renal and cardiovascular damage (Davies 1989).
Intakes of 10 times the recommendation should
not be exceeded (Marks 1989). High calcium
intakes may enhance the toxicity of vitamin D.
vitamin k
Vitamin K is the general name of com-
pounds containing a 2-methyl-1,4-naptho-
quinone moiety. Vitamin K is needed for
formation of prothrombin in the blood, and
defective blood coagulation is the only major
sign of deficiency (McCormick 1986). No evident
associations between vitamin K and exercise
metabolism are to be found. Further, very little is
known about the safety of orally administered
vitamin K (Marks 1989).
Conclusion
Vitamins are extremely important for the func-
tional capacity of the human body (see Table
20.1). Many B vitamins participate as enzyme
cofactors in pathways of energy metabolism and
in neuromuscular functions. Folic acid and vita-
mins B 12 and B 6 are needed for haemoglobin syn-
thesis, and consequently for optimal oxygen
transport from the lungs to the working tissues.
Some vitamins (e.g. vitamins A, B 6 and C) are
required for normal immune function. Finally,
vitamins A, C and E have important antioxidant
properties.
Studies have clearly shown that the output of
many vitamin-dependent functions both in vivo
(e.g. breath pentane exhalation) and in vitro(e.g.
erythrocyte enzymes) may increase after supple-
mentation above normal dietary intake. Simi-
larly, the output of many functions is decreased
at marginal vitamin supply. However, the output
of the entire human body (e.g. athletic perfor-
mance) was only rarely related to marginal
vitamin supply or to supplementation.
The dietary intake of vitamins is not high
enough to ensure maximal output of many iso-
lated functions. However, it appears that the
vitamin intake, at least in developed countries, is
above the minimal requirement for maximal
output of the human body. Consequently, the
evidence that vitamin supplementation would
increase athletic performance is not very encour-
aging. On the other hand, the risk for toxic intake
also seems to be marginal.
The above conclusions are made with some
reservations. First, the results on marginal
supply and physical function were mostly
extrapolated from nonathletic subjects. More-
over, there were hardly any data on athletes from
developing countries. Finally, many studies had
a very low statistical power, that is, there were
too few subjects to detect anything else than sub-
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