Mean tHbmass of the athletes with values higher or lower than the 95% confidence interval for the mean (95% CI) of the respective group (female/male athletes) did not differ significantly from mean tHbmass of the athletes with parameter values within the 95% CI.
Four female players had ferritin levels lower than 25 ng/mL, reflecting low iron stores. They did not have significantly lower mean Hbconc, mean tHbmass, or mean O2max than the other 13 female players.
Mean serum ferritin in female elite hockey players was much lower (36.7 ± 19.2 ng/mL) compared to the male players (106.6 ± 47.6 ng/mL) (p < 0.05).
CHr correlated positively to Hbconc, to Hct, to MCV, and to MCH in female athletes (p < 0.05). CHr also correlated positively to MCV and to MCH in male athletes, but it correlated negatively to RBC (p < 0.05).
In female athletes, Ret (%) correlated positively to Hbconc and to Hct (p < 0.05). Ret (n) also correlated positively to Hbconc and to Hct (p < 0.05). In male athletes, Ret (n) correlated to Hbconc (p < 0.05). sTfR correlated to Ret (%) and to Ret (n) in male (p < 0.05), but not in female, athletes.
This study evaluated tHbmass, examined the correlation between tHbmass and O2max, and assessed possible influences of iron status on tHbmass and on O2max in the players of the German field hockey national teams who participated in the EuroHockey Nations Championship 2007. Where available, the results are discussed against the background of studies that have been performed in elite athletes.
Differences in blood volume between endurance trained and untrained subjects have been known for many years. Even in 1974, Dill et al. (10) described higher blood volumes in middle-distance runners compared to nonathletes. Improvements of the method to determine blood volume and tHbmass by CO rebreathing have stimulated the interest in these parameters in the past years.
Gore et al. (16) investigated 33 male runners, 12 male rowers, and 17 female rowers for tHbmass and O2max. The study demonstrated positive correlations between tHbmass and O2max in every single one of these groups of endurance trained athletes. The correlation coefficients of the 3 groups were as follows: r = 0.92 in female rowers; r = 0.79 in male rowers; and r = 0.48 in male runners. As for male and female field hockey players, the regression slopes for male and female rowers were not different.
Heinicke et al. (20) assessed tHbmass in 94 male elite athletes of different disciplines by CO-rebreathing method. The athletes were subdivided into the following groups: downhill skiers, swimmers, runners, triathletes, junior cyclists, and professional cyclists. Twelve untrained subjects served as control group. Compared to untrained subjects (mean tHbmass: 11.0 ± 1.1 g/kg) tHbmass was 35% to 40% higher in the endurance trained runners, triathletes, and cyclists (e.g., 15.3 ± 1.3 g/kg in professional cyclists). Downhill skiers had rather low tHbmass values (approximately 12.3 g/kg); swimmers had an intermediate position (approximately 13.3 g/kg). These findings are consistent with the assessed mean O2max values in the different groups, ranging between 45.3 ± 3.2 mL/min per kilogram (untrained subjects) and 67.4 ± 3.3 mL/min per kilogram (professional cyclists). Compared to the investigated groups of athletes by Heinecke et al., the male German national field hockey team had an intermediate mean tHbmass (12.5 ± 0.9 g/kg) and an intermediate mean O2max (55.8 ± 4.0 ml/min per kilogram). In both parameters the field hockey players range between downhill skiers and swimmers.
The rather low values of the downhill skiers are explainable by the fact that downhill skiing is characterized by high anaerobic training and anaerobically performed competitions. Anaerobic exercise probably does not lead to a significantly elevated erythropoietic activity. Anaerobic performance is mainly determined by muscular qualities. Because hemoglobin plays an important role in buffering the concentration of hydrogen ions that contributes to skeletal muscle fatigue in anaerobic exercise, however, a higher tHbmass and consequently enhanced buffering capacity of the blood may also have a beneficial effect on anaerobic performance. Because there is no evidence in literature for this speculation so far, the contribution of tHbmass to anaerobic performance and the possible benefit for field hockey players, especially in repeated sprint performance, will have to be analyzed in future studies.
Heinecke et al. (20) found a significant positive correlation between tHbmass and O2max in triathletes (r = 0.63), junior cyclists (r = 0.50), and professional cyclists (r = 0.67). These correlation coefficients are similar to the ones found in this study in male (r = 0.57) and female (r = 0.56) elite field hockey players. A higher correlation cannot be expected because various other factors (e.g., cardiac output, oxygen utilization of the muscle) are relevant for endurance capacity.
The exact mechanisms that lead to a higher tHbmass in endurance trained athletes compared to anaerobically trained athletes and to untrained persons are controversially discussed. Training-induced adaptations and genetic predisposition are the 2 most probable mechanisms (20).
Hbconc, Hct, or RBC did not correlate to O2max in male or female hockey players. This result is consistent with previous findings by Schmidt et al. (34) in endurance athletes.
As in the general population, anemia in athletes is frequently associated with iron deficiency. It has been well documented that anemia leads to a reduction of endurance capacity and that correction of an anemia enhances performance (15,21). However, there is also evidence that iron status, independently of erythropoiesis and oxygen-carrying capacity, has an influence on exercise performance. Because iron is not only found in hemoglobin, but also in myoglobin and in several iron-dependent mitochondrial enzymes and respiratory chain proteins, tissue oxidative capacity is discussed as the main underlying mechanism (5,15).
Hinton et al. (21), for instance, studied the effect of iron depletion on the adaptation to an aerobic training regimen in a double-blind, placebo-controlled trial. Forty-two women with iron depletion but without anemia received oral iron supplementation or placebo for 6 weeks. All women performed an aerobic training program for the last 4 weeks of the study. At the end of the training period, the decrease in the time to complete a 15-km time trial was significantly greater in the supplemented group than in the placebo group. Linear regression analysis revealed that the effect of iron supplementation on time to complete the time trial could be partially ascribed to improvements in serum ferritin level and, to a lesser extent, in Hbconc.
Friedmann et al. (15) investigated the effect of oral iron supplementation in young elite athletes with low serum ferritin but normal Hbconc on O2max in a double-blind, placebo-controlled trial. At the end of the 12-week supplementation period, significant increases in O2max and in serum ferritin were seen only in the intervention group. Total red blood cell volume that had been assessed by a CO rebreathing method did not change significantly in either of the 2 groups. It was concluded that the reason for the improvement of O2max in the iron-treated athletes could be an increased muscle oxidative capacity.
Bruner et al. (7) could demonstrate a relationship between iron status and cognitive function. They investigated the effect of an 8-week oral iron supplementation in adolescent girls with iron deficiency and without anemia in a double-blind, placebo-controlled trial. Verbal learning and memory could be improved by iron supplementation, along with improvements in ferritin and hemoglobin levels. The mechanisms by which iron deficiency might affect cognitive function are still unclear. The association between iron status and neurotransmitter synthesis and brain iron deposition has been studied mainly in animal models so far (3). It is conceivable that the performance of athletes could be limited by these mechanisms in case of iron deficiency.
In the investigated field hockey players none of the investigated parameters of iron status correlated to tHbmass. Furthermore, no associations between parameters of iron status and O2max that might be independent of erythropoiesis and of oxygen transport capacity of the blood could be demonstrated. Therefore, the common practice in some professional endurance sports to supplement iron regardless of the need (40) would be unlikely to improve tHbmass or O2max.
Serum ferritin level is the most commonly used parameter to assess iron status (39). Phlebotomy studies in normal subjects have demonstrated that 1 ng/mL serum ferritin corresponds to an iron store of 8-10 mg (13). Gropper et al. (17) assessed the ferritin levels of 70 female athletes participating in collegiate cross-country track, tennis, softball, swimming, soccer, basketball, and gymnastics. Of these athletes, 17 (24.3%) had a serum ferritin concentration ≤15 ng/mL. Fallon (12) reports on iron stores of male and female elite athletes undergoing routine medical screening at the Department of Sports Medicine at the Australian Institute of Sport over a period of 3 years; 15.5% of all female and 4.2% of all male athletes athletes had a serum ferritin less than 30 ng/mL. Dubnov et al. (11) screened 103 adolescents and adults from 8 national basketball teams for anemia and iron status. Iron depletion, defined by a ferritin level of less than 20 ng/mL, was found in 22% of study participants (15% in males vs. 35% in females, p < 0.05). Anemia was found in 25% of athletes (18% in males vs. 38% in females, p < 0.05).
In comparison to the results of these exemplary chosen studies, the investigated field hockey players had a rather low prevalence of iron deficiency. All male players had ferritin levels higher than 50 ng/mL. Four female players (23.5%) had ferritin levels lower than 25 ng/mL, 2 of them (11.8%) with levels lower than 20 ng/mL. The lowest ferritin level was 18.0 ng/mL. Only 1 (female) hockey player fulfilled the World Health Organization (WHO) definition of anemia (<13 g Hb/dL for men and <12 g Hb/dL for women) with an Hbconc of 11.9 g/dL. The probable reason for this low prevalence of iron deficiency is that the German national field hockey teams are investigated for variances of their iron stores on a regular basis. Iron supplementation is initiated in athletes with ferritin levels lower than 30 ng/mL. This strategy is based on data published by Mast et al. (25), who found an improvement of specificity and sensitivity of ferritin in the diagnosis of iron deficiency by using a diagnostic cutoff value of ≤30 ng/mL instead of ≤12 ng/mL. A cut-off value for ferritin of 30 ng/mL also has been suggested by Kurer et al. (24) in patients with chronic inflammatory rheumatic diseases. Punnonen et al. (30) even calculated a cut-off limit for ferritin of 41 ng/mL to provide the optimal diagnostic efficiency in patients with anemia who do not have an accompanying infection or inflammatory disease.
An imbalance between iron requirements of the erythroid marrow and the actual iron supply leads to a reduction of red cell hemoglobinization, which finally causes hypochromic, microcytic anemia (39). This imbalance in iron status in the first stage of erythropoiesis is called “functional iron deficiency.” To accurately assess functional iron deficiency in an early stage, Ret (n), Ret (%), and CHr have been measured in this study in addition to the traditional parameters of iron status.
CHr has been demonstrated to be a specific and sensitive marker of iron-restricted erythropoiesis in healthy individuals (6) and in patients receiving chronic dialysis treated with human recombinant erythropoietin (14). Fishbane et al. (14) found a much better specificity and sensitivity for CHr to diagnose functional iron deficiency than for serum ferritin and transferrin saturation in patients undergoing hemodialysis. Comparable data on athletes do not exist. Thomas and Thomas (39) describe a cut-off level of 28 pg for the diagnosis of a functional iron deficiency. A CHr value beyond 28 pg is considered normal. The 95% CI for CHr in the investigated field hockey players was 31.6 to 32.7 pg in women and 32.0 to 33.9 pg in men. None of the investigated athletes had a CHr of less than 29.1 pg.
The maturation time for a typical reticulocyte is 3.5 to 4 days, with only the last 24 hours spent in the circulation. Hence, changes in the reticulocyte count reflect current changes in erythropoietic activity (39). Increased erythropoietic activity is reflected by an increased reticulocyte count. If at the same time there is increased mature red cell destruction-for example through foot-strike hemolysis (1) or gastrointestinal blood loss (38) as a result of running-percentage of reticulocytes also will be increased.
Parisotto et al. (28) investigated reticulocyte parameters in elite athletes. They examined venous blood of 107 male and 48 female elite athletes from 6 sports (cycling, swimming, rowing, track and field, boxing, and cross-country skiing) during regular training and a group of 23 nonathletes for CHr and Ret (n) to establish reference ranges (95% CI) for these parameters. They did not distinguish between male and female subjects. The set-up reference range (95% CI) for Ret (n) for elite athletes was 46-51 × 109/L. For nonathletes it was 40-52 × 109/L. The 95% Cls of CHr were 30.7 to 31.2 pg in athletes and 30.4 to 31.4 pg in nonathletes. The 95% CIs for Ret (n) (43.7-60.7 × 109/L in female and 45.0-65.2 × 109/L in male field hockey players) and CHr (see earlier) in our subjects were higher than in the athletes investigated by Parisotto et al.
Differences between the studies might partly be explained by varieties between fully automated hematologic systems, which particularly occur when different marking methods or dyes are used for the identification of reticulocytes (2). Differences in iron status of the assessed groups of athletes might be another reason for discrepancies.
The membrane-associated transferrin receptor mediates the cellular iron uptake by binding and internalization of diferric transferrin. The expression of this protein is regulated by the intracellular iron turnover and the intracellular iron stores. A small fraction of the receptors appears in soluble form in the blood and can be measured as soluble transferrin receptor (sTfR) (39). Unlike ferritin, sTfR is not an acute-phase protein and is not affected by infections and inflammatory states. It is therefore discussed to be a more stable index of iron status than ferritin, especially after prolonged exercise (27). Because erythroid precursor cells contain 80% of total body transferrin receptor mass, sTfR is directly correlated to the total mass of erythroid precursors and responds promptly to iron-deficient erythropoiesis (22). Not only erythroid precursors but all cells of the body produce sTfR-in particular, rapidly dividing cells. Thus, sTfR is not only a determinant of erythroid precursor mass, but also a measure of tissue iron deficit (35).
None of our athletes had an sTfR level higher than the normal range. As expected, sTfR correlated to Ret (%) and to Ret (n) in male hockey players. It did not correlate to Ret (%) and Ret (n) in female players. Because not only reticulocytes, but especially also cells with a high cleavage rate such as the endometrium express TfR (26), the missing correlation in female players could be a result of the influence of endometrial TfR production on the sTfR level. This influence might even be increased in iron-deficient states.
The following mean values for tHbmass were found in the German field hockey national teams: 12.5 ± 0.9 g/kg in men and 10.6 ± 1.1 g/kg in women. The magnitude of tHbmass is in the middle of the range between untrained subjects and elite endurance athletes.
The correlation between tHbmass and O2max has been demonstrated in endurance trained athletes in previous studies. This study proves this correlation in male and female international field hockey players.
In contrast to tHbmass - Hbconc, RBC and Hct did not correlate to O2max. tHbmass therefore can be used as an indicator for endurance capacity in field hockey players, whereas Hbconc, RBC, and Hct may not. An athlete with a high tHbmass is likely to have an advantage compared to other athletes with respect to optimal oxygen delivery to contracting muscle.
As mentioned before, the players of the German national field hockey teams are screened for iron deficiency on a regular basis. tHbmass was not influenced by the actual iron status of the investigated athletes. Associations between iron status and tHbmass might become obvious in athletes that have been iron deficient for several weeks. It is assumed that in athletic populations with normal parameters of iron status, an additional iron supplementation will probably not lead to an increase of tHbmass. Nevertheless, because female athletes especially tend to develop low iron stores and possibly iron-deficient erythropoiesis under intensive training regimens, regular screening for iron deficiency (and iron-deficient diet) is recommended. Additionally, possible negative effects of iron depletion on physical performance that are independent of erythropoiesis have to be kept in mind.
Because of the cross-sectional character of this study, the value of reticulocyte parameters in the investigation of iron-deficient erythropoiesis in elite athletes cannot be judged conclusively. Further interventional, observational, and long-term studies are necessary.
The authors wish to thank the German Field Hockey Federation (Deutscher Hockey Bund), the coaches, the medical staff, and the players of the German field hockey national teams for their support and their cooperation. We also thank Matthew Lee-Archer (Australian Red Cross Blood Service, Brisbane) for reviewing the manuscript.
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Keywords:© 2010 National Strength and Conditioning Association
physiology; blood; metabolism; oxygen uptake; athlete; field hockey