Hypothesis: Oral vitamin D supplementation causes downregulation of skin synthesis of Vitamin D, resulting in fibromyalgia and related conditions

In the 1990s in college, I had a roommate who suffered greatly from chronic fatigue syndrome. Since then I have periodically considered what might have contributed to her symptoms. I recall that she drank a lot of milk, and most US milk is supplemented (by law) with vitamin D supplements. 

The human body usually synthesizes (i.e., makes) Vitamin D in the skin, but it can also absorb it from the intestines. The principle of homeostasis indicates that when there is a high oral intake of Vitamin D, the body is consequently likely to decrease the expression and/or activity in the skin of enzymes that synthesize Vitamin D. The problem with this happening is that those same proteins and enzymes might have other important functions besides producing Vitamin D.

I have a hypothesis that the widespread intestinally-delivered supplementation with vitamin D, mostly via fortified milk and multivitamins, is depressing Vitamin-D-synthesizing processes in the skin such as to contribute to an increase in chronic fatigue syndrome ("CFS") and fibromyalgia (which often occurs along with CFS). 

Fibromyalgia is noted for pain and sensitivity all over the body, including specifically in skin tissue. 

Fibromyalgia consists of widespread pain with similarities to neuropathic pain in clinical findings, pathophysiology, and neuropharmacology. Pain is the predominant symptom and allodynia [i.e., "pain resulting from a stimulus (such as a light touch of the skin) which would not normally provoke pain"] and hyperalgesia [i.e., "increased sensitivity to pain or enhanced intensity of pain sensation"] are common signs. Extreme fatigue, impaired cognition and nonrestorative sleep difficulties coexist in addition to other somatic symptoms. Research including neuroimaging investigations shows abnormalities in neurotransmitters and an abnormal response to pain.

"Fibromyalgia." Janice E Sumpton & Dwight E Moulin, Handb Clin Neurol 2014;119:513-27. doi: 10.1016/B978-0-7020-4086-3.00033-3; online at https://pubmed.ncbi.nlm.nih.gov/24365316.

A 2010 study in England found that people were less likely to report feeling pain on days with a lot of sunshine, which supports that something involved in the skin's response to sunlight is connected to pain levels. (See "Whether the weather influences pain? Results from the EpiFunD study in North West England," found online at https://pubmed.ncbi.nlm.nih.gov/20430867.) Vitamin D is repeatedly found to be connected to fibromyalgia in ways that are unclear. (See "Vitamin D in Fibromyalgia: A Causative or Confounding Biological Interplay?" online at https://pubmed.ncbi.nlm.nih.gov/27271665.)

Further, enzymes that synthesize Vitamin D require the element magnesium. (See "Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial" online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6693398.) In a majority of studies looking at trials of magnesium for treating fibromyalgia symptoms, magnesium did significantly improve symptoms. (See "Magnesium and Fibromyalgia: A Literature Review" online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8371721.) 

I think the evidence is strong enough to investigate this possible hypothesis further. Homeostasis is an important principle in biology, and it has not received enough attention in this world of fortified foods and daily multivitamins.