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Vitamin D synthesis is the process of creating vitamin D within the body. Humans must consume most vitamins and minerals in the foods they eat, but vitamin D can be synthesized in the skin during exposure to direct sunlight. This fat-soluble vitamin is essential to maintain normal calcium metabolism, and a deficiency of the vitamin can cause a number of adverse conditions.
As vitamin D is uncommon in foods, the main source is vitamin D synthesis, wherein cholesterol is transformed into the vitamin. Diets low in cholesterol, then, can lead to decreased levels of vitamin D. While exposure to sunlight allows humans to synthesize the vitamin themselves, factors such as time of year, cloud cover, smog and the use of sunscreen decrease the body’s ability to absorb ultraviolet light. Lack of proper sun exposure may also lead to depleted levels of the vitamin.
To make the biologically inactive vitamin ready for use, vitamin D synthesis must occur. There are two processes that vitamin D must undergo before the body can access it. First, it is transformed to 25-hydroxyvitamin D, also known as calcidiol, in the liver. Next, the kidneys transform the calcidiol into the active 1,25-dihydroxyvitamin D, or calcitriol. When vitamin D is transformed into calcitriol, it can be used to regulate calcium levels in the blood.
In foods, vitamin D is primarily found in fish, and fish liver oil. Small amounts can be found in beef liver, cheese and egg yolk, all of which are considered to be foods high in cholesterol. In many developed nations, vitamin D also is often added to other foods, such as vitamin enriched milk or breakfast cereals. There are vitamin D supplements available for people with a deficiency of the vitamin as well.
Vitamin D deficiency can cause pain in the muscles and joints, decreased immune system function and trouble stabilizing mood. Eventually, serious disorders such as rickets and osteomalacia can develop as well. While these conditions are usually the result of decreased intake of vitamin D, occasionally they can also arise from a decreased capacity for vitamin D synthesis.
In rickets, a serious childhood condition, bones fail to mineralize. Without proper mineralization, growing bones in weight-bearing limbs, such as the legs, become bowed. Bones in the ribcage can also show malformations. In adults, lack of vitamin D can lead to osteomalacia, which makes the bones soft. Even though bones are no longer growing, they are still being rebuilt as old cells die and new cells come forth to replace them.