Vitamin D is very important for patients. Studies have found that 22% – 65% of CFS patients are technically deficient ( less than 20 ng/mL) . While for Fibromyalgia 61%- 80% of Fibromyalgia patients are technically deficient.
In terms of results, significant improvement is seen when blood level of 25(OH) D exceeded 50 ng/ mL (125 nmol/L). What type of improvement do studies report?
- Decrease or elimination of headaches
- Decrease or elimination of hypersomnia
- Decrease or elimination of orthostatic intolerance
- Decrease or elimination of impaired memory
- Decrease or elimination of palpitation
- Decrease or elimination of mood disturbance
- Decrease or elimination of restless leg syndrome
- Treatment with high-dose vitamin D resulted in clinical improvement in all FM patients
- There are also studies finding some patients went into remission seen with 2000-10000 IU/day (with magnesium and phosphate supplements)
A level of at least 50+ ng/mL (125 nmol/L) appears to be a critical threshold for improvement. This is in the normal range, but towards the upper end.
A related vitamin D measure, that of 1,25 D, is usually not done — but evidence suggest that it should be monitor as an ongoing status for CFS, FM and other autoimmune diseaase. A 2009 study found for most autoimmune disease, this is High: 1,25-D > 110 pmol/L. The top of the normal range is about 75. It appears to be a good clinical indicator that someone has CFS/FM/IBS/chronic Lyme or other autoimmune conditions.
The higher the level, the more severe the symptoms. Getting this measure monthly may help monitor treatment.
In general, minimum supplementation should be 5000 IU or more. The older you are, the higher the dosage needs to be.
- Vitamin D3
Vitamin D and the Gut
- Vitamin D deficiency changes the intestinal microbiome reducing B vitamin production in the gut. The resulting lack of pantothenic acid adversely affects the immune system, producing a “pro-inflammatory” state associated with atherosclerosis and autoimmunity .
- Effects of high doses of vitamin D3 on mucosa-associated gut microbiome vary between regions of the human gastrointestinal tract .
Vitamin D, the gut microbiome, and the hygiene hypothesis. How does asthma begin? 
- “Finally, vitamin D is critical to the function of the gut microbiome (15).”
- Vitamin D deficiency predisposes to adherent-invasive Escherichia coli-induced barrier dysfunction and experimental colonic injury .
” Although current data support the viewpoint that the biomarker plasma 25(OH)D concentration must rise above 750 nmol/L to produce vitamin D toxicity” 
See also “Myth of Vitamin D toxicity” for studies on safety of higher dosages.