Objective To investigate the mediating role of vitamin D in the association between exerciseand triglyceride among adolescents, as well as its potential molecular mechanisms.

Methods This prospective cross-sectional study utilized convenience sampling, enrolling 2021-grade students from Jining No. 7 Middle School on June 5, 2023. Moderate-intensity exercise frequency was assessed via standardized questionnaires, serum 25-hydroxyvitamin D levels were measured using chemiluminescence, and triglyceride levels were determined via fully automated biochemical analysis. Spearman's rank correlation analysis was employed to examine the relationships among moderate-intensity exercise, triglyceride, and vitamin D. A mediation model was constructed using the Baron & Kenny causal steps approach, adjusting for confounders including age, sex, body mass index (BMI), dairy intake, sweet food consumption, and fast-food intake. Subgroup analyses were performed based on BMI. The significance of the mediation effect was confirmed using both the Bootstrap and Sobel tests.

Results A total of 354 adolescents meeting the inclusion criteria were enrolled, including 142 females (40.11%) and 212 males (59.89%), with a median age of 13.25(12.83, 13.83)years. Spearman's analysis revealed a significant negative correlation between moderate-intensity exercise and triglyceride levels (r=-0.21, P < 0.001), a significant negative correlation between vitamin D and triglyceride (r=-0.30, P < 0.001), and a significant positive correlation between moderate-intensity exercise and vitamin D levels (r=0.26, P < 0.001). The mediation model indicated that vitamin D partially mediated the triglyceride-lowering effect of exercise (52.22%, 95% CI: -0.027 to -0.009, P < 0.001), with the effect remaining significant after confounder adjustment (47.83%, 95% CI: -0.019 to -0.005). Subgroup analysis showed that in adolescents with BMI < 24 kg/m², vitamin D accounted for 82.01% of the mediation effect between exercise and triglyceride (95% CI: -0.022 to -0.005, P=0.005), which remained significant after adjustment (81.20%, 95% CI: -0.022 to -0.005, P=0.005). In contrast, among adolescents with BMI ≥24 kg/m², the mediation effect was nonsignificant before (3.38%, 95% CI: -0.014 to 0.009, P=0.760) and after adjustment (7.51%, 95% CI: -0.019 to 0.012, P=0.621).

Conclusions Vitamin D serves as a key mediator in the triglyceride-lowering effect of exercise among adolescents, independent of age, sex, and dietary habits. This mediation effect is particularly pronounced in adolescents with BMI < 24 kg/m². The underlying mechanism may involve vitamin D-regulated lipid metabolism-related gene expression and suppression of inflammatory pathways, suggesting that targeting vitamin D signaling could be a potential molecular strategy for early intervention in adolescent dyslipidemia.