Vitamin D

Vitamin D is the most potentially toxic vitamin in human nutrition, with an upper limit (UL) of 50 micrograms per day. We can obtain the vitamin in an inactive form from food and sunlight exposure. Ultraviolet radiation (sunlight) exposure of the skin transforms a cholesterol derivative (7-dehydrocholesterol) into an inactive form of vitamin D called cholecalciferol. To be functional, this inactive form of vitamin D must be activated by the kidneys. Therefore, kidney disease may be the cause of vitamin D-related disorders. Dietary sources of vitamin D include eggs, fortified milk, liver, butter, and margarine. Cod liver oil, which was once given commonly as a supplement, is a concentrated source of the vitamin. The adult DRI for vitamin D is 5 micrograms per day of cholecalciferol or 200 international units (IU) of vitamin D (1 microgram of calciferol equals 40 IU of vitamin D).

Vitamin D promotes growth and mineralizes bones and teeth by increasing the absorption of calcium and phosphorus. A diet with an adequate intake of calcium and phosphorus but without adequate vitamin D leads to calcium and phosphorus deficiency. The childhood deficiency disease rickets and the adult deficiency disease osteomalacia are diseases of calcium deficiency that are caused either by inadequate levels of vitamin D or the inability to convert vitamin D to the active (functional) form. However, because of vitamin D's potential toxicity, caution must be taken not to consume too much. Excess vitamin D intake may lead to vomiting, diarrhea, weight loss, kidney damage, high blood calcium levels, and death.

No existing studies suggest that high levels of vitamin D intake, either through foods or supplementation, aid athletic performance. There is also no theoretical basis for performance enhancement to occur. It is possible, however, that vitamin D may play an indirect role in injury resistance. Athletes in some sports may have dramatically lower sunlight exposure because all training sessions take place inside a building. This lower UV exposure may reduce vitamin D synthesis to a point where both growth and bone density are negatively affected. Lower bone densities are known to place athletes at higher risk for developing stress fractures, an injury that can end an athletic career.^43-45 In a survey of U.S. national team gymnasts, the factor most closely related to bone density was sunlight exposure. Those with higher densities had the greatest exposure.⁴⁶ Also, sunlight exposure was more important as a predictor of bone density in this group than vitamin D or calcium intake from food.