Association between serum 25-hydroxyvitamin D levels and pulmonary function in elderly patients with chronic kidney disease: correlation analysis and exploratory predictive modelling.

Background: Vitamin D has emerged as a significant indicator influencing pulmonary function, yet the complexity of how vitamin D affects the pulmonary function among patients with chronic kidney disease (CKD) remains incompletely elucidated. To preliminarily explore this issue, we observed the relationship between serum 25-hydroxyvitamin D [25(OH)D] levels and pulmonary function indicators in CKD patients, and initially explored the predictive model for pulmonary function abnormalities. Methods: This single-center cross-sectional study enrolled 90 CKD patients hospitalized in the Department of Nephrology, Tianjin Nankai Hospital, from July 2025 to November 2025. Serum 25(OH)D levels were measured, and pulmonary function testing was performed. Main indices included forced expiratory volume in 1 second (FEV1), percent predicted FEV1 (FEV1%), FEV1/FVC, and small-airway indices (FEF25/50/75). Diffusing capacity for carbon monoxide percent predicted (DLCO%) and diffusion impairment grade were also evaluated. Spearman correlation was used to assess associations between 25(OH)D and the above indices; multivariable linear regression was used to adjust for potential confounders. Random-forest classification models were further developed for exploratory prediction of pulmonary function abnormalities and to evaluate model performance. Results: Serum 25(OH)D levels were positively correlated with multiple pulmonary function indices, particularly predicted FEV1% (r = 0.463, P < 0.001) and DLCO% (r = 0.611, P < 0.001). Serum 25(OH)D was negatively correlated with diffusion impairment grade (r = -0.585, P < 0.001), indicating that higher 25(OH)D levels were associated with less severe diffusion impairment. In random-forest analyses, both models for predicting ventilatory impairment and diffusion impairment demonstrated certain specificity; sensitivity was higher for the FEV1% model, but overall specificity was limited, suggesting room for improvement when predicting pulmonary function abnormalities based solely on variables included in this study. Conclusions: Vitamin D deficiency was common in CKD patients, and serum 25(OH)D levels were independently associated with multiple pulmonary function indices, especially diffusion-related indices. As a cross-sectional study, causality cannot be inferred. Whether vitamin D supplementation can improve pulmonary outcomes requires further validation in prospective cohorts and randomized controlled trials.