Functional dentition is a modifier of the association between vitamin D and the frailty index among Chinese older adults: a population-based longitudinal study

BACKGROUNDS: Although vitamin D and dentition status are each associated with frailty, their combined effects on frailty have not been studied. This study aimed to evaluate the combined effects of vitamin D and dentition status on frailty in old Chinese adults. METHODS: Baseline data were obtained from the 2011–2012 wave of the Chinese Longitudinal Healthy Longevity Survey. A total of 1074 participants ≥65 years who were non-frail or prefrail at baseline were included; follow-up was conducted in the 2014 wave. Frailty was assessed by a 40-item frailty index (FI) and classified into frail (FI > 0.21), prefrail (FI: 0.1–0.21), and non-frail (FI ≤0.1). Vitamin D was assessed by 25-Hydroxyvitamin D (25(OH)D) and categorized into quartiles and dichotomies (normal: ≥50 nmol/L vs. low: < 50 nmol/L). The presence of ≥20 natural teeth was defined as functional dentition, otherwise as non-functional dentition. We used bivariate logistic regression and restricted cubic splines to examine the association between vitamin D, dentition status, and frailty. We created a multiplicative interaction between vitamin D and dentition status to test for their combined effect. RESULTS: A total of 205 (19.1%) incident frailty were identified during the 3-year follow-up. Participants with the lowest quartile of plasma 25(OH) D were more likely to be frail (odds ratio [OR] 2.45, 95% confidence interval [CI]: 1.38 to 4.35) than those in the highest quartile. Older adults with the lowest quartile of 25(OH) D and non-functional dentition had the highest odds of frailty (OR = 3.67, 95% CI: 1.02 to 13.12). We also observed that a lower vitamin D level was associated with an increased risk of frailty with a threshold of 40.37 nmol/L using restricted cubic spline models. However, vitamin D levels were not significantly associated with frailty among participants with functional dentition. CONCLUSIONS: Low vitamin D levels were associated with an increased risk of frailty in older adults. Functional dentition modified the association of vitamin D with frailty. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12877-022-02857-3.