zinc supplementation. Lukaski et al. (1983)
reported no correlation between blood zinc
levels and V
.
o2max.. In a later report, however,
Lukaski (1995) presented evidence that zinc
intake was significantly related to swim times in
collegiate swimmers. Another study reported
that 50 mg · day–1of zinc had no effect on physio-
logical changes or time to exhaustion during
a run at 70–75% V
.
o2max.(Singhet al. 1994).
Krotkiewski et al. (1982) examined the effect of
135 mg zinc · day–1for 2 weeks on measures of
knee extension strength. The supplement
resulted in a significant increase in isokinetic
strength at fast angular velocities (180 s–1) only
and in isometric endurance, but no change in
dynamic endurance or isokinetic strength at
60 or 120 s–1. However, no studies have sub-
stantiated these findings regarding strength
improvement.
The popularity of zinc supplements arises
from their purported effect of increasing muscle
mass. The reason for this belief may partially
stem from the Krotkiewski et al. (1982) study,
since the increase in strength could be due to
increased muscle mass, although this was not
assessed. Animal and human studies have
found that zinc deficiency results in a stunting of
growth that can be reversed by zinc supplements
(Prasad 1991). Also, a relationship between
zinc deficiency and lower testosterone in patients
who were ill has been reported (Prasad et al.
1981). However, zinc supplementation has not
been found to have any positive effect on testos-
terone or muscle growth in individuals with ade-
quate or near adequate status.
Nishiyamaet al. (1996) examined haematologi-
cal factors in two groups of female endurance
runners, one with impaired zinc status and one
with normal status. The zinc-deficient group had
a lower number of red blood cells, serum haemo-
globin and iron. They were then given an iron
or an iron-plus-zinc supplement. The subjects
who received the iron-plus-zinc showed a
greater increase in haemoglobin and red blood
cells. The authors suggested that zinc plays a role
in haematopoiesis and can prevent anaemia.
The effect of zinc supplementation for 6 days
on exercise-induced changes in immune function
in male runners has been assessed (Singh et al.
1994). By examining the respiratory burst activ-
ity of neutrophils after exercise, it was found that
the supplement compared to a placebo blocked
the increase in reactive oxygen species which
cause an increase in free radical damage. Free
radicals have an unpaired electron, making them
highly reactive and damaging to the cell. These
data suggest that supplemental zinc may serve
as an antioxidant, but because the supplement
suppressed T-lymphocyte activity, it may also
increase susceptibility to infection.Summary
Many athletes are not ingesting the recom-
mended quantities of zinc, and zinc status may
be compromised. However, accurate assessment
of zinc status or balance in athletes is lacking.
Studies that examined the effects of acute and
chronic exercise on blood zinc levels are equivo-
cal, and the disparate results are unexplained.
Well-controlled studies are needed to examine
the changes in blood levels of zinc induced by
various types of exercise and redistribution path-
ways. Even though exercise may result in some
loss of zinc in sweat and urine, it is not known
whether the body will adapt to this loss by
increasing retention.
Zinc supplementation at levels in excess of the
RDA may have negative consequences (Lukaski
1997). Excessive zinc can inhibit copper absorp-
tion, reduce high-density lipoprotein levels, and
prevent an exercise-induced increase in high-
density lipoproteins (Lukaski 1997). Female
athletes and many male athletes, especially vege-
tarians and those maintaining low body weights,
should be concerned that they ingest foods rich
in zinc, or take a multivitamin-mineral supple-
ment with micronutrient concentrations equal or
less than the RDA.Copper
Copper is a component of many metalloenzymes
in several key reactions (Hunt & Groff 1990).
The copper-containing protein, ceruloplasmin,
serves as a multifaceted oxidative enzyme play-