Nonlinear association between serum vitamin D levels and asthma in U.S. adolescents ‒ Evidence from NHANES 2013-2018

Background: while vitamin D has known immunomodulatory effects, its relationship with asthma remains inconclusive. To investigate the association between serum 25-hydroxyvitamin D [25(OH)D] levels and asthma risk in U.S. adolescents using nationally representative data, with emphasis on nonlinear dose-response relationships. Methods: data were analyzed from 2,793 adolescents aged 12-18 years participating in NHANES 2013-2018. Asthma status was determined via self-report, and serum 25(OH)D was measured using LC-MS/MS. Multivariable logistic regression and restricted cubic spline (RCS) models were applied to evaluate linear and nonlinear associations, adjusting for relevant demographic, behavioral, and dietary covariates. Results: no significant linear association was found between categorized vitamin D status and asthma. However, RCS analysis revealed a statistically significant U-shaped association between continuous serum 25(OH)D levels and asthma risk (p for nonlinearity = 0.019). The lowest predicted odds of asthma occurred at approximately 45.3 nmol/L, with increased risk observed at both lower and higher vitamin D levels. Key risk factors for asthma included family history (OR = 3.51), low birth weight (OR = 1.78), and reduced dietary vitamin D intake (OR per µg = 0.97). Conclusion: a U-shaped relationship was observed between serum vitamin D levels and asthma risk in adolescents, suggesting that both deficiency and excess may be associated with higher susceptibility. These findings underscore the need for individualized, balanced vitamin D strategies and nonlinear modeling approaches in asthma research and public health policy.