| Hong Kong Sports Institute (HKSI):
Dr Yvonne Yuan, Sport Biochemist
Subnormal haemoglobin (Hb) level, also known as anaemia, is frequently reported among athletes. As an optimal amount of Hb is essential for transportation of oxygen within the body, Hb plays a critical role in athletic performance. There are two kinds of anaemia that are particularly common among athletes. They are dilution anaemia and iron deficiency anaemia.
Dilution anaemia
Dilution anaemia, also known as sports anaemia, is caused by the expansion of plasma volume in response to endurance training and does not pose serious problem to athletes. Since dilution anaemia is only a normal physiological adaptation, no treatment is needed. The only thing that the coach and athlete can do about it is to recognise its presence and be able to differentiate it from other pathological anaemia.
Iron deficiency anaemia
Extensive research literature indicates that elite athletes (especially distance runners) have a lower iron store when compared to sedentary individuals. Approximately 29% of male endurance runners and 82% of female distance runners have been identified as having iron deficiency. Women athletes, who lose iron through both menstruation and high exercise stress, suffer a more complicated problem.
Iron deficiency is an imbalance in the input and output of the mineral. Compared to the average person, athletes are prone to have increased blood loss, decreased iron intake and poor iron absorption.
Evidence suggested that blood loss from the gastrointestinal tract is an important source of negative iron balance in athletes. Foot strike hemolysis and similar phenomena are also believed to cause accelerated iron loss in athletes, especially runners. Furthermore, the possibility of accelerated iron loss in sweat by athletes cannot be completely eliminated. The relatively high intake of nonsteroidal anti-inflammatory drugs (NSAIDs) by athletes for treating musculoskeletal problems has further intensified the problem. Gastritis, which causes blood loss, is often induced by NSAIDs.
Some athletes, especially female athletes who participate in sports that emphasise leanness (e.g. gymnastics, diving, figure skating, distance running), run a high risk of iron deficiency due to a low calorie intake. Studies have confirmed that athletes on a vegetarian diet have a significantly lower ferritin level than those who consume ample quantities of red meat as the heme iron from meat is more readily absorbed by the body when compared to the nonheme iron found in vegetables.
Evidence suggested that absorption disturbance contributes to poor iron status of the athletic population. In a study using radioactive iron, it was found that the absorption rate of iron-deficient runners was only half of that of sedentary counterparts.
What can the laboratory tell us?
The golden standard test for iron deficiency is the absence of iron staining in bone marrow or a positive response after a therapeutic trial of oral iron. However, the former technique is too invasive and the latter can be too time consuming.
Haematological markers, such as haemoglobin (Hb), mean cell volume (MCV), mean cell haemoglobin (MCH), haematocrit (Hct), and red cell distribution width (RDW), only change in the later stage of iron deficiency. They can be indictors on the severity of the problem but cannot help identify iron deficiency at its early stage of development.
Among the biochemical indices, serum iron, transferrin saturation, and ferritin have been used widely to monitor iron status both in athletic and sedentary populations. However, interpretation of these laboratory findings is influenced by factors that are not always controllable.
Traditionally, ferritin has been extensively used for monitoring iron status. However, being an acute-phase protein, its plasma concentration can increase markedly as a result of strenuous exercise. The maintenance of a chronic acute-phase response during periods of intensive training may mask iron deficiency in some athletes. Serum iron exhibits diurnal variations, with higher concentrations late in the day. Its level is also readily changed after taking oral iron supplements. Timing of blood collection for serum iron measurement is therefore critical to the interpretation. Transferrin is an iron transport protein receiving and binding iron for delivery to receptors at recipient cells. In iron deficiency, transferrin levels increase. However, its level is readily increased by oral contraceptives and it will not be elevated in iron-deficient states in which there is severe protein malnutrition.
In view of all the potential problem of using the above parameters for assessing iron status, the use of soluble transferrin receptor (sTfR) has been suggested . The majority of the transferrin receptor in the body are attached to the cell membrane and responsible for transferring iron into the cells. When a cell needs iron, transferrin receptor increases. Soluble transferrin receptor (sTfR) arises from proteolysis of the intact protein on the cell surface and therefore is measurable from the serum or plasma. Serum sTfR has been reported to be higher in patients with iron deficiency. As sTfR is not affected by acute or chronic inflammatory conditions, it is obviously a preferred parameter to be used in the athletic population.
In view of the fact that most of the commonly used parameters of iron status are affected by inflammation, it has been suggested that
C-reactive protein (an inflammatory marker) should be used as a screening tool to identify presence of inflammation. Recently, a research project has been conducted by the HKSI with one of its aims, to confirm if this suggestion can improve diagnostic performance of the laboratory findings.
In conclusion, the choice of markers for iron deficiency has never been straightforward. This is particularly true for athletic populations. Accurate interpretation of the laboratory findings will depend very much on the understanding of the limitations of the parameters being used. |
