Association between vitamin D and fatigue in patients with rheumatoid arthritis: a cross-sectional study

OBJECTIVES: In rheumatoid arthritis (RA), fatigue is an important complaint with a significant impact on quality of life. Vitamin D has modulatory effects on cells of the immune system and may potentially affect RA disease activity and thereby RA-related fatigue. The purpose of this study was to explore associations between fatigue and vitamin D status in patients with RA. DESIGN: Hypothesis-generating cross-sectional study. SETTING: Scheduled follow-up visits at a hospital-based general rheumatology clinic. PARTICIPANTS: Patients (n=169) with established RA. PRIMARY OUTCOME MEASURES AND ANLYSES: Fatigue, assessed by the Chalder fatigue questionnaire, and serum concentrations of 25-hydroxyvitamin D (25(OH)D), assessed by liquid chromatography–tandem mass spectrometry. Associations were analysed by correlation, and multivariate linear regression with adjustments for age, sex, body mass index, RA disease activity as measured by the Disease Activity Score 28-joint count C reactive protein (DAS28-CRP), psychological distress, pain and sleep. Fatigue was also compared across four groups based on the levels of serum 25(OH)D with cut points at 30, 50 and 75 nmol/L using one-way analysis of variance. RESULTS: Two-thirds of the patients (116/169, 69%) were classified with low RA disease activity, that is, a DAS28-CRP score below 3.2. Their mean (SD) serum 25(OH)D concentration was 56.3 (21.2) nmol/L, with 77 (45.6%) having values below 50 nmol/L and 12 patients (7.1%) below 30 nmol/L. The correlation between fatigue and serum concentrations of 25(OH)D was weak and not statistically significant, r = −0.14 (95% CI: −0.29 to 0.03, p=0.08). In the multivariate model, fatigue was significantly associated with RA disease activity, psychological distress and pain, but not with serum 25(OH)D. Fatigue did not differ across groups with varying levels of serum 25(OH)D. CONCLUSION: This cross-sectional study found no evidence of association between vitamin D and fatigue in patients with RA.