Vitamin D, also called the sunshine vitamin, plays a vital role in calcium's absorption in the body. Together they are essential in building and maintaining bone strength. When vitamin D is deficient, calcium cannot be properly absorbed. When this condition occurs in children it is called rickets, and in adults it is known as osteomalacia. While the skin can synthesize vitamin D upon exposure to sunlight, vitamin D is also available in some dietary sources such as cod liver oil, eggs, fish, and fortified milk.
Reduced Intake
According to MayoClinic.com, vitamin D deficiency may develop due to reduced intake of the vitamin for a prolonged period coupled with lack of sun exposure. Approximately five to 15 minutes of sun exposure can stimulate the skin to produce enough vitamin D to prevent deficiency, but patients with darker skin may need more time. Vitamin D is stored in the liver, so a deficiency will only manifest itself after these stores are depleted. The intake of adequate amounts of calcium and phosphorous is also necessary for prevention of rickets and osteomalacia.
Malabsorption
The Merck Manuals Online Medical Library mentions diseases where food is not absorbed properly from the gastrointestinal tract as potential cause of vitamin D deficiency. Conditions like removal of a part of the stomach or intestine and diseases of the intestine such as celiac disease impair the body's ability to absorb fats. Vitamin D, being a fat-soluble vitamin, is not absorbed except with fats and other fat soluble vitamins such as vitamins A, E, and K.
Heredity and Metabolic Causes
The Merck Manuals Online Medical Library notes that some cases of resistant vitamin D and calcium deficiency are due to hereditary conditions affecting the pathway of metabolism and mechanism of action of vitamin D. Some patients can not convert vitamin D to its active form, either due to a hereditary defect or due to chronic kidney disease. In other instances, patients develop rickets because of a hereditary defect in the vitamin D receptors in their bodies. In effect, these patients have high circulating levels of active vitamin D but their organs cannot make use of it because of the defective receptors.
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