Vitamin D

Vitamin D is a fat-soluble vitamin found in foods and also synthesized in the body after exposure to ultraviolet rays from the sun. Several forms of vitamin D have been described, but the two major physiologically relevant ones are vitamin D2 and vitamin D3. 


Vitamin D2 (ergocalciferol) is a synthetic form of vitamin D that is produced by irradiation of plant steroid ergosterol in plants. Vitamin D3 (cholecalciferol) is the naturally occurring form of vitamin D produced from 7-dehydrocholesterol when the skin of animals and humans is exposed to sunlight. Both forms are used in supplements.  The main function of vitamin D is in maintaining calcium and phosphorus homeostasis by its influence on the intestines, kidneys, and bones. Other physiologic functions of vitamin D in the brain, heart, pancreas, mononuclear cells, activated lymphocytes, and skin are still unknown, but its biologic function has been identified as a potent antiproliferative and prodifferentiation hormone.  In children, vitamin D deficiency results in a bone disease called rickets, which is characterized by failure to properly mineralize bone tissue. The physical symptoms of rickets include bowed legs, knock knees, curvature of the spine, and thoracic and pelvic deformities. In adults, vitamin D deficiency leads to demineralization of the skeleton causing osteomalacia. Vitamin D deficiency causes a decreased in calcium concentrations in blood, which increases the production and secretion of parathyroid hormone (PTH).  


Elevated PTH concentrations lead to a normal bone matrix turnover, resulting in a mineralization defect. Vitamin D deficiency may be caused by poor dietary intake or limited exposure to sunlight. Vitamin D interacts with iron; therefore, iron deficiency can induce a concomitant decrease in vitamin D absorption.  The recommendations for vitamin D, as AIs, for 19–50 year-old and 51-year-old adults are 5 and 10 μg/d, respectively. Vitamin D contents of food and dietary supplement labels are expressed as IU. 

The biological activity of 1 μg vitamin D is equal to 40 IU. Natural sources of vitamin D in foods are few. Fish and fish oils are the richest sources. Almost all of the vitamin D intake in the United States comes from fortified milk products and other fortified foods such as breakfast cereals. Vitamin D toxicity may occur with chronic intake of large doses of vitamin D supplements or fish oils, but overexposure to sunlight does not cause vitamin D toxicity. The symptoms of vitamin D toxicity include nausea, vomiting, poor appetite, constipation, weakness, and weight loss. Consequences of severe hypervitaminosis D can lead to hypercalcemia, hypercalcinuria, and possible calcification of soft tissues such as the kidneys.12 The UL for vitamin D established by IOM is 50 μg/d for all children and adults. There is little evidence that the deficiency or supplementation of vitamin D influences exercise performance. Vitamin D deficiency was indicated in one-third of female gymnasts aged 10–17 years (n =18). Because vitamin D does help maintain bone strength and mineralization through regulation of calcium and phosphate homeostasis, monitoring of vitamin D status may be needed in some groups of athletes, such as gymnasts and possibly other indoor athletes. However, intake of large doses may result in vitamin D toxicity.