Vitamin D is a group of fat-soluble prohormones, the two major forms of which are Vitamin D2 (or ergocalciferol) and Vitamin D3 (or cholecalciferol), that is naturally present in very few foods, added to others, and available as a dietary supplement. It is also produced endogenously when ultraviolet rays from sunlight strike the skin and trigger Vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts Vitamin D to 25-hydroxyVitamin D, also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyVitamin D, also known as calcitriol.

Calcitriol plays an important role in the maintenance of several organ systems. However, its major role is to increase the flow of calcium into the bloodstream, by promoting absorption of calcium and phosphorus from food in the intestines. Vitamin D is essential for promoting calcium absorption in the gut and maintaining adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and prevent hypocalcemic tetany(a muscle spasm caused by reduction in the concentration of calcium circulating in the blood). It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient Vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, Vitamin D also helps protect older adults from osteoporosis.

Vitamin D has other roles in human health, including modulation of neuromuscular and immune function and reduction of inflammation. Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by Vitamin D.

Vitamin D is found in the following foods: Cheese, Butter, Cream, Fortified milk (all milk in the U.S. is fortified with Vitamin D), Fish, Oysters, Fortified cereals, Margarine.

Vitamin D deficiency can lead to osteoporosis in adults or rickets in children. Too much Vitamin D can make the intestines absorb too much calcium. This may cause high levels of calcium in the blood. High blood calcium can lead to calcium deposits in soft tissues such as the heart and lungs. This can reduce their ability to function. Kidney stones, vomiting, and muscle weakness may also occur if you have too much Vitamin D.

Vitamin D is also known as the "Sunshine Vitamin" because the body manufactures the Vitamin after being exposed to sunshine. Ten to 15 minutes of sunshine 3 times weekly is enough to produce the body's requirement of Vitamin D. However, many people living in sunny climates still do not make enough Vitamin D and need more from their diet or supplementation.

The Food and Nutrition Board at the Institute of Medicine recommends the following dietary intake for Vitamin D as cholecalciferol. (One microgram of cholecalciferol is the same as 40 IU of Vitamin D.)

Infants 0 - 6 months: 5 micrograms per day (mcg/day)
7 - 12 months: 5 mcg/day

Children 1 - 13 years: 5 mcg/day

Adolescents and Adults Males and Females age 14 to 50: 5 mcg/day

Males and Females age 51 to 70: 10 mcg/day Males and Females age over 70: 15 mcg/day

Specific recommendations for each Vitamin depend on age, gender, and other factors (such as pregnancy). In general, those over age 50 need higher amounts of Vitamin D than younger persons. Most people should be able to get all the Vitamin D they need from their diet and by getting a little sun. Ask your health care provider which amount is best for you.

Resounding evidence proving the effectiveness of Vitamin D in slowing the onset of breast, colon, and other cancers is convincing a growing body of doctors and physicians to utilize the sunshine Vitamin in their arsenal of cancer treatment weapons. In the last several years, numerous epidemiological studies have illustrated the correlation between Vitamin D deficiency and serious disease, including cancer. Researchers are now focusing attention on elevated levels of "therapeutic" Vitamin D, far above the government's daily recommended amounts, for use in disease treatment and prevention.

Although the link between low Vitamin D status and chronic disease has been well known for some time, a new study (December 2009) has shed further light on the role of Vitamin D in cases of hypertension and congestive heart failure. Researchers from the Medical College of Wisconsin analysed genetic data from 617 individuals stored at the Marshfield Clinical Personalized Medicine project, a DNA databank. They separated the group equally into a healthy control group, those with high blood pressure and those with high blood pressure and congestive heart failure, and it became clear that there was a striking correlation between those who suffered from both ailments and a deviation in the CYP27B1 gene, which reduces the rate that the body converts stored Vitamin D into its active form.

Many in the medical profession are beginning to recognize that people who take cholesterol-lowering statin drugs are becoming Vitamin D-deficient. Cholesterol is required by the body to synthesize Vitamin D and statin drugs are are responsible for eliminating it, leading many to speculate that statin drug users do not have enough cholesterol to process Vitamin D.

Contrary to popular belief, cholesterol actually plays an important role in maintaining health. It regulates proper hormonal levels and is the precursor substance for the production of Vitamin D. Cholesterol also works to digest and absorb fats, nutrients, and Vitamins.

When converting sunlight into Vitamin D, cholesterol in the skin acts as the catalyst for this important process. Vitamin D is crucial for mineral metabolization and is said to target over 2000 human genes. Deficiency is linked to over 17 varieties of cancer as well as heart disease, autoimmune diseases, muscle and bone problems, and other serious diseases.

In the study (January 2010), researchers found a clear connection between Vitamin D deficiency and muscle pain. Over 64 percent of patients with muscle pain who were taking statin drugs were also deficient in Vitamin D. Those with muscle pain in general were found to be deficient in Vitamin D.

When study participants who reported muscle pain were given 50,000 IU of Vitamin D a week for 12 weeks, more than 92 percent of them were completely relieved of all muscle pain. The prescribed supplementation also raised blood levels of Vitamin D to normal levels.

It is also known that statin drugs are responsible for depleting CoQ10 (known as the Energy Enzyme, CoEnzyme Q10 has been promoted as a supplement for supporting cardiovascular health and increasing energy as well as a potent antioxidant) levels, a vital substance that metabolizes energy in the body. Both CoQ10 and Vitamin D supplementation are recommended for anyone who takes statin drugs. A minimum of 2,000 IU of Vitamin D and between 100 and 200 mg of CoQ10 daily are appropriate doses.

Studies have shown that taking CoQ10 by itself helps to maintain proper cholesterol levels without the need for statin drugs. While keeping bad cholesterol (LDL) levels low is beneficial, it is important to keep good cholesterol (HDL) levels high. CoQ10 works well at maintaining healthy levels of both. Some other alternatives to keeping cholesterol levels in check include supplementation with niacin and policosanol. In conjunction with a healthy diet low in refined sugars and bad fats, these natural alternatives are both safe and effective. Exercise and a diet rich in omega-3 fatty acids are also good suggestions.

Scientists at the University of Copenhagen have discovered (March 2010) that Vitamin D is crucial to activating our immune defences and that without sufficient intake of the Vitamin, the killer cells of the immune system – T cells – will not be able to react to and fight off serious infections in the body.