Vitamin D limits inflammation in vitro and in animal models of autoimmune diseases, but it has not shown benefit in humans' observational studies or clinical trials.

Omega-3 (ω-3) fatty acid supplementation limits symptoms in some autoimmune diseases in humans. But neither of these supplements has been studied for preventing autoimmune disease in randomised trials. In the placebo-controlled VITAL study, 25,000 adults were randomised to take vitamin D (cholecalciferol; 2000 IU daily), ω-3 fatty acids (1 g daily), both, or neither for 5 years.

Investigators assessed the effects of these interventions on new-onset autoimmune disease. The difference between groups was barely statistically significant. The newly diagnosed autoimmune disease was less common in patients taking omega-3 fatty acids than in the placebo but not statistically significant. Those who took both supplements had a lower incidence of autoimmune disease.