Serum vitamin D levels and olfactory function in adults: a cross-sectional study with dose-response and ROC analysis

The purpose of this study was to examine the relationship between serum 25(OH)D levels and olfactory function in adults, given the emerging recognition of vitamin D as a neurosteroid with potential influence on sensory pathways. Specifically, the study aimed to determine whether vitamin D status is associated with olfactory threshold and identification performance, to explore the presence of a biologically plausible dose-response pattern, and to assess the predictive value of serum vitamin D levels for olfactory dysfunction.

In this cross-sectional study, 161 adults were enrolled, including 98 with vitamin D deficiency and 63 with sufficient levels. Serum 25(OH)D was measured by HPLC and categorized into four groups (< 10, 10-20, 20-30, ≥ 30 ng/mL). Olfactory threshold and identification were assessed using the Sniffin' Sticks rapid screening kit. Group comparisons, subgroup analyses, correlation tests, multiple linear regression, and ROC analysis (threshold < 7) were performed.

Vitamin D-deficient participants had significantly lower threshold and identification scores than controls (p = 0.008 and p < 0.001). Both scores increased stepwise across vitamin D subgroups (p < 0.001). Serum 25(OH)D correlated positively with threshold (r = 0.302) and identification (r = 0.432) (both p < 0.001); in the deficiency group, the correlation persisted only for threshold. Vitamin D was an independent predictor of identification (β = 0.041; p < 0.001). ROC analysis indicated that severe vitamin D deficiency (cutoff ≈ 10 ng/mL) was strongly associated with threshold-level olfactory impairment (AUC 0.636; specificity 95%).

Serum vitamin D level is associated with olfactory function, particularly odor identification, and shows a biologically plausible dose-response pattern. Vitamin D appears to be a specific risk indicator rather than a standalone diagnostic marker for olfactory dysfunction.