The obesity paradox is mostly driven by decreased noncardiovascular disease mortality in the oldest old in China: a 20-year prospective cohort study

National and international recommendations of healthy body mass index (BMI) are primarily based on evidence in young and middle-aged populations, with an insufficient representation of the oldest old (aged ≥80 years). Here, we report associations between BMI and mortality risk in 27,026 community-dwelling oldest old (mean age, 92.7 ± 7.5 years) in China from 1998 to 2018. Nonlinear curves showed reverse J-shaped associations of BMI with cardiovascular disease (CVD), non-CVD and all-cause mortality, with a monotonic decreased risk up to BMIs in the overweight and mild obesity range and flat hazard ratios thereafter. Compared to normal weight, overweight and obesity were significantly associated with decreased non-CVD and all-cause mortality, but not with CVD mortality. Similar associations were found for waist circumference. Our results lend support to the notion that optimal BMI in the oldest old may be around the overweight or mild obesity range and challenge the application of international and national guidelines on optimal BMI in this age group. This study shows that body mass indexes (BMIs) in the overweight or mild obesity range are associated with a decreased risk of mortality in the oldest old in China, supporting the notion that optimal BMIs are age dependent and challenging national and international guidelines on healthy BMI.