ASP drink was improved. However, when the body's pool of vitamin C is greater than 1500 mg, the efficacy of kidney reabsorption decreases, and vitamin C is excreted into the urine (Hunt and Groff, 1990). Thus, increased vitamin C in the urine may simply indicate that the athletes had high levels of vitamin C already, and the excess "spilled over" into the urine.
Vitamins A, D, E, and K are fat-soluble vitamins, and these can be stored in appreciable amounts in the fat stores of the body. Because vitamin D is involved in calcium metabolism and vitamins A and E can function as antioxidants, these supplements may be important to exercise or work performance. Because no studies have been uncovered that examined the relationship of vitamin K (a vitamin necessary for blood clotting) with exercise performance or heat stress, this vitamin will not be discussed in the following sections.
Night blindness was recognized by the ancient Egyptians and was treated by adding liver to the diet or by topically applying liver extract to the eyes (J. A. Olson, 1990). In 1914 the compound now known as vitamin A was found to prevent night blindness. Interestingly, the early Egyptian remedies had been lost over the years so that in the nineteenth century, night blindness plagued armies throughout the world (J. A. Olson, 1990).
Vitamin A designates a group of compounds including retinol, retinaldehyde, and retinoic acid. The body's need for vitamin A can be met by intake of preformed retinoids with vitamin A activity, which are generally found in animal products (National Research Council, 1989). Also, the need can be met by ingesting carotenoid precursors of vitamin A (beta-carotene, alphacarotene, and cryptoxanthin) commonly found in plants (National Research Council, 1989). The primary function of vitamin A is for maintenance of vision. Vitamin A is also involved in the growth process and the body's immune response. Beta-carotene, the major carotenoid precursor of vitamin A, plays a role as an antioxidant.
Blood levels of vitamin A (retinol) provide a relatively good index of total body stores. When the liver stores of vitamin A are low, the plasma levels fall (Olson, 1984). The few studies that have examined vitamin A status of athletes found no deficiencies (Guilland et al., 1989; Weight et al., 1988). The absence of deficiencies is most probably due to the body's relatively large storage capacity for vitamin A.
Only one study has examined the effect of vitamin A supplementation on exercise performance. Five men were placed on a vitamin A-deficient