Impact of smoking and preexisting illness on estimates of the fractions of deaths associated with underweight, overweight, and obesity in the US population.

Studies of body weight and mortality sometimes exclude participants who have ever smoked or who may have had preexisting illness at baseline. This exclusionary approach was applied to data from the National Health and Nutrition Examination Surveys to investigate the potential effects of smoking and preexisting illness on estimates of the attributable fractions of US deaths in 2000 that were associated with different levels of body mass index (BMI; weight (kg)/height (m2). Synthetic estimates were calculated by using postexclusion relative risks for BMI categories in place of BMI relative risks from the full sample, holding the relative risks for all other covariates constant. When the postexclusion relative risks were used, the attributable fractions of deaths associated with underweight and with higher levels of obesity increased slightly and the attributable fractions of deaths associated with overweight and with grade 1 obesity decreased slightly. The relative risks for BMI categories did not show large or systematic changes after simultaneous exclusion of ever smokers, persons with a history of cancer or cardiovascular disease, and persons who died early in the follow-up period or had their heights and weights measured at older ages. These analyses suggest that residual confounding by smoking or preexisting illness had little effect on previous estimates of attributable fractions from nationally representative data with measured heights and weights.