What does vitamin D do?

Question: What does vitamin D do?

Vitamin D is an endocrine hormone, just like testosterone or thyroid hormone. Like other hormones, vitamin D has many subtle functions. For the sake of discussion, let's break them down into two categories: skeletal and cellular . Understanding the distinctions between the two functions is critical to figuring out how to supplement yourself.

1. Skeletal Function of Vitamin D. Vitamin D regulates calcium in the body. Vitamin D actually refers to a family of compounds that are all derived from the action of UVB radiation on cholesterol. As these compounds move through your system, they are metabolized by the liver and kidneys. The resulting derivatives of vitamin D, called metabolites, are the ones that bind to the vitamin D receptor and result in transcription of hundreds of genes. The most important vitamin D metabolite is called calcitriol, or 1,25-dihydroxyvitamin D3.

Calcitriol works as a hormone, turning on genes in the intestine associated with absorption of calcium and phosphate from the diet (1). In the skeleton, bones are constantly turning over as calcium is resorbed from the bloodstream. Calcitriol is also critical for the development of specialized bone cells called osteoblasts and osteoclasts (1).

The simple answer to the question "what does vitamin D do?" is that it prevents a bone dysfunction called rickets. Rickets and calcium regulation are a problem because calcium is a poorly soluble salt. Without some complicated molecular machinery around, the calcium you get from milk and supplements will sit in your intestine and never find it's way into your bones. Vitamin D turns on a series of proteins that detect, capture, and transport calcium to where it is needed.

What does vitamin D do for your bones?

2. Cellular Effects of Vitamin D. Vitamin D receptors are actively expressed in numerous cells which have little to do with the skeleton. So it comes as little surprise that vitamin D has numerous functions beyond bone-building. But if you want to know what does vitamin D do outside of just regulating calcium in the body, the answers get complicated very quickly (1,2,3).

Reams of epidemiology data show strong correlations between low vitamin D levels and prostate, breast, and colorectal cancers, not to mention diabetes, heart disease and CNS diseases like Alzheimer's and Parkinson's. So what does vitamin D do to prevent these diseases? Well, quite a few things. The functions are largely related to cell proliferation, inflammation, and immunity.

As with calcium regulation, vitamin D is a hormone, so it works by turning on a number of genes at once. In the case of cancer, vitamin D operates on multiple levels to shift cells from a malignant state of hyperproliferation to a more normal, differentiated state like the other cells in your body (3). Vitamin D also upregulates proteins like thioredoxin reductase and selenoprotein P, which regulate the antioxidant balance in the body and ultimately help to prevent cancer.

Because antioxidant balance and oxidative stress are intimately related to inflammation, we can start to see how vitamin D becomes linked to diabetes, heart disease, and diseases of aging. Low vitamin D is associated with a weakened immune system (1). Vitamin D has recently been found to help mature cells in the immune system in order to ward off infections (3).

Vitamin D prevents a number of diseases

...and some forms of cancer

Why understanding the two functions of Vitamin D is important
Vitamin D supplementation is critical to prevention of rickets and bone dysfunction. Unfortunately, the recommendations for supplementation are based primarily on prevention of rickets and bone dysfunction, and not on the more recently uncovered roles of vitamin D in preventing cancer and other illnesses. Recommendations are not grounded in a complete picture of what does vitamin D do. Numerous studies have shown that considerably higher levels of vitamin D are required for the anti-cancer benefits than are adequate for prevention of rickets (4).

Getting the right vitamin D dose

Why athletes need to be concerned about low vitamin D

Are you at risk for vitamin D deficiency?

Or are you concerned about taking too much vitamin D?

1. Dusso, AS, Brown, AJ, Slatopolsky, E. "Vitamin D." Am J Physiol Renal Physiol. 289 (2005) F8-F28.
2. Holick, MF. "Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis." AM J Clin Nutr. 79 (2004) 362-71.
3. Lin, R, White, JH. "The pleiotropic actions of vitamin D." BioEssays 26 (2003) 21-28.
4. Vieth, R. "What is the optimal vitamin D status for health?" Progress Biophys Mol Biol. 92 (2006) 26-32.

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