A Semiparametric Analysis of the Relationship of Body Mass Index to Mortality

I used a semi-parametric analysis of the relationship between body mass index (BMI) and mortality to assess the adequacy of conventional BMI categories for planning public health programs to reduce mortality. I linked supplements from the 1987 and 1989 versions of the National Health Interview Survey to the 1995 Multiple Cause of Death File to obtain mortality information. I constructed nonlinear estimates of the association between BMI and mortality using a semiparametric regression technique. The mortality risk among "normal" weight men (i.e., those in the BMI range of 20 to 25 kg/m(2)) was as high as that among men in the mild obesity category (BMIs of 30-35 kg/m(2)), with a minimum risk observed at a BMI of approximately 26 kg/m(2). Among women, the mortality risk was smallest at approximately 23 to 24 kg/m(2), with the risk increasing steadily with BMIs above 27 kg/m(2). In each specification, the slope of the line was small and volatile through the BMI range of 20 to 35 kg/m(2), suggesting negligible risk differences with minor differences in weight for much of the population. Traditional BMI categories do not conform well to the complexities of the BMI-mortality relationship. In concurrence with conclusions from previous literature, I found that the current definitions of obesity and overweight are imprecise predictors of mortality risk.