Vitamin D (ergocalciferol, or vitamin D₂) has been investigated for its role in immune system modulation and, by extension, its potential impact on autoimmune disorders. There is scientific interest and some supportive evidence from epidemiological, mechanistic, and early interventional studies suggesting that vitamin D deficiency may be associated with an increased risk or severity of various autoimmune diseases, including multiple sclerosis, type 1 diabetes, and rheumatoid arthritis. Vitamin D influences both innate and adaptive immune responses; for example, it can modulate the activity of T cells and dendritic cells, potentially reducing inappropriate immune activation linked to autoimmunity.

However, while observational studies often show a correlation between lower vitamin D levels and higher prevalence or severity of autoimmune diseases, causation remains uncertain. Randomized controlled trials supplementing vitamin D in patients with autoimmune diseases have yielded mixed results, with some improvements noted in disease markers or relapse rates, but many studies are limited by small sample sizes or methodological variability. Current clinical guidelines may recommend vitamin D supplementation in autoimmune patients who are deficient, primarily to support general health, but not as a definitive treatment for the autoimmune condition itself.

In summary, there is scientific rationale and some supportive evidence for vitamin D’s use in autoimmune disorders, but the strength of evidence is moderate (score: 3/5), and large, definitive clinical trials are still needed to establish its efficacy as a primary treatment.