WHAT IS VITAMIN D?

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Vitamin D is a steroid vitamin - a group of fat-soluble prohormones - which encourages the absorption and metabolism of calcium and phosphorous. However, people who are exposed to normal quantities of sunlight will not need vitamin D supplements because sunlight promotes sufficient vitamin D synthesis in the skin. Unfortunately, because of the inherant risks involved with sun exposure - such as skin cancer -  many of us remain covered up during the sunniest periods of the year to the detriment to our health.

Five forms of vitamin D have been discovered, vitamin D1, D2, D3, D4, D5, but the two forms that matter to humans the most are vitamins D2 (ergocalciferol) and D3 (cholecalciferol).

As mentioned previously, people who get plenty of sun exposure do not require supplemental vitamin D. This is because sunlight naturally increases vitamin D synthesis when it strikes our bare skin. Although the recommended dietary allowance for vitamin D is 200 IU per day for adults, there is some evidence that elderly people need 800 to 1,000 IU per day for maximum effects on preserving bone density and preventing fractures. Sun-deprived people should take no less than 600 IU per day and ideally around 1,000 IU per day.

So, vitamin D for humans is obtained from sun exposure, food and supplements. It is biologically inert and has to undergo two hydroxylation reactions to become active in the body. The active form of vitamin D in the body is called Calcitriol (1,25-Dihydroxycholecalciferol).

Calcitriol promotes the absorption of calcium and phosphorus from food in the gut and reabsorption of calcium in the kidneys - this increases the flow of calcium in the bloodstream. This is essential for the normal mineralization of bone and preventing hypocalcemic tetany. Hypocalcemic tetany is a low calcium condition in which the patient has overactive neurological reflexes, spasms of the hands and feet, cramps and spasms of the voice box (larynx). A severe lack of vitamin D can also cause the childhood bone disorder Rickets in which bones soften and become prone to fractures and deformity. Rickets affects mainly children, although the disorder may also affect adults (osteomalacia).

Which foods contain vitamin D?

Cod liver oil is an excellent dietary source of vitamin D, as are vitamin D-fortified foods. Traces of vitamin D are found in egg yolks and butter. Top foods for vitamin D also include shiitake and buttin mushrooms, mackerel, sockeye salmon, herring, sardines, tuna and unfortunately cod live oil! However, the majority of vitamin D in the body is created during a chemical reaction that starts with sunlight exposure to the skin.

Vitamin D deficiency

Vitamin D deficiency is more common in strict vegetarians (who avoid vitamin D-fortified dairy foods), dark-skinned people, alcoholics, and people with liver or kidney disease. People with liver and kidney disease can make vitamin D but cannot activate it.

Vitamin D deficiency is more common in people suffering from intestinal malabsorption, which may have occurred following previous intestinal surgeries, or from celiac disease. People with insufficient pancreatic function (e.g., those with pancreatitis or cystic fibrosis) tend to be deficient in vitamin D. Vitamin D deficiency is also common in individuals with hyperthyroidism (Graves' disease), particularly women.

In children, vitamin D deficiency is called rickets and causes a bowing of bones not seen in adults with vitamin D deficiency. Vitamin D deficiency is common among people with hyperparathyroidism, a condition in which the parathyroid gland is overactive. In a study of 124 people with mild hyperparathyroidism, vitamin D levels were below normal in 7% of them and suboptimal in 53% of them. Vitamin D deficiency is also common in men with advanced prostate cancer. In one study, 44% of 16 men with advanced prostate cancer had decreased blood levels of vitamin D.

One in seven adults has been reported to be deficient in vitamin D. In one study, 42% of hospitalized patients under age 65 were reported to be vitamin D deficient. In this same study, 37% of the people were found to be deficient in vitamin D, despite the fact they were eating the currently recommended amount of this nutrient. Vitamin D deficiency is particularly common among the elderly. Age-related decline in vitamin D status may be due to reduced absorption, transport, or liver metabolism of vitamin D.

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Photo care of http://www.topnews.in/health/vitamin-d-levels-linked-health-blood-vessels-211571 and http://www.scientificpsychic.com/health/vitamins.html
Based on an article from http://www.medicalnewstoday.com/articles/161618.php and http://www.evitamins.com/encyclopedia/assets/nutritional-supplement/vitamin-d/how-it-works and http://www.medicalnewstoday.com/articles/176941.php