Hematocrit (HCT) is essentially the
percentage of your blood that is made up of red blood cells. When
hematocrit is tested, blood is considered to have two main components:
red blood cells (RBCs) and plasma. Plasma is mostly water, along with
some plasma proteins, blood glucose, etc. Red blood cells are the
hematocrit portion. As you probably know, red blood cells are the
portion of the blood that carry oxygen. So hematocrit is a representation
of the oxygen carrying capacity of the blood. What are normal values?
Normal* values for men are typically about 40-50%, and normal for women is
Since increased HCT, and increased oxygen
carrying capacity can lead to performance gains, some racers have been
caught using EPO and other drugs too boost their hematocrit/red blood cell
count. But there can be too much of a good thing. HCT actually increases
the viscosity of the blood. So if your HCT gets too high, your blood can
actually get so thick that your heart has difficulty pumping it, and you
can actually have a heart attack. This is why the UCI will remove a
rider from competition for "health reasons" if he or she has a hematocrit
What are factors that effect hematocrit?
Blood loss and cancer are some of the disease
states that can lower HCT, but training can also have an effect. Training
can actually lower your percent hematocrit. I know this doesn't make
intuitive sense, but stick with me here. Responses to training can
include an increase in RBC production, but blood plasma levels also increase. If plasma
levels increase more than RBC, then the percentage of HCT goes down, but
oxygen carrying capacity has actually increased. The charts below give a
visual representation of this. In the after training graph, the units of
hematocrit (these graphs are not in %) have gone up, but so have the units
of plasma. Therefore hematocrit is a smaller portion of the whole
blood. On occasion, endurance athletes are actually diagnosed with a low
HCT level , when HCT levels are just fine**.
Altitude can also effect HCT levels.
Typically, living at altitude results in an increase in HCT. This is
because RBC levels are increased just a little, but plasma levels also go
down. There fore HCT expressed as as percent, will go up. If you go from
sea level to altitude, you may find yourself make an unusually high number
of visits to the bathroom. This is because your are ridding yourself of
some of that plasma. (remember plasma is mostly water)
So training at altitude will increase my hematocrit, and help my
Probably not. Research indicates that
training at altitude has no effects on sea level performance, and it may
actually be a detriment. This is thought to be because the decreased
oxygen at altitude leads to decreases in many desirable training
adaptations that occur during training. So the current thought is that
the most desirable scenario is to live high, and train low.
In this scenario, the athlete receives the benefits of high altitude
without the disadvantages of high altitude. This has led some athletes
to the use of altitude
tents. There seems to be conflicting evidence as the the
usefulness of these tents. Some studies suggest that sleeping in the
tents 8 hours a night is not enough, while some studies indicate increased
performance after use of the tent.
* What is considered normal may vary slightly from
lab to lab.
** This is not medical advice, if you have any
questions please speak to your doctor!