Nonlinear association between serum 25-hydroxyvitamin D concentrations and lung function in the United States adult population with COPD.

Vitamin D has been studied for its potential protective effects against lung function decline in patients with COPD. However, conflicting results exist, the actual effect of vitamin D on lung function remains controversial. This study aimed to determine whether serum 25-hydroxyvitamin D (25-OHD) levels are correlated with lung function in community-dwelling individuals with COPD. We used data from the National Health and Nutrition Examination Survey (NHANES) to explore the relationship between serum 25-OHD concentration and the severity of airflow obstruction. Multivariable logistic regression, stratified analysis with interaction, restricted cubic splines (RCS), and threshold effect analysis were used to investigate the association between serum 25-OHD concentration and lung function.A total of 1384 patients with COPD were analyzed. Accordingly, the association between serum 25-OHD concentrations and lung function exhibited an inverse L-shaped curve in the RCS. Threshold analysis showed that in participants with 25-OHD concentrations < 90 nmol/L, lung function decreased with decrease in serum 25-OHD concentrations, whereas no significant association was observed in those with serum 25-OHD concentrations ≥ 90 nmol/L. An inverse L-shaped relationship was observed between serum 25-OHD concentration and airflow obstruction severity in the adult population with COPD in the United States.