This study found low erythrocyte concentrations in trace minerals of great importance for cellular functions in the subjects who trained. Cr is an essential trace element related to the metabolism of carbohydrates , altering the levels of blood glucose, potentiating the action of insulin, and influencing the metabolism of carbohydrates, lipids and protein anabolism .
It is possible that the mechanism by which exercise improves the response to insulin is related to an alteration in the metabolism of Cr. Thus, an acute exercise increases the urinary losses of Cr . Clarkson (1991), indicates that little Cr is consumed in the general population, which suggests that athletes may have a deficit in this element . However, Maynar et al., 2018 showed that serum concentrations in different sports modalities were significantly higher in athletes than in the control group .
In the present study, there were no significant differences in the concentrations among the three study groups in erythrocytes, and there was no correlation with the level of training, although the concentrations in the erythrocytes of athletes were significantly lower than in the control group.
The content of Cu in the red cells (0.79 ± 0.07 mg/L) was similar to those found by Lu et al., (2015) with a similar technique (erythrocyte non-Ficoll measurement) in Swedish subjects . In the erythrocyte, almost 60% of Cu is linked to Cu-Zn SOD, which has been proposed as one of the best indexes to evaluate the body’s status regarding this element.
Studies that examine the effects of intense acute exercise on Cu levels show different results [32, 33]. Women who run a marathon increase their plasma levels after it without changes at the erythrocyte level . In another study, running a marathon produces small increases in the plasma concentration of Cu, but at the same time, a decrease in the level in the whole blood . Maynar et al. (2018) found similar serum concentrations in athletes of aerobic and anaerobic modalities to the control group, and only found levels significantly higher than the control group and the other two groups of athletes in soccer players (aerobic-anaerobic) .
In the present study, lower concentrations of Cu were found in the erythrocytes of professional athletes compared to the other groups, presenting a high negative correlation (r = − 0.790, p < 0.001) with the degree of training. It is known that the erythrocyte contains the enzyme Cu-Zn-SOD and that according to the study by Mena et al. (1991) its enzymatic activity is higher in amateur and professional cyclists with respect to sedentary subjects, but it is also observed that Cu-Zn-SOD decreases its activity the greater the level of training of the cyclist .
In theory, the function of copper enzymes is essential for physical performance. For example, mitochondrial cytochrome-c oxidase catalyzes the final step in the aerobic respiratory chain. Also, three copper enzymes (ceruloplasmin, cytosolic superoxide dismutase, and extracellular superoxide dismutase) have essential antioxidant functions that can reduce the free radicals formed during physical activity that would produce oxidative stress and could lead to fatigue and delay in muscle recovery .
Mn is a trace element present in all tissues of terrestrial and aquatic organisms. Its average concentration in erythrocytes is 22 ± 7.4 μg/L as measured by ICP-MS (erythrocyte non-Ficoll measurement) . It is a component of several enzymes, including arginase, glutamine synthetase, phosphoenolpyruvate decarboxylase, hexokinase, xanthine oxidase and Mn-SOD. The glycosyltransferases and xylosyltransferases, which are involved in the synthesis of proteoglycans and therefore in the formation of bone, are sensitive to Mn.
Several studies have observed that physical exercise increases the activity of Mn-SOD, which could be related to changes in serum concentrations of this element . This is significant because Mn-SOD is an antioxidant enzyme located at the level of the mitochondria that would neutralize the superoxide radicals that form inside it. Therefore, it is suggested that the practice of physical exercise and the consequent increase in the activity of Mn-SOD can induce cardioprotection and in general protection of cellular mitochondria against free radicals (Reactive Oxygen Species related to damaging cells, lipids and DNA) produced by sports practice .
But in addition to the Mn-SOD, Mn is part of enzymes that are fundamental in gluconeogenesis, and this is important especially in athletes of aerobic physical activities and also in the formation of urea, which is crucially important in aerobic physical activities . Maynar et al. (2018) found that Mn in serum was much higher in aerobic than in sedentary people. However, in athletes with a greater anaerobic component, these concentrations were lower, even compared to the control group . However, we have not found studies that tell us about the values at the cellular level in athletes.
In our study, we found for Mn something similar to that seen for Cu, that is, lower concentrations of Mn in the erythrocytes of professional athletes compared to the other groups, presenting a high negative correlation (r = − 422; p < 0.001) with the degree of training. However, unlike Cu, there is no Mn-SOD inside the erythrocyte because there are no mitochondria in the erythrocyte, so the lower concentration of this element would probably be due to a smaller amount of this element in the body of the athlete, which manifests itself in the erythrocyte, that could act as a reservoir.
The concentrations of Mo in erythrocytes are 0.52 ± 0.28 measured by ICP-MS . Mo functions as an enzymatic cofactor of three enzymes (aldehyde oxidase, sulfite oxidase, and xanthine dehydrogenase), which catalyze the hydroxylation of several substrates . The aldehyde oxidase oxidizes and detoxifies several pyrimidines, purines and pteridines and related compounds. Thus, sulfite oxidase catalyzes the transformation of sulfite to sulfate, from cysteine and methionine or directly from the diet. Xanthine dehydrogenase catalyzes the conversion of hypoxanthine to xanthine and from xanthine to uric acid. Therefore, molybdenum is involved in the purine cycle and the final production of uric acid, considered an antioxidant in the human body .
The higher concentrations of molybdenum would facilitate the formation of uric acid and thus avoid the damage that superoxide anions would produce generated by xanthine oxidase in the ischemia-reperfusion processes that occur in athletes’ muscles during intense activity .
In this study, we found significantly higher concentrations of Mo in the MTG and HTG group than in the controls. However, there was no correlation between this element and the training level of the subjects, which indicates that the erythrocyte changes of the element would not be related to the training.
Blood Se, which represents about 3% of total body Se [42, 43], is distributed in plasma, red blood cells and white blood cells. The erythrocyte values of Se, with ICP-MS, are 0.15 ± 0.03 mg / L . In red blood cells Se, as in other cells, prevents the deterioration of the membrane cytoskeleton . Its initial role in these cells is to protect the hemoglobin from oxidation. Also, it is well established that most of the Se is bound to hemoglobin, and a small part is in glutathione peroxidase (GSH-Px), the main action of which is to protect the body from the cytotoxic activity of hydroperoxides and free radicals, since they prevent the formation of lipid peroxides in the cell membrane . Therefore, it has antioxidant properties and plays an essential role in the defense against free radicals, which occurs largely in situations of trauma and overexertion, as well as during strenuous exercise.
Contrary to Cu and Zn, the variations in erythrocyte Se are strongly dependent on dietary habits . In a state of equilibrium, there is a close relationship between the erythrocyte and plasmatic Se .
Pograjc et al. (2012) found that at the end of the training period of the soldiers in their study, the level of Se in whole blood decreased, but that of plasma did not change or increased slightly . This could be the result of a decrease in Se concentration in erythrocytes, but not at the expense of a reduction in the activity of GSH-Px. So, there must be another form of storing interchangeable Se in erythrocytes. The decrease in total blood concentrations could indicate that some of the erythrocyte Se would be transferred to the tissues during training .
In the study by Mena et al., (1991), an increase in GSH-Px activity was observed in the erythrocytes of professional cyclists, keeping this relationship with the degree of training . Therefore, something similar to that indicated by Pograjc et al. (2012) must have happened in our study where there was a significant correlation of Se with the degree of training (r = − 275, p < 0.05) accompanied by decreases in the athletes’ erythrocytes, although without reaching statistical significance. This, together with the fact that the values found in all the groups were lower than those found by Lu et al. (2015) in their study in non-athlete subjects, would indicate a possible deficiency of the element in our subjects that, given its low levels and its importance in the organism, would prevent further decreases.
In erythrocytes, Zn is a cofactor of carbonic anhydrases, SOD, and 8-aminolevulinate dehydratase. In addition, zinc is bound to the erythrocytic membrane, hemoglobin, and other proteins, as well as to small molecules . So far, contradictory reports have been presented regarding erythrocyte Zn in physically active subjects [34, 49, 50] so that no firm conclusions can be drawn. In trained athletes erythrocyte values of Zn are lower . Some articles hypothesize about redistribution in tissues  and most refer to poor homeostatic control of Zn in red blood cells. Some studies report that Zn increases in plasma and decreases in erythrocytes after intense running on a treadmill or an extraordinary effort on a cycle ergometer. These studies propose that such plasma increases are due to a Zn leakage from muscles damaged by physical activity .
It can be assumed that functional displacements of Zn can occur between tissues during exercise, as occurred with Se. For that reason, it seems quite difficult to determine the effects of exercise on the concentration of Zn and other elements .
In the present study, the erythrocyte concentrations of Zn showed a high negative correlation with the degree of training (r = − 0.678, p = 0.000), which would indicate that they are dependent on sport training. Their level in those who practiced physical exercise was lower than those in the control group, being the lowest in the athletes with the highest degree of training.
This could indicate that our athletes could present a deficit in their nutritional status as indicated by De Carvalho et al. (2012) in their study, performed on swimmers. These authors observed plasma Zn concentrations below normal levels and erythrocytic Zn levels at borderline or within the lower reference limits. The low or limiting levels of this mineral, since before initiating training, suggest that the athletes started the training phases with existing deficiencies pointing to the need to re-evaluate the Zn requirements of athletes .
This possible deficiency, similar to that of the other essential elements, could have an important impact on an athlete’s performance, which might be significantly reduced in different aspects as seen above.