Optimal form of operationalizing BMI in relation to all-cause and cause-specific mortality: the original Whitehall study.

OBJECTIVE: The shape of the association between BMI (kg/m(2)) and mortality has important methodological implications as it partially determines the optimal form for operationalizing BMI for use in analyses. We examined various BMI operationalizations in relation to mortality from all causes and specific causes. METHODS AND PROCEDURES: A clinical examination with measurements of height and weight was conducted at baseline (1967-1970) for 18,860 working men aged 40-69, in the total cohort and 7,865 men in the healthy subcohort, that is, those who had no unexplained weight loss, no cardiovascular (CVD) or respiratory disease, were nonsmokers and did not die during the first 5 years of follow-up (the original Whitehall study). A mean follow-up of 35 years for mortality gave rise to 13,498 deaths of which 4,766 were in the healthy subcohort.
RESULTS: There was a dose-response relation between BMI and CVD and coronary heart disease (CHD) mortality in the total cohort and healthy subcohort, with an increasingly steep slope at the high end of the BMI distribution. For noncardiovascular, cancer, and respiratory mortality, an excess risk was also associated with a BMI <18.5; in the healthy subcohort, this was true only for respiratory deaths. The association between BMI and all-cause mortality was J-shaped in the total cohort and healthy subgroup and even after excluding underweight participants. DISCUSSION: For associations with all-cause and cause-specific mortality, a linear and quadratic term in combination provided a more parsimonious BMI operationalization than the WHO definition, obese-nonobese dichotomy or BMI treated as a continuous linear variable.