The major biological function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D increases absorption of calcium from the small intestine, helping to form and maintain strong bones. It promotes bone mineralization in concert with a number of other vitamins, minerals, and hormones.
However, it is quite clear that vitamin D does far more than promote optimized bone health. In addition to the strong evidence provided about prevention of cancer, vitamin D has also been associated with improvement in the following conditions:
• Heart Disease
• Infertility and PMS
• Fatigue, Depression and Seasonal Affective Disorder
• Autoimmune Disorders
• Syndrome X
Vitamin D is not "just" a vitamin: it regulates steroidal hormones that influence literally thousands of different genes in your body. Vitamin-D receptors are found in almost every type of human cell, from brain tissue to your bones.
Vitamin D up-regulates your ability to fight infections and chronic inflammation by producing over 200 antimicrobial peptides, including a naturally occurring broad-spectrum antibiotic. For this reason, it can be highly effective against colds and influenza.
The best place to get vitamin D is from sunlight; but darker-skinned people require 10 to 20 times more sun exposure than lighter-skinned people do to build up the same amount of vitamin D. Light-induced synthesis occurs in the skin when ultraviolet light reacts with a form of cholesterol, converting it to cholecalciferol. This molecule is then altered by the liver and kidneys to form the physiologically active vitamin D.
Some key points to remember
The vitamin D in supplemented milk and in many cheap vitamins may be vitamin D2 (ergocalciferol). It is not nearly as good as the vitamin D created in your body when you expose your skin to sunlight or that found in animal-based food sources (like cod liver oil, fish and egg yolk) which is called vitamin D3 (calciferol).
It is also very important to realize that the RDA of vitamin D of 600 IU (recently upgraded from 400IU) is absolutely inadequate for most people who do not have exposure to regular sunshine. Remember that RDA values were established to prevent deficiency diseases--not for optimal health!
Most people who are deficient will need up to 10,000 units per day for a short time to build their vitamin D levels up to healthy ranges.