Get to Know the Vitamin D

You think you have enough vitamin D in your body? You’re in for a surprise. You think you know what vitamin D does for you? You’re in for a shock. Many people don’t have enough vitamin D, and almost nobody knows all that it might do for you. In fact, scientists are discovering new possible roles for it almost daily.

If vitamin D were a house, it might be the most desirable house on the block. It’s turning out to provide possible benefits for your body that you could never have imagined.

Understanding What Vitamin D Is and How It Works

When is a vitamin not a vitamin? When it’s vitamin D.

A vitamin is defined as an essential nutrient that a living being must acquire in tiny amounts from the diet. A vitamin is a chemical that’s essential for your body but that your body can’t make; it must be ingested. By this definition, vitamin D isn’t a vitamin at all. Consider this – your skin can make vitamin D when it’s exposed to sunlight, so your body doesn’t have to acquire it from food.

If vitamin D isn’t really a vitamin, what is it? It becomes a hormone called calcitriol (active vitamin D) after your body metabolizes it. A hormone is a chemical in your body that regulates your physiology.

But old names are hard to change, so even though the substance can be made in your kin and becomes a hormone, experts still call it vitamin D.

Forming vitamin D in the body

Vitamin D comes in two forms:

• Vitamin D2: The form found in plants
• Vitamin D3: the form found in animals

Both forms of vitamin D are created when the ultraviolet rays of the sun act upon a form of cholesterol. In certain plants, the ultraviolet rays convert a molecule called ergosterol into vitamin D2, which is also called ergocalciferol. In humans, vitamin D starts as a substance in the skin called 7-dehydrocholesterol. The ultraviolet B rays from the sun convert 7-dehydrocholesterol into vitamin D3, or cholecalciferol.

However, neither vitamin D nor vitamin D3 are active yet. In fact, vitamin D does nothing by itself; it’s completely inactive, and that may make you wonder what all the fuss is about. But it’s what vitamin D turns into that becomes important. Vitamin D travels through the bloodstream to the liver, where it’s turned into 25-hydroxycholecalciferol (25 (OH) D or calcidiol). This is a prohormone or precursor for the vitamin D hormone. The vitamin D prohormone travels through the bloodstream to the kidneys, where it’s turned into the active form, 1,25-dihydroxycholecalciferol (1,25 (OH)2 D3 or calcitriol). 1,25 (OH)2 D3 is the active vitamin D hormone. It is released back into the bloodstream where it then regulates how your body uses calcium and phosphorus.

Some controversy has arisen over whether vitamin D2 is as active as vitamin D3 when it’s ingested, but the consensus is that D3 is two or three times as potent in raising the level of 25-hydroxycholecalciferol.

Because the liver and the kidneys are involved in the production of calcitriol, diseases of these organs may affect your ability to make this hormone.

Although the kidneys produce most of the calcitriol that ends up in the blood, there is some evidence that the conversion of 25 (OH)D3 into 1,25(OH)2D3 may occur in other tissues in the human body. The production of calcitriol in these tissues is low in comparison to the kidney, and calcitriol made in these tissues is probably not released back into the serum. This calcitriol acts within the tissue where it’s made:

• Cells of the immune system (macrophages, dendritic cells)
• Brain
• Breast
• Colon (large intestine)
• Endothelial cells (inner lining of blood vessels)
• Pancreas
• Parathyroid glands
• Placenta
• Prostate
• Skin

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