1990). Erythrocyte NAD concentration or levels of 2-pyridone may be more sensitive indicators of niacin depletion (Swendseid and Swendseid, 1990).
Some evidence suggests that exercise may increase the niacin requirement (Keith, 1989). Because most adult athletes have shown no evidence of niacin deficiency, the increased requirement probably is satisfied by the athlete's diet. Chronic ingestion of niacin above the recommended dietary allowance (RDA) (National Research Council, 1989) is not recommended, because large doses are often associated with undesirable side effects, such as flushing, liver damage, increased serum uric acid levels, skin problems, and elevated plasma glucose levels (Hunt and Groff, 1990). Niacinamide in large doses is not harmful. Acute ingestion of nicotinic acid (3 to 9 g per day) has also been shown to prevent the release of fatty acids (Keith, 1989; National Research Council, 1989), which may adversely affect endurance performance.
In a double-blind placebo-controlled experiment, Hilsendager and Karpovich (1964) found that 75 mg of niacin had no effect on arm or leg endurance capacity. Bergstrom et al. (1969) compared the perception of a work load before and after subjects were given niacin, 1 g intravenously and 0.6 g perorally. After the supplementation, the subjects perceived the work load to be heavier. Niacin can decrease free fatty acid mobilization (Carlson and Oro, 1962; Williams, 1989), which may explain the negative effects of the niacin supplement. A decrease in free fatty acid mobilization would force the muscle to rely more on its muscle glycogen stores. In fact, Bergstrom et al. (1969) found that muscle glycogen content was lower in postexercise biopsy samples taken from subjects who had received the niacin supplements than with control subjects.
The only information with regard to niacin requirements in a hot environment comes from an early study that found that nicotinic acid was lost in the sweat in significant amounts (100 µg per 100 ml; Mickelsen and Keys, 1943). However, later studies did not agree with this finding (Mitchell and Edman, 1951; Robinson and Robinson, 1954). Nicotinic acid is considered to be lost in concentrations of 20 µg or less per 100 ml of sweat (Mitchell and Edman, 1951). As with thiamin and riboflavin, niacin intake should be proportional to energy intake (6.6 mg niacin per 1000 kcal). If energy intake is increased to meet the demands of exercise or work in a hot environment, then niacin should be increased as well.
Vitamin B6 is composed of three natural compounds—pyridoxine, pyridoxamine, and pyridoxal (Merrill and Burnham, 1990)—that function in protein hemoglobin, myoglobin, and cytochromes. The coenzyme form of B6 is and amino acid metabolism; in gluconeogenesis; and in formation of