Both dietary and supplemental vitamin D reduce the risk of RA, which is an autoimmune disease—a disease in which the body’s immune system attacks its own tissue. In a study of 29,000 women, those who ranked in the top third of vitamin D consumption had one-third less risk of RA. It is likely that a greater vitamin D intake would have produced much better results, since it is virtually impossible to ingest sufficient vitamin D from food and multivitamins.
In studies performed on mice, vitamin D was shown to inhibit the progression of rheumatoid arthritis and minimize or prevent symptoms. The same is true in humans. In subjects diagnosed with a form of the disease known as inflammatory arthritis, the lower the vitamin D levels are, the higher is the disease activity. Vitamin D's anti-inflammatory properties and its ability to reduce the autoimmune response are likely responsible for the improvement in RA.
Investigations also find that RA is more common in winter, consistent with the idea that vitamin D is a major factor in reducing the risk. In a report from researchers in Ireland, it was shown that 70% of patients had low vitamin D levels and that 26% were severely deficient. However, in that report, 21 ng/ml was considered as the deficiency level and 10 as the severe deficiency level. A level of 21 is dangerously deficient. The ideal level of vitamin D is 50-60 ng/ml. Using those numbers, it is likely that all of these patients ranged between deficient and severely deficient.
In our health institute/resort, we observed that guests with arthritis often regained full range of motion in their joints from a week to a month after beginning a program. I assumed that our anti-inflammatory vegetarian nutrition was responsible for the positive results. Now I realize that many of the benefits came from sunlight exposure during outdoor exercise.
RA prevention and relief are two more reasons to obtain regular, non-burning sunlight exposure. Remember that sunscreens can prevent 99% of vitamin D production by the skin.
 Merlino, L. et al. Vitamin D intake is inversely associated with rheumatoid arthritis: Results from the Iowa Women’s Health Study. Arthritis & Rheumatism 2004;50:72-77.
 Cantorna, M. et al. 1,25-Dihydroxycholecalciferol inhibits the progression of arthritis in murine models of human arthritis. J Nutr1998;128:68-72.
 Patel, S. et al. Serum vitamin D metabolite levels may be inversely associated with current disease activity in patients with early inflammatory polyarthritis. Arthritis Rheum 2007;56;2143-49.
 Cutolo, M. et al. Vitamin D in rheumatoid arthritis. Autoimmune Rev 2007;7:59-64.
 Cutolo, M. et al. Circannual vitamin D serum levels and disease activity in rheumatoid arthritis: Northern versus Southern Europe. Clin Exp Rheumatol 2006;24:702-4.
 Haroon, M. Report to European Union League Against Rheumatism , June 13, 2008.