Comparison of serum 25-hydroxyvitamin D levels between patients with multiple chemical sensitivity and healthy controls: A case-control study.

Vitamin D deficiency has been associated with a range of neurological and allergic conditions. Whether such an association exists in multiple chemical sensitivity (MCS) has not been clarified. This study aimed to compare serum 25-hydroxyvitamin D (25[OH]D) concentrations between patients with MCS and healthy controls. We conducted a case-control study including 80 patients with physician-diagnosed MCS and 5,518 controls. Serum 25(OH)D concentrations were compared using a general linear model with bias-corrected and accelerated bootstrap resampling (1,000 iterations), adjusting for age, sex, season of blood collection, smoking status, body mass index, alcohol intake, and physical activity. Vitamin D deficiency (< 20 ng/mL) was highly prevalent in both groups (78.8% in MCS vs. 75.3% in controls). Median serum 25(OH)D concentrations did not differ significantly between groups (14.6 vs. 15.6 ng/mL, p = 0.622). Adjusted analyses confirmed no statistically significant difference (adjusted difference = 1.07 ng/mL, 95% CI: -0.18 to 2.46, p = 0.119). Despite the high prevalence of vitamin D deficiency, patients with MCS did not differ significantly from controls in serum 25(OH)D concentrations. Although serum 25(OH)D concentrations did not differ significantly between groups, the findings do not exclude a mechanistic role of vitamin D. Local dysregulation of vitamin D receptor signaling or tissue-specific activation may contribute to neuroimmune sensitization in MCS, highlighting the need for system-level investigations.