## 4 Discussion
When differences were analysed along the assessment in the extracellular compartments, no differences in plasma concentrations were observed. However, there was an increase in urinary Mn excretion in both groups. In relation to urinary Mn values, increases in excretion were previously reported in athletes after a six-month training period [18]. Likewise, in the previous study, they observed a decrease in serum Mn concentrations after six months of training. However, when the acute effect of physical exercise was evaluated, there was a decrease in Mn excretion [18; 19]. Both the level of training and the type of sport could affect Mo concentrations. Lower erythrocyte concentrations have been observed in athletes with high levels of training. This could be due to the absence of Mn-SOD inside the erythrocyte because there are no mitochondria in this cell [39]. On the other hand, depending on the type of sport, higher serum concentrations have been observed in aerobic athletes. The higher Mn concentrations in athletes could be related to a higher organic utilisation of this mineral in mitochondrial Mn-SOD production processes, among others. The higher basal blood Mn concentrations in aerobic athletes compared to athletes of other sports could be related to probably lower Fe levels, a fact that is common in long-distance athletes, as a consequence of the impact of body Fe levels on Mn metabolism [20]. The gradual low plasma concentration observed in the present investigation could be caused by an increase in Fe absorption [22; 36], which would be related to increased Mn excretion, revealing a possible renal adaptation with training. Another reason for decreased plasma Mn concentrations could be an increase in Mn-SOD activity, which has been shown to increase at the end of the season [40]. As mentioned above, free radicals are produced as a result of oxidative metabolism. Increased free radicals are associated with increased lipid peroxidation and accumulation of metabolites that can lead to injury and cell death. Among other enzymes, Mn-SOD controls free radical damage, and it is known that one of the adaptations to physical training is an increase in its activity [41]. As for intracellular concentrations, there were different changes according to sex. In the men, a gradual increase of Mn concentrations in platelets and erythrocytes was observed. However, in the women, decreases in Mn concentrations were observed in both intracellular compartments. No studies have been found that evaluate the evolution over a training season/period of erythrocyte and platelet Mn concentrations. Previously, it has been reported that sedentary subjects showed higher erythrocyte Mn concentrations compared with active or very physically active populations [36]. It should be noted that Mn-SOD would exist in very small proportions inside erythrocytes, since there are no mitochondria in this type of cell. Therefore, the evolution in erythrocyte Mn concentrations could be due to the different Mn intake between sexes [36]. Regarding platelet Mn concentrations, there are no reports in the scientific literature on Mn concentrations in this compartment throughout the season in athletes. Perhaps, as occurs with erythrocytes, sex differences in intake could explain the trend of Mn in this biological matrix. Regarding sex differences, higher Mn concentrations have been observed in plasma and urine in men compared to women. Despite absorbing less Mn, absorbed Mn had a longer half-life in the men compared to the women [42]. However, in a population of 1417 U.S. subjects, it was observed that the women excreted more Mn relative to urinary creatinine values [43]. As mentioned above, Fe deficiency influences Mn metabolism by increasing absorption. Since the main route of Mn elimination is bile [42], these data may show that hepatic secretion of Mn into bile is more active in women than in men. The influence of sex on whole-organism Mn turnover warrants further investigation of whether sex influences hepatic metabolism of Mn and its excretion in bile.