Vitamin D is also known as the "sunshine" vitamin because it is actually manufactured in the human skin when in contact with the ultraviolet light in the sun’s rays. The sunlight interacts with 7-dehydrocholesterol to form cholecalciferol, which is then transferred to the liver or kidneys and converted to active vitamin D. Wintertime, clouds, smog, and darkly pigmented skin reduce the body’s production of the "sunshine" vitamin.

This fat-soluble vitamin, when ingested, is absorbed through the intestinal walls with other fats with the aid of bile. Mineral oil binds vitamin D in the gut and reduces its absorption. From the blood, calciferol is taken mainly to the liver, where it is utilized or stored. Vitamin D is also stored in the skin, brain, spleen, and bones. Vitamin D intake must be more finely tuned in regard to the right therapeutic level than most other vitamins, and it is considered by many authorities to be the most potentially toxic vitamin. Symptoms of vitamin D toxicity can easily occur when vitamin D is taken in large amounts or with excessive sun exposure. (It is possible that part of sun poisoning symptoms are due to vitamin D toxicity.)

Sources: Provitamin D is found mainly in animal foods. D3, or "natural" vitamin D, is found in fish liver oil, which is the traditional source of both A and D. Cod liver oil is a commonly used source. Egg yolks, butter, and liver have some D, as do the oily fish, such as mackerel, salmon, sardines, and herring. Most homogenized milk and some breakfast cereals are "fortified" with synthetic vitamin D to give children, particularly, sufficient amounts. The plant foods are fairly low in D, with mushrooms and dark leafy greens containing some. Strict vegetarians who do not get adequate exposure to sunlight need to be concerned about getting their 400 IUs of vitamin D daily.

Functions: Vitamin D helps to regulate calcium metabolism and normal calcification of the bones in the body as well as influencing our utilization of the mineral phosphorus. Calcium and phosphorus together with other minerals make up our bones. Vitamin D3 helps increase the absorption of calcium from the gut, decreases excretion from the kidneys, stimulates resorption of calcium and phosphorus from bone, helps put them into teeth, and helps to maintain normal blood levels of calcium and phosphorus. With these functions, vitamin D is closely tied to the work of the parathyroid glands. Vitamin D is most important in regulating calcium metabolism in the body. Even with adequate calcium and phosphorus intake, if our vitamin D intake is low, we will have poor calcification of our bones; whereas, with good vitamin D intake, we will have better calcification even with low calcium and phosphorus intake. This function is especially important in menopausal women, for whom many doctors prescribe straight calcium without vitamin D, which is not likely to do much good unless they are sunbathing, an activity that doctors no longer recommend. Actually, taking calcium, magnesium, and vitamin D all together is probably ideal for best bone health. Phosphorus is usually readily available in adequate amounts in most diets.