Objective: To examine the effect of reverse causality and confounding on the association of BMI with all-cause and cause-specific mortality.
Research methods and procedures: Data from two large prospective studies were used. One (a community-based cohort) included 8327 women and 7017 men who resided in two Scottish towns at the time of the baseline assessment in 1972-1976; the other (an occupational cohort) included 4016 men working in the central belt of Scotland at the time of the baseline assessment in 1970-1973. Participants in both cohorts were ages 45 to 64 years at baseline; the follow-up period was 28 to 34 years.
Results: In age-adjusted analyses that did not take account of reverse causality or smoking, there was no association between being overweight (BMI 25 to <30 kg/m(2)) and mortality, and weak to modest associations between obesity (BMI > or =30 kg/m(2)) and mortality. There was a strong association between smoking and lower BMI in women and men in both cohorts (all p < 0.0001). Among never-smokers and with the first 5 years of deaths removed, overweight was associated with an increase in all-cause mortality (relative risk ranging from 1.12 to 1.38), and obesity was associated with a doubling of risk in men in both cohorts (relative risk, 2.10 and 1.96, respectively) and a 60% increase in women (relative risk, 1.56). In both never-smokers and current smokers, being overweight or obese was associated with important increases in the risk of cardiovascular disease.
Discussion: These findings demonstrate that with appropriate control for smoking and reverse causality, both overweight and obesity are associated with important increases in all-cause and cause-specific mortality, and in particular with cardiovascular disease mortality.