Vitaimn D -Cholecalciferol

Technically not a "vitamin," vitamin D is in a class by itself. Its metabolic product, calcitriol, is actually a secosteroid hormone that targets over 2000 genes (about 10% of the human genome) in the human body. Current research has implicated vitamin D deficiency as a major factor in the pathology of at least 17 varieties of cancer as well as heart disease, stroke, hypertension, autoimmune diseases, diabetes, depression, chronic pain, osteoarthritis, osteoporosis, muscle weakness, muscle wasting, birth defects, periodontal disease, and more.
Vitamin D's influence on key biological functions vital to one's health and well-being mandates that vitamin D no longer be ignored by the health care industry nor by individuals striving to achieve and maintain a greater state of health.

Cholecalciferol is formed in the skin when ultraviolet light of the correct wavelength, UVB, strikes bare skin. Enormous quantities of cholecalciferol are rapidly made in the skin if: the sun is high in the sky (midday and the summer season), your skin is not covered by clothes or sunblock, you stay in the sun until your skin just begins to turn pink (not red), you are not behind glass. Glass blocks virtually all UVB, preventing vitamin D from being made.
Studies show that if you go out in the summer sun in your bathing suit until your skin just begins to turn pink, you make between 10,000 and 50,000 units of cholecalciferol in your skin. Professor Michael Holick of Boston University School of Medicine has studied this extensively and believes a reasonable average of all the studies is 20,000 units. That means a few minutes in the summer sun produces 100 times more vitamin D than the government says you need! As discussed in other pages, this is the single most important fact about vitamin D.
After it is made in the skin, or taken by mouth, cholecalciferol is transported to the liver where it is metabolized into calcidiol or 25(OH)D. Calcidiol is now thought by some scientists to have steroid hormone properties. It certainly helps maintain your blood calcium levels. But calcidiol's main importance is that it is the storage form of vitamin D. Calcidiol is what fills your vitamin D gas tank. If your serum calcidiol level is less than 40 ng/mL, your tank is low and should be filled up, keeping it that way unless you have a rare medical condition called vitamin D hypersensitivity.
In order to understand why you should keep your vitamin D tank full, you need to understand the next step in the metabolism of cholecalciferol. After your liver turns cholecalciferol into calcidiol, calcidiol follows one of two pathways. The first pathway takes priority—as your life literally depends on it—but the second pathway is causing all the excitement. However, if your tank is low, most of your calcidiol takes the first pathway.
Calcitriol Made in Kidneys

The first pathway leads to the kidney, where calcidiol is turned into calcitriol. Calcitriol is a potent steroid hormone, in fact, it is the most potent steroid hormone in the human body. A steroid hormone is simply any molecule in the body that is made from cholesterol and that acts to turn your genes on and off. They are always important to health, always need to be handled with care, and are often quite potent.
Calcitriol made by the kidney circulates in the blood to maintain your blood calcium levels. Calcium is vital to the function of the cells in the body, without enough calcitriol in the blood calcium levels will fall and illness will set in. Therefore, the first priority for calcidiol is to go to the kidney where it makes enough calcitriol to secrete into the blood in order to regulate serum calcium.
More Calcitriol Produced in Tissues

The second vitamin D pathway leads to your tissues and that is where all the action is. All of the amazing health benefits of vitamin D discovered in the last 10 years are from vitamin D going down the second pathway. If any calcidiol is left over—that is, if your tank is full and your kidneys are getting all the calcidiol they need to maintain serum calcium—then calcidiol is able to take another pathway, one that leads directly to the cells. This path is only now being fully understood and is causing excitement all around the world, especially concerning cancer. These are the autocrine (inside cell) and paracrine (around the cell) functions of the vitamin D system.
These functions are crucial to understanding why you should keep your vitamin D tank full. If you only have a small amount of calcidiol in your blood, virtually all of it goes to your kidney, which then makes extra calcitriol to keep your serum calcium levels from falling. Almost no calcidiol gets to your tissues to make tissue calcitriol.
Tissue Calcitriol A Cancer Fighter

But when your tank is full, the left over calcidiol goes to the many cells in the body that are able to make their own calcitriol to fight cancer—and they do so with gusto! In fact, they appear to make as much calcitriol as they can. The more calcidiol they get, the more calcitriol they make. The step is not rate-limited by its product (calcitriol) and is thus uncontrolled. No other steroid hormone system in the body works this way; the manufacture of calcitriol in the tissues is unique. This is the second most important fact about vitamin D.
Other steroids limit their own production by inhibiting the very chemical reactions that make them. For example, a chemical reaction in the body turns cholesterol into progesterone, a female hormone. When enough progesterone is made, progesterone shuts down (inhibits) the chemical reaction so no more progesterone is made. This is called negative feedback. This occurs with all other steroids, somewhere in the metabolic process. If it didn't, the body would not be able to precisely regulate steroid hormone levels.
It does not appear to occur with calcitriol in the tissues! Throughout the entire range of normal calcidiol levels, tissue calcitriol levels continue to increase.
This is a crucial piece of information, because it has such profound implications for the normal state of human affairs. Just as modern humans have been living (and dying) with historically low levels of calcidiol in their blood, their tissues have been living (and dying) with historically low levels of calcitriol. And calcitriol is the most potent steroid hormone in the human body. It turns genes on and off at a dizzying rate, genes that are either making proteins that are essential to fighting cancer or genes that are making proteins that are promoting diseases like cancer.

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Good post Jasthace!

I get 5000-6000IU per day minimum not counting food.

Nice post ...!!!!!