One of the main exposure sources to trace elements is diet . In this survey, nutritional intake was analysed by recording each gram of each food ingested during the evaluation days. Once the total daily amounts of each aliments were summed, the amount of each specific mineral was obtained using several databases [13–15].
In this survey participated sedentary population (CG) as well as high level sportsmen of different modalities (Aerobic, Anaerobic and Mixed). A critical issue in this kind of surveys is a proper delineation of experimental groups in accordance with the predominant metabolic demands (aerobic, anaerobic or aerobic-anaerobic metabolisms), because each specific sport induces different biological adaptations according to the metabolisms developed in of each activity. RERmax could be an interesting variable to differentiate groups due to the link between RER values and anaerobic CO2 production . Data about RERs in Table 2 manifests proper differences among sports groups and was used as criteria to verify that sportsmen groups was accurately delineated.
Sportsmen have, in comparison to sedentary population, greater energetic demands, due to their physical training. This fact is reflected in the obtained results, being the caloric intake (Kcal/d) significantly higher (p < 0.01) among sportsmen, but only in the AEG. Theoretically, sportsmen should present, due to an augmented food intake, greater mineral intake, but, contradictorily, the obtained results have shown similar mineral values. This fact was previously analysed in a review of Gupta and Gupta . In this review, it was reported that although takin a balanced and varied diet there is a great percentage of possibilities to have a lack of some elements. The main reason of this fact is the chemical composition of grounds, poor in several minerals. This idea can explain why the sportsmen of this survey have similar nutritional intake of minerals in spite of ingest sensibly higher amounts of foods. Another possible explanation could be that athletes ingested low amounts of foods rich in the studied elements.
Despite similar nutritional intake of minerals. it can be observed significant differences in serum concentrations of essential trace metals between sportsmen and sedentary people. However, mineral seric values were within reference concentrations  with the exception of Cr, which presented concentrations under normal values in the CG. This fact could be due to a diminution in the intake, as Clarkson  indicated, fact that has not been reflected in the obtained results.
Seric Cr concentrations were higher (p < 0.001) in SPG in comparison to CG. Berger et al. , found in marathon runner’s values of this element similar to the results obtained in this survey. However, they had not control group. They also reported high Cr concentrations before and after the race, being sensibly higher than in our sportsmen. Normal or sensibly augmented Cr values could be highly relevant to sportsmen because of the link of this element with insulin, a critical hormone for high level physical demands [1–3].
If sportsmen are classified by metabolic modalities and compared to the CG it can be observed that all sportsmen, independently of the nature of the exercise, presented greater Cr values than controls. The highest Cr values are reported in the AEG, followed by the AE-ANEG, and being the lowest in the ANEG. This fact could be linked to the insulin-related Cr role, being critical in aerobic modalities, where insulin regulates the substrates metabolism; in the other two modalities, this hormone takes a less important role. It is well known that the practice of physical exercise decreases the blood glucose and insulin levels. These effects may be observed even in only one exercise session and may last for several hours after the end of the effort. If regular practice of physical exercise is performed, a chronic blood glucose diminution may occur .
Long-term Cr supplementation effects have been studied involving long periods of time (12–24 weeks). It has been associated high daily doses of chromium (400 μg·day− 1) with significant reductions in body fat mass of 5% [22–25]. This fact indicates that the effects of chromium on body re-composition may occur; however, these effects require high dosage, long supplementation, periods.
In the present survey, the highest Cr serum concentrations have been found in AEG, which is the group with the lowest body fat amounts. This fact can be mainly due to physical exercise by itself but, in all cases, is in accordance with the previous reports and it could strengthen the hypothesis of Rubin et al. , who indicated that body composition changes, body fat diminutions or free fat weight increments can be produced by physical exercise by itself or by the exercise-associated serum Cr increments.
In the case of Cu, no differences were found between the SPG and the CG. However, if SPG participants are separated by metabolic modalities, serum Cu concentrations are significantly higher (p < 0.01) in the AE-ANEG in comparison to the CG and higher in the ANEG than in the AEG (p < 0.01). Cu has been previously studied in plasma of high level aerobic and anaerobic sportsmen compared to a control group (who performed moderate physical activities) . The results of this previous research showed that anaerobic sportsmen had significantly higher concentrations than controls and aerobic sportsmen. The results obtained here are in accordance to this previous report, being the highest concentrations in football players who performed aerobic-anaerobic training programs. The Cu concentrations in the AE-ANEG concentrations were also higher (p < 0.01) than in the CG. The increase on these concentrations could be due to exercise-induced rhabdomyolysis, as consequence of daily training. When muscle tissues impact increases, as occur in high-intensity activities, this effect is augmented .
It could be stated that aerobic exercises develop more reductions in blood Cu levels among athletes . Resina, Gatteschi, Rubenni, Giamberardino, and Imreh  found among soccer players lower serum ceruloplasmin levels and lower serum ceruloplasmin biological activity than in the control group. However, serum Cu levels were comparable in both groups. These results suggest that more attention should be focussed to serum Cu and ceruloplasmin among soccer players.
Mn is a compound of superoxide dismutase enzyme (Mn-SOD). Thereby, in several researches, it has been observed that physical exercise increases the Mn-SOD activity in the myocardium and in the mitochondria [31–33]. This is a critical fact among sportsmen because Mn-SOD is a mitochondrial antioxidant enzyme which neutralize superoxide radicals formed during the exercise. Furthermore, Mn develops important functions in gluconeogenesis and urea formation processes as an active compound of several involved enzymes. These processes are essential, becoming critical in aerobic sportsmen [31, 33]. Sánchez, López-Jurado, Aranda, and Llopis  in a work developed in the south of Spain, studied among sedentary and physically active populations Mn blood concentrations. These authors found the highest concentrations in active participants. The higher Mn concentrations in sportsmen would be linked to an increased organic utilization of this mineral in the mitochondrial Mn-SOD production processes, among others . Higher basal blood Mn concentrations in aerobic athletes in comparison with other metabolic sportsmen could be linked to a probably lower iron (Fe) levels, fact that is common in long distance sportsmen, as a consequence of the impact of Fe body levels in Mn metabolism [35, 36]. In this sense, it has been observed increased blood Mn levels in Fe deficitary cases [37, 38]. The fact that the AEG had the higher concentrations, followed with the AE-ANEG and the ANEG in our study, verifies and reinforces the previous reports.
Ni controls tissular levels of phospholipids, triglycerides, urea, glucose, glycogen and ATP [4, 5]. Due to these roles is critical among sportsmen to maintain high organic concentrations of this mineral. Berger et al.  found in marathon runners similar blood Ni levels that in the present survey. However, they had no control group.
The highest (p < 0.001) Ni concentrations in this survey have been reported in sportsmen. This fact could be linked to their high training volume as well as to the higher daily quantities of metabolic reactions needed to maintain high performance levels, specially among aerobic sportsmen. As previously indicated, it is known that Ni regulates tissular concentrations of substrates and metabolites, which are significantly altered during long-duration trainings and competitions. The obtained data reinforces this idea, being in aerobic participants the highest concentrations, followed by anaerobic and aerobic-anaerobic exercises (p < 0.001).
About the role of Se in sportsmen, it is known that physical activity generates an increase in free radicals of oxygen production, being this the fact which conduces sportsmen to improve their antioxidant system, as adaptive mechanism. A specific adaptive change is the behaviour of the enzyme GSH-Px, a Se protein .
The obtained results manifest lower Se concentrations in all sportsmen in comparison to CG. Sánchez et al.  also found lower Se values in active population in comparison to sedentary people. Se intake from food was not enough to maintain the constant levels of blood Se during training. Because the estimated intake of 48 μg·day-1 was nevertheless higher than the lower recommended level (30 μg·day-1), it should be considered that in sportsmen population the recommendation should be increased . The main reason of this affirmation is that Se requirements are increased among athletes .
This could be an important cause of the low Se concentrations found in our sportsmen. An augmented production of GSH-Px and a major synthesis of other Se-proteins could be a reasonably justification of the lower Se values among sportsmen.
In the present study the lowest concentration was found in the ANEG (p < 0.001) followed by the AE-ANEG and the AEG. Pograjc et al.  found decreased Se plasma levels during training of intensive physical activity. This fact suggests a slowly biologically active decreasing Se pool. The results of the study of Akil et al.  indicated that decreased liver glycogen levels in acute swimming exercise can be restored by selenium administration. It is possible, so, that the cause of the lowest Se concentrations in the ANEG and the AE-ANEG was a greater and more rapid replacement of muscular glycogen necessary for a high performance in these modalities. The results of the study of Akil et al.  indicate that the increase in free radical production and lactate levels due to acute swimming exercise in rats might be offset by selenium supplementation. Selenium supplementation may be important in that it supports the antioxidant system in physical activity. Physical activities which produce lactate and high amounts of free radicals are anaerobic, so, as the previous work indicated, anaerobic sportsmen would need more Se quantities to overcome the exercise-induced metabolic situations as well as to avoid oxidative damage. This could explain the obtained data.
A nutritional plan rich in Se containing foods may be beneficial for both athletes who exercise regularly and in patients with increasing oxidative stress .
Finally, regarding the limitations of this survey it has to be mentioned that to know the exact metabolic behaviour of each element, more matrixes have to be analysed, like urine, plasma, or sweat. The inclusion of these other matrixes could inform about the degree of elimination and redistribution of each element. In further surveys, multi-matrix studies should be carried out tin order to deep in the knowledge of the exact effect of diet and exercise in the organic trace elements values.