Overall and abdominal obesity and risks of all-cause and cause-specific mortality in Korean adults: a pooled analysis of three population-based prospective cohorts

Background: Studies found a J-shaped association between body mass index (BMI) and mortality. However, it is unclear whether the association is driven by biases, particularly confounding by fat-free mass. Methods: We conducted an individual-level pooled analysis of three cohorts of Korean adults (aged ≥ 40 years; n ¼ 153 248). Mortality was followed up through December 2019. Anthropometric data were directly measured at baseline. Fat and fat-free mass were predicted using validated prediction models. Using Cox proportional hazards models, we estimated the associations of BMI and waist circumference (WC) with all-cause and cause-specific mortality. To account for biases, we excluded participants aged ≥ 70 years, deaths that occurred within 5 years of follow-up and ever smokers, and adjusted for fat-free mass index (FFMI). Results: During the follow-up of up to 18 years, 6061 deaths were identified. We observed J-shaped association of BMI (nadir at 22–26) and monotonically positive association of WC with all-cause, cardiovascular, and cancer mortality among Korean adults without a history of cancer or cardiovascular disease. In the BMI analysis, excluding ever smokers and adjusting for FFMI attenuated the excess mortality in underweight participants and transformed the J-shaped association into a monotonically positive shape, suggesting an increased mortality at BMI > 22.0. Excluding participants aged ≥ 70 years and deaths that occurred within 5 years of follow-up did not change the results. In the WC analysis, the monotonic positive associations did not change after the control. Similar results were observed among participants with a history of cancer or cardiovascular disease. Conclusions: Our data suggest that both overall and abdominal body fat are associated with increased mortality in Korean adults.