## 4 Discussion
However, in male adults (not football players), it has been shown that the execution of short series of high-intensity exercises (90% HRmax) alternated with similar periods of rest (1:1) can lead to a greater release of serum BDNF than continuous high-intensity exercise (70% HRmax) [16]. During a protocol of 1 min of effort at 100% VO2 peak and 1 min of rest (1:1), a significant increase in BDNF level was observed immediately at the end of the exercise, although it decreased 60 min later [15]. During anaerobic exercises of high intensity, BDNF can be released to counteract the lack of oxygenation in the brain. Between 70 and 80% of the surrounding BDNF is contributed by the brain [42]. Likewise, it has been stated that lactate can promote the release of BDNF [43], and therefore BDNF concentration increases after high-intensity exercise, even after maintaining its level in post-exercise rest periods [14]. Serum BDNF levels after physical efforts may decrease due to their use in lipid metabolism that occurs at the peripheral level [42]. Another factor that can modulate BDNF secretion is hydration status and neuromuscular fatigue [35]. On the one hand, serum BDNF levels increased in a small group of young football and rugby players, who ran for 60 min at 75% of the heart rate reserve in a thermoneutral environment (18 \({}^{\circ}\)C), as well as in a high-temperature environment (32 \({}^{\circ}\)C), drinking water or sports drinks for hydration. On the other hand, when this same group was prohibited hydration in a high-temperature environment (32 \({}^{\circ}\)C), serum BDNF levels decreased immediately after the run [35]. Coaches and athletes should address that heat could inhibit BDNF release and that fluid intake can be a protective factor [37]; therefore, hydration protocols should be defined during training and competition to allow regular acute BDNF responses. BDNF polymorphisms have been evaluated [33; 38]. Considering the implications of gene expression on BDNF release levels, two studies in football players suggested that players with BDNF CC homozygote polymorphism expression sprinted quicker and jumped farther [33]. A study assessed BDNF (rs6265) CC (also known as Val 66 Met)and its relationship with physical tests, and found that the homozygous BDNF (CC) was associated with faster running speed and jumping capacity [33]. In contrast, in a recent study, the Val 66 Met gene was shown to be associated with poor motor performance in basketball players [44]. In another study, the relationship between BDNF Val 66 Met polymorphism with repeated heading was explored, and showed that players with Met (+) showed an association between high exposure to heading and low radial diffusivity [38], which indicated an affection in the myelination of the axons, which could be reflected even in functional tests [38]. A previous systematic review has also shown that this polymorphism (BDNF Val 66 Met) could affect the release and concentration of BDNF [40]. Although it has been known that BDNF stimulates myelination, a high frequency of heading can affect it [38]. Given the characteristics of football, players have frequent contacts between the ball and their heads. Although this area has not been as well addressed in studies and the conclusions are not precise, an association has been reported between head butting and structural level affectations determined by neuroimaging techniques, as well as in the release or restrictions of neurotransmitters [13]. Repeated heading can cause increases in serum BDNF associated with microtrauma [34]. Increased peripheral BDNF and NGF levels have been associated with the recovery of possible damage generated by trauma in the central nervous system caused by contact with the ball [13,34]. Considering these results, soccer players should be monitored for the amount and severity of headings they perform during training to avoid microtrauma and subsequent inhibition of BDNF release. These actions should be considered, especially in young populations and masters, who have fewer protective factors.