The general view is that endurance athletes, and especially cyclists, often suffer from iron deficiency. Iron deficiency can cause anemia which reduces oxygen transport to the muscles and therefore reduces performance. But anemia in endurance athletes is much less common than generally assumed. Approximately 3% of the endurance athletes develop an iron deficiency anemia. This percentage corresponds to the occurrence of iron deficiency anemia among the normal population.


We speak of anemia when the hemoglobin level is less than 8.5 mmol / L for men or less than 7.5 mmol / L for women. In some countries, the hemoglobin is expressed in g / dl. The conversion factor from mmol / L to g / ml is 1.6. Thus, a hemoglobin value of 10.0 mmol / L is equivalent to 10.0 x 1.6 = 16 g / dl.

Hemoglobin is abbreviated as Hb.


Blood consists of a solid substance: the blood cells and of liquid: blood plasma. Under the influence of long term sustained endurance effort the so-called sports anemia arises whose main features include a sharp rise in the blood plasma and a low Hb. The plasma volume can -in extreme cases- increase up to 25%. Due to the increase of the plasma volume endurance athletes have a much larger circulating blood volume than sedentary humans. Non athletes have a blood volume of 5 liters. Endurance athletes have a blood volume from 6 to 7 liters. Plasma increase is a training effect; it is a physiological adaptation of the body to far-reaching endurance training.

Left tube: blood cells and plasma separated after centrifugation.
Right tube: full blood.

The increase of plasma volume is not followed by an increase in the number of red blood cells. Therefore many endurance athletes have low hemoglobin values without iron deficiency.

The degree of increase in plasma is dependent on the size and the intensity of the endurance training. The final effect is that the blood becomes thinner, so less viscose, which is also reflected in a decrease in the hematocrit. Also, the decrease of the hematocrit should be viewed as a physiological adaptation of the body to far-reaching endurance training. During the Tour de France, a stage race over three weeks, the average hematocrit of the riders reduces during the race, with two to three percentage points. So a rider who starts the Tour with a Ht of 45 has at the end of the Tour a Ht of 42 at the end. In the period of rest after the Tour the situation becomes normal again and the Ht returns, after about a week, to 45.

Sports anemia is a physiological phenomenon that does not require treatment. But because sports anemia is often considered to be a real iron deficient anemia iron therapy is often prescribed. That is not necessary and highly inadvisable because iron therapy may be associated with serious side effects.

Therefore if a rider has a low Hb always have ferritin determined. Ferritin gives a good impression of the iron store of the body. A low Hb with a normal ferritin value is a sports anemia: a treatment is not needed.

A rider with low Hb as well as a low ferritin level has iron deficient anemia: start treatment with iron therapy.