Vitamin D is a fat soluble family of compounds known as vitamins D1, D2 and D3. It is obtained from food, and, uniquely among the vitamins, it can be made in the skin through the action of sunlight.
The biologically active form of vitamin D is a hormone known as calcitriol.

BONE - The most important role of vitamin D is to regulate the absorption and use of calcium and phosphorus, which are vital for normal growth and development of bones and teeth. Vitamin D stimulates intestinal absorption and re-absorption in the kidneys as well as maintaining blood levels of calcium and phosphorus. It enables bones and teeth to harden by increasing the deposition of calcium and may also assist in the movement of calcium across body cell membranes.

CELL GROWTH - Vitamin D is involved in normal cell growth and maturation and so may play a part in cancer prevention. In laboratory experiments calcitriol does seem to have anticancer properties, inhibiting the growth of human colon cancer, skin cancer, breast cancer and prostate cancer in test tube experiments.

THE IMMUNE SYSTEM - Vitamin D may also be involved in immune system regulation and play a part in the prevention and treatment of infectious diseases. Parts of the bone marrow which produce immune cells are receptive to vitamin D.

FERTILITY - Vitamin D is essential for strong pelvic bones and therefore plays an indirect role in fertility.

HORMONES - Vitamin D plays a role in the secretion of insulin by the pancreas thus aiding in the regulation of blood sugar. It also affects the parathyroid gland and a hormone that it produces.

NERVOUS SYSTEM - Vitamin D ensures the functioning of healthy nerves and muscles by regulating the level of calcium in the blood. Calcium is vital for normal nerve impulse transmission and muscle contraction.

Absorption: - When taken orally vitamin D is absorbed with fat through the intestinal walls. Vitamin D can be stored the fat cells of the liver, skin, brain and bones in amounts sufficient for many months consumption. Exposure to sunlight in spring, summer and autumn makes up for any shortfall in dietary vitamin D and even short exposure to sunlight during these times is adequate although there may be problems in winter months in some climates. Babies under 12 months have stores of vitamin D which they accumulate while in the womb.

The production of vitamin D in the body is blocked by anything which blocks ultra violet light including skin pigment, smog, fog, sunscreen, windows and hats.

Deficiency: In cases of vitamin D deficiency the body increases production of a hormone that removes calcium from the bones. In children, this results in rickets where the bones are so soft that they become curved from supporting the weight of the body. The equivalent in adults is osteomalacia with bone pain and tenderness and muscle weakness.

Other signs of deficiency include hearing loss, which is due to a softening of the bones in the inner ear, senile osteoporosis where the bones become lighter and less dense, and severe tooth decay.

Older people may be at risk of vitamin deficiency as they do not absorb or manufacture the vitamin D in their bodies as well as younger people. Others at risk of deficiency include alcoholics, people who don’t drink milk or get much sunlight, those with absorption problems and darker skinned people living in colder climates. Vitamin D is converted in the liver and kidneys to its active form so sufferers of kidney and liver diseases may also be at risk of vitamin D deficiency.

Vitamin D deficiency may play a role in inflammatory bowel disease, the development of breast and colon cancers and stroke, high blood pressure and rheumatoid arthritis as deficiency is often found in the diets of sufferers. However, over consumption of vitamin D may aggravate arthritis symptoms.