Posted on 24 May 2012 20:42
Sports anemia is not a true anemia. It is an anemia-like condition that is normal in endurance and ultra-endurance athletes, and is widely considered to be a normal adaptive response to endurance exercise. The condition manifests as endurance athletes having lower hemoglobin levels (Hb) than nonathletes. These athletes will often look slightly anemic compared to others and this decrease in plasma Hb levels can range from 5% in recreational runners to 20% in elite marathoners. There are two mechanisms for this pseudoanemia and the latter described is the best explanation for a chronic low hemoglobin level.
One way that aerobic athletes experience temporary low hemoglobin may be due to the destruction of older red blood cells through a sudden increase in aerobic exercise. This would lower the blood cell count and cause the body to have to clean up the dead cells, which would lower the blood's iron level temporarily.
Also, regular aerobic exercise expands the plasma volume, thus diluting the RBC and Hb levels of the blood. This is termed dilutional pseudoanemia. This probably occurs as an adaptive response to the acute loss of plasma volume that occurs early in each workout. The reduction in plasma volume, or hemoconcentration, is due to three factors:
1) The mean arterial blood pressure and the muscular compression of venules is increased, which both increase capillary hydrostatic pressure.
2) Tissue osmotic pressure is increased because of the generation of lactic acid and other muscle metabolites, increasing osmotic pressure.
3) Sweat is produced, resulting in fluid loss.
It is thought that the body responds to this repeated hemoconcentration by releasing renin, aldosterone, and vasopressin so as to conserve water and salt. Albumin is also added to the blood to help pull in more water, expanding the plasma volume.
As early as one day after a strenuous aerobic exercise bout, plasma volume can expand by up to 10%. The amount of the rise, and thus the reduction of Hb levels, correlates with the intensity and frequency of exercise, so the hardest training athletes will tend to have the lowest hemoglobin levels, likely making it appear as if they are training too hard. Any increases in plasma volume, however, will disappear as fast as they came, once exercised is ceased.
Since hemoglobin is responsible for delivering oxygen to the muscles and any anemic condition would seem to reduce aerobic endurance, dilutional pseudoanemia being an adaptive response may seem to present a paradox. However, as the plasma volume rises so to does the cardiac stroke volume, which more than makes up for any loss in Hb per unit of blood, so that more, not less, oxygen is delivered to the muscles.1
After a few months of consistent exercise during which large shifts in volume or intensity are not made, blood concentrations tend to return to normal. However, long-term sports anemia is present in some elite endurance athletes.2
Diagnosis and Problems with Sports Anemia
The major problem with the sports anemia is that it is a misnomer. For one, as described above, it is not a true anemia. Also, the term would make it seem as if there is one anemia unique to sports, when, of course, athletes can experience anemia due to all the normal causes. It is also problematic because certain athletes do seem to be at increased risk for iron deficiency anemia, namely female athletes, long distance runners, and vegetarian athletes.2 It is more important that practitioners be aware of this increased risk than to be aware of a pseudo-anemic phenomenon called sports anemia.
Diagnosis of sports anemia can only be made after all other likely causes have been eliminated, together with the knowledge that the patient is an endurance athlete or engages in regular aerobic exercise. Although hemoglobin levels may be low, other blood parameters will test normal.
Once diagnosed, the condition should does not need to be treated or prevented, as it is a beneficial and normal response to exercise.1
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