Brazil has been the most active country in the international football transfer market since FIFA started collecting transfer information from member associations worldwide in 2010. Brazilians soccer players are transferring out of Brazil on average younger, almost one-third of all transactions were under 18 years old22. However, few studies describe immature red blood cell parameters in these athletes during training periods 14.
Reticulocytes are immature red blood cells that remain 1–2 days in the bloodstream and provide a good index of hemoglobin in newly produced red blood cells in response to training21. In our study, we observed significant increases in Ret-He induced by training (Table 1). To our knowledge, is the first study that shown this behavior in young soccer athletes. Ret-He is used successfully in the diagnostic of functional iron deficiency and iron deficiency anemia in non-exercised individuals.21,23 Ret-He represents a direct measurement of iron for hemoglobin synthesis in the reticulocyte and has no limitation in contrast to biochemical iron status parameters para such as ferritin and transferrin that are strongly influenced by inflammation.3 Our Ret-He values were increased after the training period with no significant changes in the reticulocytes count. This behavior showing early response for this parameter compared to reticulocytes as previously observed after 48 hours after iron therapy in deficiency treated patients. 24
Soccer is characterized by high neuromuscular demands with sprints, changes in direction, and jumps. Young soccer athletes play one or two matches per week. Thus, elite soccer training programs are planned to prepare the players to repeat these high efforts several times a week during a whole season 25. Important hormones that stimulate the erythropoiesis, such as erythropoietin, testosterone, and growth hormone have shown an increase as a training adaptation.26,27 We observed slight differences in traditional hematological indices MCV, MCH, MCHC and RDW (Table 1). However, the mean differences were lower than the biological variation for physically active individuals.20 Lippi et al. observed similar behavior for MCV and RDW after moderate endurance exercise and suggested that these changes could be potential implications on ABP.28
Adequate nutritional status during training periods is a fundamental requirement for positive adaptations and optimal physical fitness in young athletes.36 For endurance modalities, oxygen transportation is crucial to achieving better performance. Hemoglobin is the oxygen carrier to the muscles and the iron is the central component in heme proteins. Hemoglobin and iron status were positively associated with cardiorespiratory and muscular fitness in young athletes.36 During our study period, the hemoglobin concentration remains stable with no significant change. Plasma volume adaptation and the short training period interval of our athletes may explain this behavior.8
The RCV observed for traditional RBC parameters (Table 2) were similar when compared to physically active individuals.20 As previously reported in different sports by Banfi et al. [Hb], Ht e RBC have almost identical variation, in contrast to reticulocytes parameters that present high variation.29 Besides, at the end of competition season reticulocytes can decrease between 5 and 21% without alteration in [Hb].14 It is important to state that our athletes were at the beginning of the season and most of the hematological changes are observed in the heaviest training periods.14,29
Reticulocytes evaluation in soccer players is important because of its usefulness in detecting sports anemia. During some stages of endurance training, plasma volume changes promote hemodilution, instead of hemoglobin concentration could be falsely decreased. Furthermore, athletes could experience exercise-induced hemolysis due to mechanical trauma or oxidative stress. Our reticulocytes biological variation data (Table 2 and Figure 1) showed a low within and high between-subject biological variation. This behavior suggests the RCV than population-based reference intervals to better interpret the athlete’s results.30 Endurance athletes have similar results for reticulocytes between-subject variation (CVB = 28%), but higher values for CVI = 21,1% and RCV95% = 42,8% than young soccer players.5 In a large professional soccer player population, Malcovati et al. calculated a slightly higher reticulocyte within-subject coefficient of variation (CVI = 16,3%).31 These differences may be explained by the effect of age, ethnicity, sports discipline, training phases, and analytical variation.5,31,32 Also, intraindividual biological variation for reticulocytes is higher in soccer players compared to inactive subjects.33,34
Immature reticulocytes are cells with high RNA content which results from the increased rate of bone marrow red cell production.14 In athletes, high IRF values can be seen in hemolysis, altitude training, and blood manipulation with erythropoietic stimulants.35 The biological variation for IRF in our soccer players was slightly higher than observed in the non-athletes study population, repeating the same pattern for reticulocytes and RBC parameters. 34