High-altitude training. Aspects of haematological
adaptation.
Berglund B
Department of Medicine, Karolinska Hospital, Stockholm, Sweden.
Physical training at high altitude improves performance at high altitude.
However, studies assessing
performance improvements at sea level after training at higher altitudes
have produced ambiguous
and inconclusive results. Hypoxia-induced secondary polycythemia is
a major contributor to
increased work capacity at altitude. The common finding upon exposure
to hypoxia is a transient
increase in haemoglobin concentration and haematocrit because of a
rapid decrease in plasma
volume followed by an increase in erythropoiesis per se. Both nonathletes
and elite endurance
athletes have maximal reticulocytosis after about 8 to 10 days at moderate
altitude. Training periods
of 3 weeks at moderate altitudes result in individual increase of haemoglobin
concentration of about
1 to 4%. A more accentuated increase in haemoglobin can be obtained
with longer sojourns at
moderate altitude. The normal erythropoietin reaction upon exposure
to hypoxia comprises initially
increased levels followed by a decrease after about 1 week. Thus, the
maintenance of a high
erythropoietin concentration is not a prerequisite for a sustained
increase in erythrocyte formation at
high altitude. The main pharmacological modulator of erythropoietin
production seems to be
adenosine. But modulators such as growth hormone and catecholamines
may also potentiate the
effect of hypoxia per se on erythropoietin production. On the other
hand, there is a risk that the
stress hormones may induce a relative depression of the bone marrow
particularly in the early phase
of altitude training when the adaptation is minimal and the stress
reaction is most accentuated. The
most important 'erythropoiesis-specific' nutrition factor is iron availability
which can modulate
erythropoiesis over a wide range in humans. Adequate iron stores are
a necessity for haematological
adaptation to hypoxia. However, at moderate altitude, there is a need
for rapid mobilisation of iron
and even if the stores are normal there is a risk that they cannot
be mobilised fast enough for an
optimal synthesis of haemoglobin. Data from healthy athletes training
at moderate altitudes suggest a
true increase in haemoglobin concentration of about 1% per week. Complete
haematological
adaptation occurred when sea level residents have similar haemoglobin
concentrations at moderate
altitude compared with residents. The normal difference in haemoglobin
concentrations can be
estimated to be about 12% between permanent residents at sea level
and at 2500m above sea level.
This difference indicates a necessary adaptation time of about 12 weeks.
If the training period at
moderate altitude must be shorter, several sojourns at short intervals
are recommended. The
important factor in haematological adaptation in athletes at moderate
altitude is hypoxia.