Vitamin D is a fat-soluble vitamin (soluble in fat) synthesized in the human body from a derivative of cholesterol in the action of ultraviolet radiation of light. It exists in two forms: D2 (ergocalciferol) or D3 (cholecalciferol).
Vitamin D occurs in the absorption of calcium and phosphorus by the intestines and in their reabsorption by the kidneys, under the influence of PTH. This is a real hormone. Its effects are offset by calcitonin.
A sufficient amount of vitamin D is particularly necessary during early childhood to prevent rickets, breast milk being relatively poor. However, a sufficient quantity is necessary for adults to avoid osteomalacia.
Structure of Vitamin D
To obtain the structure of vitamin D3, add a hydroxyl group (-OH) on carbon 1 and 25, so you will have 1.25 alphadihydroxycholécalciférol, alias vitamin D3 or calcitriol.
Vitamin D2 or ergocalciferol is a product irradiation ergosterol by ultraviolet rays.
Vitamin D3, or cholecalciferol is the natural vitamin, it is concentrated in the liver oils such as fish. All milks are poor.
Vitamin D is formed especially when we expose our bodies to the sun (heliotherapy). At the skin, ultraviolet rays allow the formation of vitamin D3 from derivatives of cholesterol present normally in the body. This source is very variable depending on sun exposure (seasons, fog, region, clothing), thickness and skin pigmentation. There is, in theory, never overdose on vitamin D when exposed to sunlight, the latter also involved in the destruction thereof. In certain situations such as renal failure, it is necessary to artificially increase the dietary intake of vitamin D. This is done from pharmaceutical preparations that allow accurate dosing of intakes of vitamins D (1 mg of vitamin D = 40 000 units, correspondent, following the marks, either of vitamin D2, is to vitamin D3). A healthy person with a varied diet and a normal exposure to the sun usually does not need extra vitamin D unless it is black or dark skin.
A regular exposure to the sun guarantees, in most people, a good supply of vitamin D, vitamin D may occur in certain situations in which vitamin D will be used in preventive use.
It is defined by a rate of 25-hydroxyvitamin D less than 75 nmol / l (30 ng / ml). With this limitation, the failure affects more than one billion people on earth and more than half of postmenopausal women. However, a severe deficiency, the cause of rickets and osteomalacia is defined by a rate of less than 25 nmol / l (10 ng / ml).
The main causes of failure are insufficient exposure to sunlight and dark phototype. The main risk groups are:
* Elderly people living in institutions and the very elderly in general.
* People with dark skin or black.
* Breast-fed infants. Breast milk contains relatively little vitamin D. The artificial milk are always rich in vitamin D.
* Those little exposed to the sun for medical reasons or cultural (Islamic veil covering the face).
* The winter months for higher latitudes because UVA responsible for the synthesis of vitamin D are low.
Vitamin D produced by sun exposure, however, is present during the 2 months after exposure thanks to reservations made by the body. This characteristic is that many people feel muscle weakness between January and April because during the months from October to April, UVA responsible for the synthesis of vitamin D are low. A vitamin D deficiency causes weakness and muscle pain or fatigue, and at a later stage, osteomalacia in adults and rickets in children. It could increase the risk of fracture while supplementation with vitamin seems only marginally reduce this risk.
It would also be associated with an increased risk of cardiovascular disease.
Metabolism of vitamin D
The vitamin D made by food, fat-soluble, is partially absorbed into the back end of the small intestine, emulsion with bile salts (vitamin deficiency in the event of obstruction of the bile principal or fat malabsorption). After absorption, it is transported by traffic at certain organs where it is stored (possibility of preventing rickets by discontinuous doses of vitamin D).
Summary of vitamin D
If exogenous supply is insufficient, the agency itself synthesizes vitamin D derived from cholesterol. Thanks to the action of ultra-violet light, cycles of 7-déhydro-choléstérol is broken. The molecule s'isomérise spontaneously cholecalciferol still inactive. Followed (in the liver) a first hydroxylation on carbon 25 by the enzyme 25-hydroxylase, which gives 25-hydroxy-cholecalciferol, which is still inactive. The real vitamin D results from a new hydroxylation (at the kidney) of the molecule on carbon 1. The latter reaction is catalyzed by the enzyme alpha-1-hydroxylase. This results in the 1.25-dihydroxy cholecalciferol or vitamin D. Therefore, an alcohol which has added two hydroxy group, so a triol: calcitriol.
Regulation of synthesis
The parathyroid hormone (PTH) stimulates the gene expression of the enzyme alpha-1-hydroxylase. It promotes hydroxylation on carbon 1 and therefore stimulates the production of the active form of vitamin D. However, the absence of PTH promotes a different hydroxylation that does not have the active form. It is (at the kidney) on carbon 24 by the enzyme 24-hydroxylase, which gives the 24.25-dihydroxy-cholecalciferol less active than 1,25-dihydroxycholecalciferol.
Vitamin D helps calcium absorption by the intestine, the reabsorption of calcium and phosphorus by the kidneys and production of a bone of good quality. There is a time for action between the time of administration of vitamin D and when the absorption of calcium increases in its effect.
The action depends on vitamin D available and the burden of bone calcium. It determines the calcium on bone dose physiological then it frees, too high dose (hypervitaminosis), causing hypercalcemia.
During growth, it has a site of action. This is the area where the cartilage metaphyseal conjugation turns into bone tissue.
Finally physiological dose, it reduces calciurie (urinary excretion of calcium).
At the molecular level, vitamin D attaches to a receiver near the DNA in the cell nucleus, thus facilitating the transcription of certain genes, which would explain the varied effects observed.
* There seems to be an inverse correlation between blood levels of vitamin D and the risk of developing multiple sclerosis. This correlation has been found that among people with white skin.
* The blood concentration of vitamin D is inversely correlated with the prevalence of hypertension, diabetes and obesity.
* Several arguments, indirect, suggesting that vitamin D is involved in cancer prevention. On the one hand there is much less of colo-rectal cancer in the countries of South and North (northern hemisphere), and on all continents. On the other hand, the administration of vitamin D3, or cholécalciférols modified inhibit carcinogenesis in rodents induced (more than ten experiments reported). However, the only clinical trial conducted extensive in volunteers (U.S.) showed no protection. Other studies of lesser magnitude, however, seem to contradict the previous with a reduction of almost 60% of the occurrence of cancers-run in postmenopausal women.
Slowdown of ageing
Women who have the most vitamin D in the body would have longer telomeres than those who lack vitamin which could have beneficial effects on aging.
Health authorities American and French believe they need only 200 IU of vitamin D per day to be healthy, a level considered ridiculous by international specialists.
Food Content mg per 100g
Oil halibut 50 to 100
Oil carp 25
Liver oil mackerel 5
Oil tuna 5 to 15
Liver oil salmon 1
Oil cod liver 0.6000
Chicken Liver 0.0020
Read also Vitamin C