(1976) reported a similar observation among Finnish Skolt children. Hannon et al. (1976) reported a transient reduction of thiamin intake by women during their sojourn to 4,300 m (14,098 ft). However, there was an increased intake during Operation Everest II (Rose et al., 1987) and during the military exercise in Bolivia (Edwards et al., 1991). It has been suggested that 10 to 14 days are required for consumption of a thiamin-deficient diet to result in the appearance of deficiency symptoms.
Due to thiamin's role in the metabolism of carbohydrate, the RDA for thiamin is based on total anticipated energy intake from carbohydrate and some amino acids (NRC, 1989). There have been many reports of increased intake of carbohydrate, at the expense of fat intake, at high altitudes (Frisancho, 1981; Ward et al., 1989b). Thus, the suggested micronutrient intake goal of this vitamin for those working either in the cold or at high altitudes is set at 3 mg, which is twice the existing RDA of 1.5 mg/d and higher than the MRDA of 1.6 mg/d. The suggested micronutrient intake goal of 3 mg/d is a prudent dose considering the low toxicity (NRC, 1989) of this vitamin.
Niacin as nicotinamide adenine dinucleotide (NAD) functions mainly to produce ATP from glycolysis and from the Krebs cycle. It is also necessary for the conversion of pyruvate to acetyl-CoA, in the hexose monophosphate shunt, and in the synthesis of fatty acids from acetyl-CoA. A deficiency of niacin could impair glycolysis and the Krebs cycle, whereas excessive niacin supplementation may suppress the release of free fatty acids from adipose tissue through decreased lipolysis, resulting in a decreased availability of a major fuel source for utilization during strenuous exercise (Bulow, 1981; Carlson et al., 1963). Thus, if muscle glycogen levels are low, as may occur during prolonged physical exertion in the cold or at altitude, excessive niacin supplements could actually impair physical performance (Williams, 1989).
An increase in niacin intake was reported among participants in Operation Everest II (Rose et al., 1987) during their simulated exposure to high altitudes and by U.S. Army soldiers during the exercise in Bolivia (Edwards et al.,