Association of serum 25-hydroxyvitamin D with C-reactive protein across body mass index categories in adults

Background: Vitamin D insufficiency and systemic inflammation frequently coexist with excess adiposity, but their relationship across the full body mass index (BMI) spectrum remains incompletely characterized.
Aim: To evaluate serum 25-hydroxyvitamin D [25(OH)D] and C-reactive protein (CRP) across BMI categories and determine whether CRP is independently associated with 25(OH)D.
Methods: This cross-sectional analysis included 200 adults allocated to five BMI groups (normal weight, overweight, obesity class I, obesity class II, and obesity class III; n = 40 per group). Group differences were tested with Kruskal– Wallis analysis and post hoc multiple comparisons. Associations between CRP and 25(OH)D were assessed using Spearman correlation in the total sample and BMI strata. A multivariable linear regression model adjusted for age, sex, smoking, BMI, triglycerides, high- and low-density lipoprotein cholesterol, glycated hemoglobin, and insulin resistance.
Results: Serum 25(OH)D decreased markedly across BMI categories (H = 139.74, p < 0.001), whereas CRP increased (H = 122.68, p < 0.001). The pooled association between CRP and 25(OH)D was strong and inverse (ρ = −0.68, p <0.001). The association remained significant in normal-weight participants (ρ = −0.78, p < 0.001), participants with overweight (ρ = −0.50, p < 0.001), and the combined obesity group (ρ = −0.24, p = 0.008). The multivariable model was significant (R² = 0.528; adjusted R² = 0.502; p < 0.001), and CRP remained independently associated with lower 25(OH)D (B = −0.483, standardized ρ = −0.211, p < 0.001).
Conclusions: Higher BMI was accompanied by lower 25(OH)D and higher CRP. The inverse CRP–25(OH)D relationship persisted after adjustment for demographic and metabolic covariates, although the cross-sectional design does not establish causality.
Keywords: obesity; inflammation; adiposity; cardiometabolic risk; hypovitaminosis D