metabolic functions even in well-nourished
humans: after chronic vitamin E supplementa-
tion (typical dose, >100 mg · day–1), indices of
exercise-induced lipid peroxidation, mainly
serum malondialdehyde concentration and
breath pentane exhalation, are reduced (Jenkins
1993; Rokitzki et al. 1994; Kanter & Williams 1995;
Tiidus & Houston 1995). There are some interpre-
tation problems, however, mainly because of the
lack of specificity and/or reliability of most indi-
cators of lipid peroxidation (Kanter & Williams
1995).
In one study (Simon-Schnass & Pabst 1988),
vitamin E supplementation helped to maintain
aerobic working capacity at very high altitude
(>5000 m). Other studies have not conclusively
proven that vitamin E intake exceeding daily rec-
ommendations would have any beneficial effects
on athletic performance (Rokitzki et al. 1994;
Kanter & Williams 1995; Tiidus & Houston 1995).
safety of elevated
vitamin e intake
Vitamin E, in contrast to two other fat-soluble
vitamins (A and D), is apparently not toxic for
healthy individuals (Machlin 1989). The safety
factor for long-term administration is at least 100
times the recommended daily intake—that is, at
least 1 g daily in oral use (Marks 1989). Over-
doses of vitamin E are contraindicated only in
individuals receiving vitamin K antagonists
(Machlin 1989).
Other fat-soluble vitamins
vitamin a
The two natural forms of vitamin A are retinol
and 3-dehydroretinol, of which retinol is the
more abundant in the human body (Bates 1995).
All higher animals can convert plant-derived
carotenes and cryptoxanthin to retinol. The most
common and effective provitamin in the human
diet is b-carotene. Retinol is transported in
chylomicrons from the gut, and later bound to a
protein (retinol-binding protein, RBP). Several
276 nutrition and exercise
hundred milligrams of retinol are stored in the
liver (McCormick 1986).
The best known function of retinol is as an
essential component in vision. In vitamin A defi-
ciency, worsening of night vision is an early clini-
cal sign (McLaren et al. 1993). Both retinol and
b-carotene are capable of scavenging singlet
oxygens and hence act as antioxidants (Bates
1995). Vitamin A is also important for immunity.
The literature provides no evident data on
relations between vitamin A status and physical
performance.
Chronic toxicity of retinol will cause joint or
bone pain, hair loss, anorexia and liver damage.
The safety level for chronic use is estimated to be
10 times the recommended daily intake—that is,
10 mg retinol daily (Marks 1989). Because of an
increased risk for spontaneous abortions and
birth defects (Underwood 1989), the safe level
during pregnancy might be only four to five
times the daily recommendation (Marks 1989). b-
carotene, in contrast to retinol, is not toxic. This
provitamin is stored under the skin and it is con-
verted to retinol only when needed.vitamin d
In the diet, vitamin D occurs mainly as cholecal-
ciferol (D 3 ), which can also be synthesized in skin
after ultraviolet irradiation (Fraser 1995). In the
liver, D 3 is hydroxylated to 25-hydroxycholecal-
ciferol (25-OH-D 3 ), and further in the kidneys
to 1,25-dihydroxycholecalciferol (1,25-(OH) 2 -D 3 ,
active form) or to 24,25-dihydroxycholecalciferol
(24,25-(OH) 2 -D 3 , inactive form). Vitamin D is
stored in several parts in the body, e.g. in the liver
and under the skin.
Vitamin D stimulates calcium absorption in
the small intestine and increases calcium reab-
sorption by the distal renal tubules. Deficiency
results in bone demineralization (rickets and
osteomalacia), and this may eventually increase
the risk for stress fractures (Fraser 1995).
Vitamin D is potentially toxic, especially
for young children, causing hypercalcaemia,
hypercalciuria, soft-tissue calcification, anorexia
and constipation, and eventually irreversible