after controlling age, smoking, and the other potential confounding factors.
Results: The mean baseline BMI was 21.7 kg/m(2), and 2763 IHD deaths were recorded during the 15-year follow-up (6.8% of all deaths) program. Among men without prior vascular diseases at baseline, there was a J-shaped association between BMI and IHD mortality. When baseline BMI was above 20 kg/m(2), there was a strongly positive association of BMI with IHD risk, with each 5 kg/m(2) higher in BMI associated with 21% (95%CI: 9% - 35%, P = 0.0004) higher IHD mortality. Below this BMI range, the association appeared to be reverse, with the risk ratios as 1.00, 1.11, and 1.14, 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.
Conclusion: Lower BMI was associated with lower IHD risk among people in the so-called 'normal range' of BMI values (20 - 25 kg/m(2)). However, below that range, the association might well be reversed.