Vitamin D deficiency is associated with higher frailty risk among middle-aged and older adults: a national population-based analysis of NHANES 2007–2018

Background Although frailty is common in older adults, the relationship between vitamin D and frailty remains unclear. Using NHANES data, this study examined the association between frailty and serum vitamin D in middle-aged and older adults.Methods Eight thousand and sixty-four participants aged 40 and older from NHANES (2007–2018) underwent cross-sectional analysis. Serum vitamin D was categorized as deficiency (<25 nmol/L), insufficiency (25–50 nmol/L), or adequate (≥50 nmol/L). Frailty was quantified using a 49-item Frailty Index (FI) based on the cumulative deficit model; frailty was defined as FI ≥ 0.21. Logistic regression and restricted cubic splines were used.Results The deficiency group had the highest frailty prevalence (55.9%), followed by insufficiency (42.9%) and adequate (37.2%). Frailty was significantly associated with deficiency (OR 2.15, 95% CI 1.42–3.25) after full adjustment, whereas insufficiency was not significantly associated with frailty (OR 1.15, 95% CI 0.95–1.38). Serum vitamin D and FI showed a U-shaped dose–response relationship, robust across subgroup and sensitivity analyses.Conclusion In middle-aged and older adults, vitamin D deficiency, but not insufficiency, is associated with higher frailty risk. Serum vitamin D may have a U-shaped association with FI. Monitoring vitamin D levels may help inform frailty management in aging populations.