Vitamin D is a group of fat soluble vitamins with a broken steroid ring; hence it is also known as secosteroid.
Vitamin D consists of Ergocalciferol, Cholecalciferol (or calciol), 22 dihydroergocalciferol, calcidiol or 25-hydroxycholecalciferol, calcitriol and vitamin D5.
The first three are known as vitamin D2, vitamin D3, vitamin D4.
Calcitriol is also known as 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitaminD3. This is the main form of vitamin D that is used in the body for formation of bones and teeth.
Calcidiol or 25-hydroxycholecalciferol or 25-hydroxyvitamin D is a prehormone that is synthesized by liver by hydroxylation of vitamin D3 (cholecalciferol).
The concentration of this form in the blood reflects the vitamin D status of the body. Also, this form is the major storage form of vitamin D in the body.
Ergocalciferol form of vitamin D is a plant product where as cholecalciferol comes from animals.
The analogues of vitamin D5 are used as antitumor agents.
Vitamin D, like vitamins A, E and K, is a fat soluble vitamin and therefore can be stored in the body.
Although referred to as a vitamin, vitamin D3 is not a vitamin in a strict sense; since vitamins are substances that cannot be synthesized by the body and must be obtained in the form of diet. Vitamin D is synthesized by the skin of human body when the ultraviolet rays of sunlight (specifically UVB rays) strike the skin, delivering the energy for the reaction to take place. Since the vitamin is synthesized by sunlight it is given the name “sunshine vitamin”.
7-dehydrocholesterol in the skin changes into cholecalciferol with sunlight exposure. This cholecalciferol (aka calciol) is converted in the body by liver to 25-hydroxycholecalciferol or calcidiol which stays in the blood in the same form and is converted at the time of need into calcitriol which is the active form of vitamin D3.
Vitamin D proper (1, 25-dihydroxy vitamin D) is also famous for its function as a hormone. Since it contains a steroid nucleus it is more properly known as a “steroid hormone”. This is the most potent form of vitamin D in the body which works by stimulating the receptors inside the cell nuclei.
Ergocalciferol and cholecalciferol are two important forms of vitamin D in human body; both are changed first into 25-hydroxy vitamin D and then utilized after second hydroxylation as the main vitamin in the body.
Vitamin D in the body is involved in calcium homeostasis. Vitamin D helps in intestinal absorption of calcium, plus absorption of phosphate and magnesium ions in the kidneys.
When the blood calcium levels go down, parathyroid hormone in the body is stimulated, which activates vitamin D3 by its second hydroxylation in the proximal convulated tubule cells of the kidneys and converts it to vitamin D which goes to intestine for calcium absorption and performs other of its functions.
Vitamin D suppresses the formation of type I collagen in the body. This form of collagen is found primarily in the bones. The vitamin also activates bone eating cells called osteoclasts. These functions of vitamin D are to maintain normal calcium levels in the serum. These functions of vitamin D are not of crucial importance. The most important function probably is to provide calcium and phosphate level so that bone mineralization can take place.
Vitamin D has been shown to reduce the risk of cancer, rheumatoid arthritis and multiple sclerosis; multiple sclerosis is a disease in which the body destroys the myelin (fatty) covering of the nerve fibers in brain and spinal cord.
Vitamin D may also help in recovery from tuberculosis and reduce the chances of having a heart attack.
Vitamin D receptors are present in most of the cells of the body which when activated, cause the cells to proliferate and differentiate into their respective types.
Vitamin D deficiency in the body may lead to rickets in children and osteomalacia in the adults. These deficiency states may cause the bones to weaken (owing to the bone formation function of vitamin D) which lead to multiple bone fractures.
Low levels of vitamin D may also be a result of anomalous function of parathyroid glands which produce parathormone, which activates vitamin D in kidneys; another reason for vitamin D deficiency may be chronic kidney disease.
Low sunlight exposure or usage of SPF containing sunscreens or products may also lead to decreased vitamin D levels in the body. Low exposure may be seen in women who veil their bodies and elderly people who stay indoors and have poor diets.
SPF containing products or sunscreens with value greater than SPF-8, block the entrance of UV rays in skin and therefore block the synthesis of vitamin D.
The darker your skin, the difficult it is for sunlight to enter; therefore dark skinned people have less formation of vitamin D than light toned people.
Liver diseases may also pose a threat to vitamin D synthesis.
Genetic diseases which involve production of aberrant vitamin D receptors may cause resistance to vitamin D actions in the body.
For people who have a hard time absorbing vitamin D in the body, due to certain disease states or fat malabsorption, vitamin D supplements may be used to replenish the vitamin stores in the body.
Vitamin D may be given to treat rickets, osteomalacia, sometimes osteoporosis, familial (hereditary) hypophosphatemia and hypocalcemia etc. osteoporosis is a disease which causes thinning of bones and makes the bones prone to fractures.
Patients with renal failure may also use vitamin D supplements for maintaining the normal levels of vitamin in blood.
Vitamin D also has some function in boosting up the immune system and is also known to prevent respiratory functions.
Important sources of vitamin D include fatty fishes such as tuna, salmon and mackerel; other sources are cheese, mushrooms, beef liver and egg yolks.
Fortified foods such as fortified breakfast cereals, margarines, milk, yogurts and soy beverages also contain vitamin D.
Fortified foods are the foods to which are added certain micronutrients (vitamins-for instance vitamin D) which they normally and naturally do not contain.
Too much vitamin D on the other hand may cause hypervitaminosis of vitamin D, which may lead to ectopic calcification in lungs or heart; in kidneys it may lead to formation of kidney stones. Other side effects include nausea, vomiting, constipation, loss of appetite etc.
The recommended dose of vitamin D in adults is about 600IU/day; in people above 70 it is 800IU/day; in pregnant and lactating mothers, 600 IU/day; in infants 400 IU/day.
Vitamin D levels should be properly maintained in the body to keep it functioning normally. To gain normal levels of vitamin D, supplements or fortified foods may be used. Fishes are an important source of vitamin D; people who are purely vegetarian should realize that may be prone to getting diet related vitamin D deficiency and therefore they should consume vitamin D from other sources mentioned above.