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Monday, May 18, 2009

The ABC's of Vitamins - Vitamin D

And so we come to Vitamin D, which had been the object of some squabbling over whether it should even be called a vitamin (the other camp voted for it to be a hormone), but alas, a vitamin it became. Vitamin D is a fat soluble vitamin like vitamin A, and it has two primary forms: ergocalciferol (D2) and cholecalciferol (D3) both of which are secosteroids.

D2 is the form found in plants and is created by the irradiation (from the sun, not a lab) of ergosterol in the plant. D3 is the form people produce through photosynthesis in the skin by activation of the sunlight on 7-dehydrocholesterol. (And you thought only plants performed photosynthesis!) Both D2 and D3 are biologically inactive and have to be converted to their hormonal (thereby active) forms through two hydroxylations to create dihydroxylated vitamin D.

Not all solar rays have the required energy to penetrate the epidermis and create vitamin D3, so there are a number of factors that affect the production. First, we need UVB rays, which we also know are related to skin cancer, so sunscreen (with a SPF as low as 8) does reduce the amount of vitamin D produced. Also, skin pigmentation plays a role; while pale ol’ me needs only about 15-30 minutes of exposure, someone who has very dark skin may need 3 hours to produce an equivalent amount of vitamin D.

Age also plays a role in the production of vitamin D, the younger you are, the more vitamin D is produced in the skin. As we age, our epidermis thins out and negatively affects our ability to photosynthesize. Luckily we can get our vitamin D from supplementation (recommended: 200 IU/day if you are under 50, 400 IU/day if you are over 50, and 600 IU/day if you are over 70). Just be aware, there is no toxicity from naturally produced vitamin D, but there is the chance of toxicity from supplements – at high levels it can cause hypercalcaemia and ultimately kidney failure. We can also get vitamin D from fortified milk, fatty fish, egg yolks and butter.

What does vitamin D do for us? It is required for the homeostasis of calcium and phosphorus, bone remodeling (takes minerals away and puts them back to create new bone), and the modulation of cell proliferation (increase in number) and differentiation (different kinds of cells). Vitamin D also plays a role in our immune system like macrophage activation (those cells that gobble up pathogens) and as an immunoregulator (your T-cell response). Perhaps best known is its relationship with calcium; they are inextricably connected, calcium absorption depends on vitamin D.

Well, we have covered all of the water soluble and half the fat soluble vitamins. Only vitamins E & K are left to tackle. I'm going to be taking a quick vacation for the Memorial weekend so look for my next post around the end of May or beginning of June. Until then, eat well & be well!

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