The essential nutrient vitamin D is normally produced after exposure to sunlight. The skin produces previtamin D, which is then transported to the liver where it undergoes a series of reactions that end in its conversion to the active form 25-dihydroxyvitamin D that can be used by the body. Vitamin D in the body mainly acts on the bones, but it also has some effect on the muscles and immune system. Someone who becomes critically deficient in vitamin D can develop the diseases rickets, which typically occurs in children and can affect bone development, or osteomalacia, which causes bone and muscle weakness in susceptible adults.
Inadequate Sun Exposure
According to Merck Manuals Online Medical Library, the most common cause of a severe vitamin D deficiency is inadequate exposure to sunlight. People who live above 42 degrees north latitude are more prone to developing a critical deficiency. This occurs because sunlight in the far north is typically insufficient to allow skin synthesis of the vitamin between the months of November and February. Other individuals prone to inadequate sun exposure are traditional Muslim women whose skin remains covered the majority of the time, breast-fed babies who are not given significant sun exposure and older individuals who may be housebound or hospitalized yet have higher requirements for vitamin D than younger people. People who have darker skin may need longer sun exposure and may be more at risk for developing deficiencies due to inadequate sunshine.
Malabsorption Disorders
Individuals with malabsorption disorders have difficulty absorbing fat. Because vitamin D is fat-soluble and normally enters the body with fat in the small intestine, this may lead to an insufficient absorption of this vitamin. One well-known disorder that involves fat malabsorption that can lead to a critical vitamin D deficiency is the genetic disorder cystic fibrosis. Some forms of Crohn's disease can also cause fat malabsorption and result in insufficient vitamin D in the bloodstream.
Conversion Problems
Conversion problems occur when the body has difficulty changing the inactive forms of vitamin D into the active 25-dihydroxyvitamin D that can be utilized by the body. Kidney and liver disorders can cause conversion problems. People who are taking the drug rifampin or anticonvulsants such as phenobarbitol and phenytoin may develop conversion disorders as a side effect of these drugs. Rare hereditary disorders such as type I hereditary vitamin D-dependent rickets and X-linked familial hypophosphatemia are another source of conversion disorders.
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