with known nutritional deficiencies. In contrast,
several other studies did not find an associa-
tion between increased erythrocyte glutathione
reductase activity and maximal oxygen uptake
(Suboticanec-Buzinaet al. 1984; Weight et al.
1988a, 1988b; Suboticanec et al. 1990; Singh et al.
1992a, 1992b), exercise-induced lactate appear-
ance in the blood (Weight et al. 1988a, 1988b;
Fogelholmet al. 1993b), work efficiency (Powers
et al. 1987) or grip strength (Suboticanec et al.
1989).
safety of elevated
riboflavin intake
As with thiamin, there is no evidence of any
harmful effects even with oral doses exceeding
100 times the recommended daily intake (Marks
1989). Riboflavin in large doses may cause a
yellow discoloration of the urine which might
obviously cause concern in people not aware of
the origin of the colour (Alhadeff et al. 1984).
Vitamin B 6
chemistry and
biochemical functions
Vitamin B 6 is a common name for pyridoxine,
pyridoxamine and pyridoxal (McCormick 1986).
Pyridoxine hydrochloride is the synthetic phar-
maceutical form of vitamin B 6 (Halsted 1993). All
three chemical forms of vitamin B 6 are metaboli-
cally active after phosphorylation. The most
common cofactor in human body is pyridoxal
phosphate (PLP) (Driskell 1984). It is a prosthetic
group of transaminases, transferases, decarboxy-
lases and cleavage enzymes needed in many
reactions involving for instance protein break-
down (Manore 1994). PLP is also an essential
structural component of glycogen phosphory-
lase, the first enzyme in glycogen breakdown
pathway (Allgood & Cidlowski 1991). In fact,
muscle-bound PLP represents 80% of the approx-
imately 4-g body pool of vitamin B 6 (Coburnet al.
1988).
In addition to energy metabolism, vitamin B 6
is needed for synthesis and metabolism of many
neurotransmitters (e.g. serotonin), and in the
development and maintenance of a competent
immune system (Allgood & Cidlowski 1991).
PLP-dependent enzymes are involved in syn-
thesis of catecholamines (Driskell 1984), and
perhaps in regulation of steroid hormone action
(Allgood & Cidlowski 1991). Vitamin B 6 is also
needed for the synthesis of aminolevulic acid, an
intermediate compound in the formation of the
porphyrin ring in haemoglobin (Manore 1994).
From a physiological viewpoint, vitamin B 6
depletion could decrease glycogen breakdown
and impair capacity for glycolysis and anaerobic
energy production. Because glycogen phos-
phorylase is not a rate-limiting enzyme in
glycogenolysis, small changes in its activity
would, however, not affect glycogen me-
tabolism. Severe depletion would also affect
haemoglobin synthesis, and impair oxygen
transport in the blood. The contribution of amino
acids in total energy expenditure is not likely to
exceed 10%, even in a glycogen depleted state.
Therefore, it is unclear how an impairment in
amino acid breakdown would affect physical
performance.
The activity of two enzymes involved in ery-
throcytic protein metabolism, namely aspartate
aminotransferase (ASAT) and alanine amino-
transferase (ALAT), are used as indicators of
vitamin B 6 status (Bayomi & Rosalki 1976;
Driskell 1984). The principle of the assay, with
and without in vitrosaturation, is similar to that
explained earlier for thiamin (transketolase) and
riboflavin (glutathione reductase) (Bayomi &
Rosalki 1976).supply and metabolic functions
In male wrestlers and judo-athletes, a decrease in
the erythrocyte ASAT activity indicated deterio-
ration in vitamin B 6 supply during a 3-week
weight-loss regimen (Fogelholm et al. 1993a).
Maximal anaerobic capacity, speed or strength
were, however, not affected. Coburn et al. (1991)
showed that the muscle tissue is, in fact, quite
resistant to a 6-week vitamin B 6 depletion.