Circannual vitamin d serum levels and disease activity in rheumatoid arthritis: Northern versus Southern Europe.

Significantly lower 25(OH)D serum levels were observed in RA patients from North versus South Europe with a circannual rhythm in winter and summer time. In addition, 25(OH)D values showed a significant correlation (negative) with RA clinical status (DAS28) in both North and South European RA patients, suggesting possible effects of vitamin D among other factors on disease activity.