Body mass index at various ages and mortality in Chinese women: impact of potential methodological biases

Background: Inadequate control for potential methodological biases has been suggested as an explanation for the conflicting findings concerning the relationship between body mass index (BMI) and mortality. Methods: Using data from the Shanghai Women's Health Study, a prospective cohort study conducted in a relatively lean population, we examined the associations of BMI at various ages and average adult BMI with mortality and evaluated the impact of potential biases related to preexisting illness and cigarette smoking on the associations. Included in the analysis were 74 896 women aged 40-70 years with anthropometrics taken by trained interviewers at baseline (1996-2000). Recalled body sizes at ages 20 and 50 years were also obtained for older women. Participants were followed through April 2007 by biennial home visits and linkage with the vital statistics registry. Results: During a mean follow-up of 7.4 years, 2389 deaths occurred. In initial analyses, both low and high levels of baseline BMI were associated with an increase in mortality, whereas mortality increased monotonically with increasing levels of average adult BMI, BMI at the age of 50 years and, to a lesser extent, BMI at the age of 20 years. A direct monotonic relationship between baseline BMI and mortality emerged after accounting for potential biases. Controlling for potential biases also strengthened the positive associations of average adult BMI and BMI at ages 20 and 50 years with mortality, with a hazard ratio comparing the highest vs the lowest quartiles of average adult BMI reaching 2.19 (95% CI, 1.67-2.88). Conclusion: High BMI during adulthood consistently predicts mortality risk after accounting for potential biases.