## 4 Discussion  
In this systematic review, we examined studies on football players or football task interventions that evaluated the causality of exercise or its relationship with changes in basal BDNF level. Few studies showed the effect of a football match on BDNF concentrations [36; 37]. On the one hand, one study found a significant increase in serum BDNF level after a 90 min match in 15-year-old adolescents [39]. On the other hand, after a 60 min football match, it was reported that the BDNF level was not altered in 12-year-old adolescents [37]. Other studies have been conducted with football players as subjects, which evaluated the responses of their neurotrophic factor to different types of training [6; 35], the effect of frequent heading [34], as well as the influence of polymorphism on expression [33; 38]. BDNF concentrations increase immediately after physical aerobic, anaerobic, and high-intensity exercise [35; 39]. Some meta-analyses have reported that this response is conditioned by factors, such as the duration and intensity of the efforts, the type of exercise, frequency, and duration of the intervention, as well as gender [40; 41]. Exercise intensity must be at least 65% of VO2 max to show a linear relationship with BDNF concentration [41]. The BDNF level can increase by around 60% in the blood at the peripheral level as an acute response and there is a greater release in activities that exceed 30 min [40]. Figure 2: Main outcomes of studies analysing the effects of football playing on brain-derived neurotrophic factor levels [6; 33; 39]. The physical fitness of the participants has also been found to affect the release of BDNF, although the results were different. In 12-year-olds with high fitness, no change in serum BDNF levels was observed [37]. The physical and physiological adaptations generated by training programs can also affect BDNF responses [8]. Interestingly, in some studies, lower concentrations of BDNF have been observed in football players [6] or adults who perform more physical activities than sedentary adults [7]. Low concentrations of this neurotrophic factor have been found in football players; an inverse correlation was even found between VO2 max and BDNF, i.e., BDNF levels were higher in those with a low cardiorespiratory capacity [6]. When a sample of non-athletic men was analysed, it was found that those people who presented a vigorous level of physical activity had low BDNF levels [7]. BDNF contributes to an improvement in cognitive responses; in football players, it has been evaluated using a memory test and information processing and the domain of inhibitory control. The evidence suggests that there is better performance in the Stroop Colour Word test [36]. The physical fitness of adolescent football players can also be considered to be a factor that modulates these cognitive responses after a match [37]. A study indicated that environmental conditions could affect the release of neurotransmitters, and therefore could influence cognitive functions [35]. It has also been concluded that the immediate effects of BDNF release after anaerobic training on master football players did not seem to imply memory capacity [6]. Indeed, the outcomes of this review suggest that football playing improves neurocognitive function [38; 39]. The fittest players performed better in information processing and working memory tasks [36], suggesting a role of BDNF release in cognitive function during these sporting activities. It has been reported that both aerobic and resistance exercises trigger the peripheral release of BDNF [40]; however, it has been found that transient increases in peripheral BDNF are more prone with aerobic exercise than with resistance exercise [12]. Little evidence has evaluated changes in BDNF in high-intensity activities [2].