Vitamin D, More than just a Vitamin

This is part of our ongoing The Best Kept Secrets to Healthy Aging spotlight. Each day, we will be posting some of the great information that’s packed into our book, The Best Kept Secrets to Healthy Aging.

Today’s topic:
Vitamin D – More than just a Vitamin

The discovery of vitamin D early in the 20th century eventually led to several important discoveries that have changed how nutritionists look at vitamins. Most importantly, under ideal circumstances of sun exposure, the human body can make all the vitamin D it needs without help from the diet. But, the amount of vitamin D produced in the body depends on the length of time and the amount of skin exposed to ultraviolet B sunlight – too little sunlight, too little vitamin D. If sunlight exposure is inadequate, vitamin D becomes a dietary essential that must be consumed in order to satisfy the body’s needs. More recently, evidence has come to light regarding the widespread deficiency of vitamin D. While the majority of individuals deficient in this vitamin live in northern latitudes, where sun exposure is limited except in the summer, a number of studies report vitamin D-deficient populations in states and countries where sunshine is intense year-round. An important factor seems to be that the darker an individual’s skin, the longer the sun exposure necessary to produce vitamin D. Vitamin D deficiency is common in African Americans for this reason. Research also suggests that vitamin D deficiency may be common in India, a country where there is plenty of sunshine. What these facts tend to point to is sun-avoidance, even in the tropics, is contributing to this global epidemic of vitamin D deficiency. Dietary factors can impact vitamin D nutrition, as can possibly several factors that limit the conversion of vitamin D in the body to its active form.

Whether vitamin D is consumed through a fortified dairy food, is part of a dietary supplement or is synthesized in the skin in response to exposure to sunlight, these forms of vitamin D are inactive. They must undergo a 2-step activation process: step 1 occurs in the liver (converting vitamin D to 25-hydroxyvitamin D3 or 25OHD3), step 2 is performed by kidneys and multiple other organs in the body to convert 25OHD3 to the most active form of the vitamin, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Although the rate of the renal activation step is controlled by other hormones in response to fluctuations in plasma calcium concentration, at 25OHD32 levels achieved by most modern humans, both the hepatic step and the production of 1,25(OH)2D3 in multiple other organs depends directly on the amount of vitamin D produced by the skin or added to the blood through the diet. Only when 25OHD3 levels exceed 50 ng/ml, a level only achieved when adults take 5,000 IU per day or sunbathe regularly, does the metabolism of Vitamin D become similar to other such hormones, controlled with feedback to limit production.

Although called a “vitamin,” vitamin D actually is probably better characterized as a “prohormone” that becomes a full-blown hormone after the sequential activation steps. Vitamin D also differs from most other vitamins in another important respect. Typically, vitamins assist other molecules (usually enzymes) to do their jobs. In contrast, vitamin D does not participate in any other molecules’ functions. Instead, vitamin D interacts with messenger molecules to signal to a cell’s DNA that it needs to switch on or off a particular cell function.

Traditionally it has been believed that the functions controlled by vitamin D all involved the absorption of calcium and phosphorus by the intestinal tract and the regulation of the amounts of these minerals in the blood. These tasks require fully activated vitamin D (1,25(OH)2D3) to help control the rates of dietary calcium absorption, bone formation and resorption in the skeleton and excretion of calcium by the kidney.

Beyond this, when 1,25(OH)2D3 is made in tissues, it becomes an autonomous activator of more than 1,000 genes, thus vitamin D influences processes in organ systems throughout the body.1 Since the functions of vitamin D in multiple tissues are autonomous, it has as many mechanisms of actions as genes it regulates.2 These autonomous hormone actions, termed autocrine or inside the cell, extend far beyond the regulations of calcium and phosphorus levels, and these exciting functions of vitamin D are just now beginning to be untapped by researchers. As a nutrient, vitamin D has the potential to be a game-changer and, with more research, the vast reach of this health-giving essential nutrient will begin to be realized.

Tomorrow’s topic: The Sun, the Skin and Vitamin D

1.Cannell JJ, Hollis BW, Sorenson MB, Taft TN, Anderson JJ. Athletic Performance and Vitamin D. Med Sci Sports Exerc. 2009; 41(5):1102- 1110.
2. Cannell JJ, Hollis BW. Use of vitamin D in clinical practice. Altern Med Rev 2008 Mar;13(1):6-20

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