How Vitamin D Works
- Like most of vitamin D’s biological effects, its influence on immune cells is mediated by an intra-nuclear transcription factor known as the vitamin D receptor (VDR), which has been identified in nearly every tissue in the human body. When the active form of vitamin D enters the nucleus of a cell and binds to the VDR, it initiates a cascade of events that culminates in the transcription, or “reading,” of specific genes. (The products of gene transcription are the proteins, enzymes, and hormones that are necessary for life; the genes activated by VDRs modulate a vast array of physiologic functions, many of which haven’t been fully defined.)
- A substantial body of evidence indicates that vitamin D stimulates innate immune responses, which serve as our front line of defense against novel or unfamiliar antigens. However, in most respects, vitamin D’s actions within the immune system are inhibitory. By suppressing specific facets of immune activity and preventing runaway inflammation, vitamin D prevents the inappropriate responses that lead to autoimmunity.
- It’s interesting to note that as latitude increases, so does the prevalence of many autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, and type 1 diabetes. These conditions are more common in temperate and northern regions than in equatorial countries, suggesting a link between lower sunlight exposure (hence, decreased endogenous production of vitamin D) and autoimmunity.
- Some scientists contend that other factors could explain the higher frequency of autoimmune disease in northern climates. However, animal research and human epidemiologic studies demonstrate that vitamin D probably plays a pivotal role in preventing the immune dysfunction that triggers autoimmune tissue damage.
- Cancer cells possess two traits that characterize their unusual and, ultimately, pathologic behavior: rapid growth (proliferation) and poor differentiation (i.e., each successive generation of cancerous cells looks less like its parents). Like most other tissues, many cancers contain vitamin D receptors; through its actions on VDRs, vitamin D promotes cellular differentiation and inhibits proliferation of malignant tumors.
- Epidemiologic studies suggest that vitamin D, in adequate doses, could play an important role in preventing and/or treating cancers of the colon, breast and prostate. Malignant melanomas and certain bone cancers may also respond favorably to high-dose vitamin D. While epidemiologic studies cannot prove that higher vitamin D intake prevents cancer, the evidence is certainly intriguing.
Not surprisingly, many experts believe the Food and Nutrition Board’s guidelines are too conservative. Vitamin D toxicity is very unlikely to occur at daily doses below 10,000 IU, and much higher doses are employed for treating vitamin D deficiency states or managing autoimmune disorders. It’s possible that the Food and Nutrition Board’s tolerable upper limit for vitamin D intake could prove to be the optimal dosage for most people.
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