nicotinamide adenine dinucleotide phosphate
which serve as hydrogen acceptors and donors
in glycolysis, fatty acid oxidation, and in the elec-
tron transport system. Niacin deficiency may
possibly impair glycolysis and/or the oxidation
processes of the citric acid cycle, so both anaero-
bic and aerobic type performances may be
affected adversely. On the contrary, niacin sup-
plementation in high doses may suppress free
fatty acid release through decreased lipolysis
which would result in a decreased availability of
a major fuel source, forcing the muscle to rely
more on its glycogen stores, and this may also
adversely affect prolonged exercise performance
(Williams 1985; Clarkson 1991).
The RDA for niacin is expressed as niacin
equivalent because niacin may be synthesized
in the body from tryptophan (1 mg of niacin is
equivalent to 60 mg of dietary tryptophan). The
requirement for niacin also usually is linked to
energy intake which means that athletes who
have a large energy intake need a proportionally
higher niacin intake. The RDA for niacin in the
general population has been set at 6.6 mg ·
4.2 MJ–1(1000 kcal–1) or about 14–19 mg · day–1for
adults. The RDA for niacin for Chinese athletes
has been set at 10 times that for riboflavin: 25 mg ·
day–1. Since niacin is widely distributed in plant
and animal food sources, most athletes show no
evidence of niacin deficiency except those who
have a chronically reduced dietary intake for
weight control. Niacin deficiency symptoms may
easily be solved by a well-balanced diet without
recourse to specific supplementation. Good
sources of niacin include poultry, meats, grain
products, peanuts, yeast, fish, etc. (see Chapter
20).
Pyridoxine (vitamin B 6 )
Vitamin B 6 exists in five forms: pyridoxine,
pyridoxal, pyridoxamine, pyridoxal phosphate
(coenzyme form), and pyridoxamine phosphate
(coenzyme form). The pyridines function in
protein and amino acid metabolism, gluconeo-
genesis, and in formation of haemoglobin, myo-
globin, and cytochromes, and are also a
component of glycogen phosphorylase which
plays a key role in glycogenolysis. Because
exercise stresses metabolic pathways that use
vitamin B 6 , it has been suggested that the
requirement for this vitamin is increased in
athletes and active individuals (Manore 1994).
Vitamin B 6 is essential in coenzymes related to
nitrogen metabolism, and the requirement for
vitamin B 6 is closely related to dietary protein
intake. Currently the RDA for vitamin B 6 is
2.0 mg · day–1 for men and 1.6 mg · day–1 for
women. If the dietary protein is more than 100 g ·
day–1, then the intake of vitamin B 6 should be
more than 2 mg · day–1(US National Research
Council 1989; Manore 1994). Although the
requirement for vitamin B 6 appears to increase
when a high protein diet is consumed, vitamin B 6
is found in meats and other animal foods, and
sufficient may be provided if these foods are the
major source of protein in the diet. The incidence
of acute toxicity of vitamin B 6 is low. Intakes of
117 mg of vitamin B 6 for more than 6 months can
result in neurological impairment (US National
Research Council 1989). Chronic ingestion of 2–
6 g pyridoxine · day–1has been shown to result in
sensory neuropathy (Clarkson 1991). Some
studies reported that athletes have low or mar-
ginal dietary intakes of vitamin B 6 (Hicksonet al.
1987; Weight et al. 1988b; Guilland et al. 1989),
whereas others reported mean intakes at or
above the RDA (Faber et al. 1991; Singh et al.
1992a, 1992b, 1993). Young female athletes and
those participating in sport training emphasizing
low body weights need to be monitored regu-
larly. Deficiencies should be corrected by a well-
balanced diet, and if necessary, small amount of
supplements within the RDA (Rokitzki et al.
1994a). There is no evidence suggesting that sup-
plementation enhances athletic performance, so
the use of vitamin B 6 as an ergogenic aid is con-
traindicated. Good food sources include meat,
poultry, fish, whole grain, peanuts, soybeans,
seeds, yeast and eggs (see Chapter 20).Cobalamin (vitamin B 12 )
Vitamin B 12 is involved in a variety of metabolic