A masking effect of vitamin D on ASD symptom severity: mediation by developmental level and altered predictive features in vitamin D insufficiency.

BACKGROUND: Symptom severity in children with autism spectrum disorder (ASD) correlates with developmental level. Vitamin D is crucial to developmental level. Studies show inconsistent findings on serum 25(OH)D and ASD symptom severity. This study explores their association and the mediating role of developmental level. METHODS: A total of 699 children aged 24-71 months were enrolled. Serum 25(OH)D levels were measured and children were categorized into normal vitamin D (NVD) and vitamin D insufficiency (VDI) groups. Symptom severity and developmental level were assessed using the Childhood Autism Rating Scale (CARS) and the Gesell Developmental Schedule (GDS). Structural equation modeling (SEM) was used to analyze mediating effects, and machine learning algorithms were employed to assess the predictive importance of developmental dimensions. RESULTS: In the total sample, SEM revealed a masking effect: the indirect effect of serum 25(OH)D levels on CARS scores via developmental level was negative, whereas the direct effect was positive, offsetting each other and resulting in a non-significant total effect. The mediating effect was attenuated in the VDI group. Machine learning analysis further revealed that language dominates in the NVD group, whereas in the VDI group, the dominance of language is diminished, the predictive importance of personal-social also declines, and fine motor and gross motor show enhanced predictive importance. CONCLUSION: Vitamin D insufficiency may weaken the predictive importance of language and personal-social while enhancing motor domains. Sufficient vitamin D was associated with better developmental levels and lower symptom severity, highlighting the potential research relevance of nutritional considerations in studies of ASD symptom severity.