Mortality and optimal body mass index in African-American men and women

Although the contribution of obesity to risk of specific chronic conditions has been well defined, the relationship to all-cause mortality remains controversial. Furthermore, the potential heterogeneity of this relationship across ethnic groups has been even less well studied. We examined the impact of body mass index (BMI; kg/m(2)) on mortality in black men and women, in a national representative cohort available from the NHANES-I Epidemiologic Follow-up Study (NHEFS). The principal analyses focused on 1,980 African-American participants in the NHEFS. Many studies have demonstrated a non-monotonic relationship between mortality and body mass index(BMI), with excess mortality occurring at both low and high levels. Although much discussion and many different analyses have appeared, to our knowledge no attempt has been made to quantitatively establish the BMI at which minimum mortality(BMImin) occurs or to establish confidence intervals for this BMI, accounting for the asymmetry of the relationship. We model the nonmonotonic relationship between BMI and mortality to estimate the BMI at which minimum mortality occurs. In our analyses we consider the joint relationship of age, smoking status and BMI to mortality. We present two methodologies for estimating BMImin: a logistic regression model with a transformation of BMI to accommodate asymmetry, and a change point model as suggested by Goetghebeur and Pocock(1995). We establish confidence intervals for BMImin using the delta method and bootstrap sampling for the logistic and the profile likelihood for the change point model. We also present formal tests for the heterogeneity of BMImin by smoking status, gender, and age. Furthermore, we discuss the problem of goodness of fit statistics when the relationship between the characteristic and the outcome is non-monotonic.