Population heterogeneity in the impact of body weight on mortality.

Existing research provides inconsistent evidence for a relationship between overweight and/or obesity and mortality, and poorly studies the population heterogeneity with respect to the mortality consequence of overweight/obesity. This study investigates how overweight and/or obesity affect mortality and how these effects may vary across sociodemographic groups defined by race, sex, age, education and income by using the U.S. Third National Health and Nutrition Examination Survey (NHANES III) with linked mortality data from 1988 to 2006 (6915 respondents with 2694 deaths). Analysis from Cox proportional hazard model suggests overweight people are at lower risk of death compared to normal weight people, but this protection effect is concentrated in black men, older adults, and people in the lowest income stratum. Class I obesity does not increase mortality risk, but Class II/III obesity does and the harmful effect is concentrated in whites, young and middle adults, and people with higher education and income levels. We discuss these findings in the context of the extant literature and the long-term prospect of life expectancy in the U.S.