About Vitamin D
How is vitamin D unique?
Among the body's steroid hormone systems, vitamin D certainly is unique. Unlike other steroids, the body cannot create the activated vitamin D it needs directly from cholesterol; all of the body's activated vitamin D must come from simple vitamin D—either made in the skin or taken orally. Besides its endocrine role in maintaining blood calcium, activated vitamin D has multiple independent hormonal functions, if enough of its precursor is available. Like all steroid hormones, activated vitamin D binds to a member of the nuclear hormone receptor superfamily where the complex then acts as a molecular switch to signal its target genes. So far, we know 2,000 genes (about 10% of the human genome) are primary targets of activated vitamin D, and the list is steadily growing. If adequate precursor is available, most organs in the human body produce their own activated vitamin D, have a vitamin D receptor, and regulate their own needs independently. Thus, they do not depend on blood supply of activated vitamin D from the kidney. Lips P. Vitamin D physiology. Prog Biophys Mol Biol. 2006 Sep;92(1):4–8. Dusso AS, Brown AJ, Slatopolsky E. Vitamin D. Am J Physiol Renal Physiol. 2005 Jul;289(1):F8–28.
The pharmacology contained in this paragraph may be a bit confusing to some, but it is vitally important in understanding vitamin D. It is so important that Professor Reinhold Vieth, of the University of Toronto, wrote an entire chapter about its implications in the most current textbook of vitamin D. Unlike any other steroid hormone, substrate (precursor) concentrations are absolutely rate-limiting for activated vitamin D production. The enzyme that first metabolizes vitamin D in the liver and the enzyme in tissue where activated vitamin D is made both operate below their respective Michaelis-Menten constants throughout the full range of their normal substrate concentrations, i.e. the reactions follow first-order, mass-action, kinetics. In English, this means the more vitamin D made in the skin or taken by mouth, the more vitamin D in your blood, and the more vitamin D in your blood, the more activated vitamin D in your brain. That is, levels of activated vitamin D during brain development directly depend on the mother's vitamin D levels, which in turn, directly depend on the amount of vitamin D the mother makes in her skin or ingests orally. That is, the rate-limiting step for the production of activated vitamin D is totally dependent on human behavior, a situation that is unique among all steroids. Brain concentrations of activated vitamin D literally depend on one's behavior—be it the step into the sun, to the supplements, into the shade, or to the sunscreen. Vieth R. The pharmacology of vitamin D, including fortification strategies. In Feldman D, Pike JW, Glorieux FH, eds. Vitamin D. San Diego: Elsevier, 2005.» page: autism index 1 2 3 4 5 6 7 8 9 10 11