Vitamin D is a group of fat-soluble secosteroids. In humans, it is unique both because it functions as a prohormone - a committed intra-glandular precursor of a hormone, usually having minimal hormonal effect by itself - and because the body can synthesise it (as vitamin D3) when sun exposure is sufficient, hence its nickname, the 'sunshine vitamin'.
Although vitamin D is commonly called a vitamin, it is not an essential dietary vitamin as it can be synthesised in adequate amounts by all mammals from sunlight. Vitamin D fits within the definition of vitamin as it is ‘an organic compound required as a vital nutrient in tiny amounts by an organism.’ An organic chemical compound (or related set of compounds) is called a vitamin when it cannot be synthesised in sufficient quantities by an organism, and must be obtained from the diet.
As with other compounds called vitamins, it was discovered in an effort to find the dietary substance that was lacking in a disease, namely, rickets, the childhood form of osteomalacia. Furthermore, like other compounds called vitamins, in the developed world vitamin D is added to staple foods, such as milk, to avoid disease due to deficiency.
The active form of vitamin D is 25(OH)D3 and its primary role is to maintain blood calcium and phosphorus levels within a normal range. It acts in conjunction with two others hormones: parathyroid hormone (PTH) from the parathyroid glands and calcitonin from the thyroid glands. It’s also important for regulating cell differentiation and growth, and, according to an article by J H Kleibeuker et al, published in the European Journal of Cancer, it may also protective against colorectal cancer.
Vitamin D3 (cholecalciferol) is produced by ultraviolet irradiation (UV) of its precursor 7-dehydrocholesterol. This molecule occurs naturally in the skin of mammals and in milk. Vitamin D3 can be made by exposure of the skin to UV, or by exposing milk directly to UV, which is one commercial method. A moderate, sensible amount of sun exposure can provide a sufficient amount of vitamin D which is stored in body fat during the winter, when vitamin D cannot be produced as readily.
Food sources of vitamin D come in natural and fortified forms. Natural sources include oily fish – salmon, sardines and herring for example – egg yolks and liver. Fortified forms are included in dairy products, especially milk, cereals, and some margarine. Fish liver oil preparations are normally used as supplements for the supply of vitamins A and D.
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