Vitamin D

Vitamin D is a fat-soluble steroid hormone responsible for the intestinal absorption of calcium, iron, magnesium, phosphate and zinc. In human, dietary Vitamin D is negligible; most comes from exposure of skin to sunlight or from supplementation.

Immune function
Vitamin D improves regulatory T cell function in healthy adults and in patients with relapsing remitting multiple sclerosis, suggesting that it may play a role in both preventing and ameliorating autoimmune disease.

Chronic Fatigue Syndrome
A retrospective study found patients with CFS have lower levels of Vitamin D than the general population. However, if it was uncertain whether this is correlated with the amount of time CFS patients spend indoors or with some other factor.

Raising Vitamin D levels has reportedly resulted in remission for some CFS patients.

Recent research has found no toxicity at 10,000 IU per day and levels as high as 15,000 IU/day are probably tolerated. . At this dosage it will take 75-90 before symptom improvement is expected to be seen. Some people have seen some symptoms improved within days, but those are likely a small minority.

Fibromyalgia
A cohort of Saudi women with fibromyalgia was found to be Vitamin D deficient. Vitamin D status was inversely correlated with pain. High dose Vitamin D supplementation resulted in improvement or resolution of symptoms. A second study also found improvement of symptoms with Vitamin D supplementation.

Autoimmune disease
Low Vitamin D is a possible risk factor for asthma, SLE , multiple sclerosis, rheumatoid arthritis, type 1 diabetes, inflammatory bowel disease and autoimmune thyroid disease. Active Crohn's disease was associated with lower Vitamin D levels.

Infectious disease
Vitamin D deficiency increases the risk of viral infections.

Vitamin D receptor (VDR).
An Epstein barr virus protein EBNA-3 has an affinity for VDR and may actually block the activation of VDR-dependent genes.