third of the male and two thirds of the female
runners (but few controls) were taking iron sup-
plements, and when those taking iron were
excluded, sample sizes were small. When 100
female collegiate athletes were compared with 66
non-athletic controls, differences in iron balance
were minor (Risser et al. 1988), but only 8% of the
athletic women were distance runners.
Despite these problems, it seems likely that
among athletes, distance runners at least have
some reduction in iron stores compared to non-
athletes. So concluded a comprehensive survey
in South Carolina of 111 adult female runners vs.
65 inactive controls (Pate et al. 1993). The mean
ferritin of the runners was lower than that of the
controls (25mg·l–1vs. 36mg·l–1), and twice as
many runners as nonrunners (50% vs. 22%) had
ferritin levels of less than 20mg·l–1. Anaemia,
however, was rare (3%) in both groups.
So distance runners—especially female
runners—tend to have lower iron stores than
non-athletes and seem prone to iron deficiency
anaemia. But frank iron deficiency (ferritin
< 12 mg·l–1) among athletes, even among female
runners, is not as common as once thought, and
anaemia is not clearly more common in athletes
than non-athletes. Then, too, most ultraen-
durance athletes studied—especially males—
have adequate iron stores (Burke & Read 1987;
Singhet al. 1993). Most iron studies are on
runners; we need studies of women in ‘low-
bodyweight’ sports such as ballet, gymnastics,
diving and ice skating. But future studies of iron
status are apt to be biased by the increasing use of
iron supplements by athletes.
Causes of low ferritin level in athletes
We have established that athletic training tends
to decrease serum ferritin level (i.e. iron stores)
and that some athletes, particularly female dis-
tance runners, may be prone to iron deficiency
anaemia. Now the question becomes: Howdoes
training deplete iron stores? In theory, training
can reduce serum ferritin level in at least eight
ways, not all pathophysiological:
- haemodilution;
- increase in myoglobin mass;
- increase in red cell mass;
- inadequate iron intake;
- gastrointestinal bleeding;
- iron loss in sweat;
- iron loss in urine;
- shift of iron to liver.
Haemodilution
As reviewed above, training—especially
endurance training—can expand baseline
plasma volume by as much as 10–20%, and if the
training is regular, this expansion is maintained.
This adaptation to exercise dilutes haemoglobin
(pseudoanaemia). It seems likely, if not yet
demonstrated, that the expansion of plasma
volume in a highly fit athlete dilutes serum fer-
ritin concentration by 10% or more.Increase in myoglobin mass
When adolescent boys undergo a growth spurt,
stored iron is shifted into the increased mass of
myoglobin, lowering ferritin. This must also
occur in athletes who develop muscles by train-
ing, and surely accounts for much of the fall in
ferritin with strength training (Schobersberger
et al. 1990). This likely shifting of iron from stores
to functional compartment (myoglobin) has not
been quantified in athletes, but seems evident.
To paraphrase Yogi Berra, you can observe a lot
by just looking.Increase in red cell mass
Because cross-sectional studies show an
expanded red cell mass in athletes—men and
women—compared to non-athletes (Dill et al.
1974; Brotherhood et al. 1975; Weight et al. 1991),
it seems likely that training can increase red cell
mass. Yet longitudinal studies are inconclusive.
Restricting it to studies that employ radiola-
belled red cells, the best way to gauge red cell
mass, two studies are positive and two ‘nega-