Seasonal disease activity and serum vitamin D levels in rheumatoid arthritis, ankylosing spondylitis and osteoarthritis

Background: Vitamin D is a steroid hormone that plays essential roles in calcium and phosphorus metabolism, bone formation and mineralization homeostasis, also has a role in the maintenance of immune-homeostasis.
Objective: We aimed to investigate seasonal serum vitamin D levels and seasonal disease activity in patients with Rheumatoid Arthritis, Ankylosing Spondylitis, and Osteoarthritis.
Methods: Seventy-one Rheumatoid Arthritis patients, 72 Ankylosing Spondylitis patients, 74 knee Osteoarthritis patients and 70 healthy controls were recruited for the study. Bi-seasonal measurements of serum 25(OH)D vitamin were checked in either in July or August or September for summertime and either in December or January or February for wintertime. Disease activity were evaluated by Disease Activity Score-28, Bath Ankylosing Spondylitis Disease Activity Index, and Western Ontario and McMaster Universities Osteoarthritis Index in groups of Rheumatoid Arthritis, Ankylosing Spondylitis, and Osteoarthritis respectively.
Results:We did not find any correlation between serum 25(OH)D levels and Disease Activity Score-28, Bath Ankylosing Spondylitis Disease Activity Index, and Western Ontario and McMaster Universities Osteoarthritis Index scores in winter and summer. The difference of Disease Activity Score-28 and Western Ontario and McMaster Universities Osteoarthritis Index scores between winter and summer seasons were not significant in Rheumatoid Arthritis and Osteoarthritis patients (p>0.05). The mean Bath Ankylosing Spondylitis Disease Activity Index score was significantly higher in winter than in summer (p<0.05). Consequently we did not find any correlation between variations of seasonal serum 25(OH)D and the disease activity in the patients with Rheumatoid Arthritis, Ankylosing Spondylitis, and Osteoarthritis.
Conclusions:These results suggest that vitamin D does not have an important role in the seasonal disease activity of these diseases and that seasonal changes in disease activity may play an important role in evaluating Ankylosing Spondylitis patients rather than Rheumatoid Arthritis and Osteoarthritis patients and should be taken into account when examining these patients. These conclusions need to be validated in multicenter studies with high number of patients.

Keywords
ankylosing spondylitis, disease activity, osteoarthritis, rheumatoid arthritis, season, vitamin D