Serum 25-hydroxyvitamin D, frailty, and mortality among the Chinese oldest old: Results from the CLHLS study

Background and aims: In this study, the aim is to explore whether frailty status modified the associations of serum 25(OH)D levels with all-cause and cause-specific mortality in the oldest old Chinese population. Methods and results: A total of 1411 participants aged at least 80 years were enrolled in the Chinese Longitudinal Healthy Longevity Survey (CLHLS). Information on serum 25(OH)D level, frailty status, and covariates were examined at baseline. All-cause and cause-specific mortality status were ascertained during the follow-up survey conducted in 2017–2018 by using the ICD-10 codes. Cox proportional hazard models with stratified analyses were performed to evaluate potential associations. Over a median follow-up of 3.2 years, 722 (51.2%) participants were deceased, including 202 deaths due to circulatory diseases, and 520 deaths due to noncirculatory causes. After multivariable adjustment, the lowest quartile of serum 25(OH)D levels (Hazard Ratios (95% Confidence Intervals), 1.85 (1.45–2.36), 1.85 (1.45–2.36), 1.73 (1.31–2.29), respectively) and frailty (Odd Ratios (95% Confidence Intervals), 1.91 (1.60–2.29), 2.67 (1.90–3.74), 1.64 (1.31–2.05)) were associated with significantly higher risk of all-cause mortality, circulatory mortality, and noncirculatory mortality, respectively. In addition, we observed significant interactions among 25(OH)D and frailty on the risk of all-cause and cause-specific mortality (all P-interaction < 0.001). Similar results were found in sensitivity analyses by excluding participants who died in the first year of follow-up and using clinical cutoffs of serum 25(OH)D levels. Conclusion: Low serum 25(OH)D levels were associated with higher risk of all-cause and cause-specific mortality among the oldest old of the Chinese population, and the associations were significantly stronger in individuals with frailty.