Biology Reference
In-Depth Information
A paradox exists in exercise physiology. Athlete fitness is negatively corre-
lated with hematocrit, yet increasing the hematocrit by transfusion or by dop-
ing with erythropoietic proteins improves VO
2max
and performance [30, 31].
Exercise training expands plasma volume and decreases hematocrit and hemo-
globin concentrations, thus causing a negative correlation between hematocrit
and fitness [30-33]. Values for hematocrit, hemoglobin, and red blood cell
count are in the lower range of normal for athletes [30, 34]. In one study, ath-
letes with the lowest hematocrits had the highest aerobic working capacity and
isometric adductor strength [31].
At first it was thought that the hemoconcentration that accompanies
endurance competitions would lead to increased blood viscosity and that this
would severely limit cardiac output and thereby limit the usefulness of
rHuEPO. Hematocrit is highly correlated with blood viscosity [31]; however,
because hematocrits of 50% and more are often recorded among competing
athletes, other factors must be operating. In one study of athletes competing in
an ultra-marathon, the immediate post-race hematocrits were not different
from the baseline values, and on the day after the competition, the values were
lower than baseline [30]. Apparently replenishing lost fluid and electrolytes
during very long events is sufficient to avoid serious increases in viscosity.
The notion that dehydration during long endurance events will lead to
hemoconcentration and increased blood viscosity and,therefore, result in ath-
letes experiencing strokes and other complications of increased blood viscosi-
ty is difficult to confirm. No media reports exist of athletes experiencing such
catastrophes during endurance events. The cluster of deaths in cyclists in the
late 1980s [15] could be related to increased blood viscosity, but the athletes
were not competing at the time of death.
Autologous blood infusion increases performance
Many clinical experiments have shown that transfusions increase human per-
formance [24, 25,35-37]. The most convincing study [25] used a double-
blind, sham-infusion controlled, cross-over design. Highly trained elite ath-
letes received 900 mL of autologous red blood cells collected and frozen
approximately seven weeks earlier; 24 hours after the transfusions, the athletes
experienced a 35% increase in run-time to exhaustion, a 5% increase in
VO
2 max
, and a 7% increase in hemoglobin concentration. The authors con-
cluded that the limit to aerobic activity was the transport of oxygen to muscle.
In another study,this time using 10-km race-time as the outcome rather than
physiological measurements, six highly trained but not elite athletes improved
their mean run-times from 33.3 to 32.1 min and their hematocrits increased 5%
shortly after receiving a 400 mL autologous infusion of red cells collected 11
weeks earlier [37]. An interesting aspect of autologous infusions is that the
magnitude of the improvement is related to the baseline level of fitness.
Individuals who are moderately fit experience the greatest improvement while
