L-shaped association of serum 25-hydroxyvitamin D with all-cause and cardiovascular mortality in older people with chronic kidney disease: results from the NHANES database prospective cohort study

BACKGROUND: This study was conducted to assess the association of serum 25-hydroxyvitamin D [25(OH)D] concentrations with all-cause and cardiovascular disease (CVD) mortality in older people with chronic kidney disease (CKD) in the United States. METHODS: We identified 3230 CKD participants aged ≥ 60 years from the National Health and Nutrition Examination Survey (2001–2018). CKD was defined as an estimated glomerular filtration rate (eGFR) < 60 ml/min/1.73 m(2). Mortality outcomes were determined by linkage to National Death Index (NDI) records through December 31, 2019. Restricted cubic spline based on Cox regression models were utilized to elucidate the nonlinear relationship between serum 25(OH)D concentrations and mortality in patients with CKD. RESULTS: During median 74 months of follow-up, 1615 all-cause death and 580 CVD death were recorded. We found an L-shaped association between serum 25(OH)D concentrations and all-cause and CVD mortality, reaching a plateau at 90 nmol/L. Accordingly, per one-unit increment in natural log-transformed 25(OH)D was associated with a 32% and 33% reduced risk of all-cause mortality (hazard ratio [HR] 0.68; 95%CI, 0.56 to 0.83) and CV mortality (HR 0.69; 95%CI, 0.49 to 0.97) in participants with serum 25(OH)D < 90 nmol/L, but no considerable difference was observed in participants with serum 25(OH)D ≥ 90 nmol/L. Compared with those in the deficiency group (< 50 nmol/L), insufficient (50 to < 75 nmol/L) and sufficient group (≥ 75 nmol/L) were significantly associated with lower all-cause mortality (HR,0.83; 95%CI, 0.71 to 0.97 and HR, 0.75; 95%CI, 0.64 to 0.89) and CV mortality (HR,0.87; 95%CI, 0.68 to 1.10 and HR, 0.77; 95%CI, 0.59 to < 1.0), respectively. CONCLUSION: An L-shaped relationship between serum 25(OH)D levels with all-cause and CVD mortality was observed in elderly CKD patients in the United States. A 25(OH)D concentration of 90 nmol/L may be the target to reduce the risk of premature death.