Vitamin D
Background
Vitamin D differs from other nutrients in that the body can synthesise it, with the help of sunlight, from a precursor that the body makes from cholesterol. In tropical and subtropical areas enough vitamin D is made in the skin to meet the body's needs. However when people live at high latitudes and are covered by clothes, spend most of their times indoors and the sky is polluted with smoke, there is insufficient UV exposure in winter to make enough vitamin D. This also occurs if people are housebound or are completely covered. In these cases, vitamin D must be obtained from the diet.
Functions
Vitamin D acts like a hormone, in that it is manufactured by one organ (the skin) of the body and affects another part. The best known target organs for vitamin D are the intestines, the kidneys, and the bones. All these organs respond by making calcium available for bone growth.
- Maintains adequate levels of calcium in the blood
- Bone growth- helps to promote normal bone mineralisation
- Other vitamin D target tissues include the brain, nervous system, pancreas, skin, muscles, cartilage, reproductive organs, immune cells, and some cancer cells, which suggests that vitamin D may have numerous functions
Recommended Intakes
The recommendation of vitamin D in the USA is 10mg/day for those aged 19-24yrs, and 5mg/day for those aged 25yrs and over. Some experts consider that for those adults living a normal lifestyle no dietary intake is necessary since vitamin D can be manufactured by the body. For those confined indoors the UK panel of experts agree on an intake of 10mg/day.
Food Sources
Only a few foods supply significant amounts of vitamin D. Rich sources include egg yolks, liver, fatty fish, butter. Some milks and margarines may be fortified with vitamin D.
Deficiency
In vitamin D deficiency, the production of the calcium-binding protein in the intestinal cells declines. This means that even though there may be sufficient calcium in the diet, it passes through the gastrointestinal tract unabsorbed, leaving the bone undersupplied.
The vitamin D deficiency which is seen in children is known as rickets. Here there is reduced calcification of the growing ends of bones. Rickets only occurs in young people whose bones are still growing. As the bones fail to calcify normally there is growth retardation and skeletal abnormalities. The bones become so weak that they bend when they have to support the body's weight. One of the most obvious signs of the disease is seen in children when they walk, they develop bowed legs. Another sign of rickets is a protruding belly that results from lax abdominal muscles.
In adults, vitamin D deficiency (adult rickets) is referred to as osteomalacia. This is the corresponding decalcifying bone disease in adults whose bones ends have fused so that the bone is no longer growing. The bones of the legs may soften so that the individual becomes stooped and bowlegged.
Toxicity
With the oral intake of vitamin D, the margin between adequate and toxic intakes are quite narrow. Vitamin D is the most likely vitamin to have toxic effects when consumed in even small amounts above the recommended level on a continuous basis.
Vitamin D toxicity enhances calcium absorption, produces high blood calcium, and promotes the return of bone calcium into the blood. Excess blood calcium tends to precipitate in the soft tissue, forming stones in the kidneys. Calcification may also harden the blood vessels.
Exposure to the sun does not cause vitamin D toxicity because excessive UV light degrades the vitamin D precursor in the skin to inert compounds. | 
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