This was a huge (25,000 participants) and long (5 years) study. For those experiencing Rheumatoid Arthritis, the numbers were even more interesting – with a 40% lower rate of incidence. The study supplement with 2,000 IU of D3 daily.
"Vitamin D supplementation for five years, with or without omega 3 fatty acids, reduced autoimmune disease by 22%, while omega 3 fatty acid supplementation with or without vitamin D reduced the autoimmune disease rate by 15% (not statistically significant). Both treatment arms showed larger effects than the reference arm (vitamin D placebo and omega 3 fatty acid placebo)."
Source: The BMJ