NUTRITION IN SPORT

organs, anatomical systems or the host him/
herself to perform optimally the intentioned,
nutrient-dependent function (Solomons & Allen
1983). Vitamins—like all micronutrients—are
needed directly or indirectly (because of activity
on structural integrity) for innumerable func-
tions. Metabolic functions may be viewed from
an isolated, molecular viewpoint (i.e. a single
biochemical reaction in a single metabolic path-
way), or from a perspective of the entire human
body.
The metabolic functions of vitamins required
in sports are mainly those needed for production
of energy and for neuromuscular functions
(skills). Physical performance involve several
metabolic pathways, all including several bio-
chemical reactions. The relation between vitamin
supply and functional capacity is S-shaped or
‘bell-shaped’, depending on whether the exami-
nation is extended to megadoses (Fig. 20.1)
(Brubacher 1989). The core in the above rela-
tion is that the output (functional capacity) is
not improved after the ‘minimal requirement
for maximal output’ is reached (Brubacher 1989).

In contrast, overvitaminosis may in some

cases reduce the output below the maximal

level.

Different body functions (single biochem-

ical reactions, metabolic pathways, function of

anatomical systems, and function of the host

him/herself) reach their maximal output at

different levels of supply. In other words,

the supply needed for optimal function of an

anatomical system (e.g. the muscle) may be quite

different from the supply needed to maximize

the activity of a single enzyme (Solomons &

Allen 1983).

Short-term inadequacy of vitamin intake is

characterized by lowering of vitamin concentra-

tions in different tissues and lowering of certain

enzyme activities (Fig. 20.2) (Piertzik 1986).

However, functional disturbances (such as

decreased physical performance capacity)

appear later (Solomons & Allen 1983; Fogelholm

1995). In the opposite case, very large vitamin

intakes increase the body pool and activity of

some enzymes, but do not necessarily improve

functional capacity (Fogelholm 1995).

vitamins: metabolic functions 267

Table 20.1Summary of the most important effects of vitamins on body functions related to athletic performance.

Cofactors

for energy Nervous Haemoglobin Immune Antioxidant Bone

metabolism function synthesis function function metabolism

Water-soluble

vitamins

Thiamin X X

Riboflavin X X (X)

Vitamin B 6 XX XX

Folic acid X X

Vitamin B 12 XX

Niacin X X

Pantothenic X

acid

Biotin X

Vitamin C (X) X X

Fat-soluble
vitamins
Vitamin A X X
Vitamin D X
Vitamin E X X