## 4 Discussion  
In the present study, a significant level of reduction was observed in QA, while the 5-HIAA level increased significantly after the 12 weeks of _L.__paracasei_ HII01 supplementation (Table 2). It has been proven that lower serum levels of tryptophan, kynure acid, xanthurenic acid, and 3-hydroxyanthranilic acid are associated with ADHD adults [32]. Lower levels of 3-hydroxyshurenine in ADHD children have been related to delayed brain maturation [33]. [PERSON] et al. [34] reported changes in serum levels of kynure metabolites in ADHD subjects. Serum levels of anthranilic, kynure, and xanthurenic acid were lower in ADHD children compared to healthy controls; however, kynure and tryptophan concentrations increased. There were no changes observed in QA concentration. The statistical analysis suggested that lower anthranilic acid and high tryptophan were associated with ADHD, which might be a biomarker for ADHD [34]. It is known that the etiology of neurological complications associated with inflammation, oxidative stress, and excitotoxicity might alter the metabolites of KP. Studies suggest that neurological disease and disorder complications could activate KP inappropriately, leading to increased QA levels [35]. In the present study, levels of QA in the urine were significantly reduced after the 12-week supplementation of _L.__paracasei_ HII01 in healthy children (from 26.73 \(\pm\) 1.42 to 19.64 \(\pm\) 1.08 ng/mL) (Table 2). Therefore, we can hypothesize that probiotic supplementation protects the host by hindering the inappropriate activation of KP. However, further studies are needed to confirm these statements. Reduced serotonergic activity in the brain is related to the susceptibility to depression and mania. Reduction in serotonergic activity is directly correlated with 5-HIAA concentration [36]. Lower levels of 5-HIAA may be correlated with depression, aggressive or violent behavior, and obsessive-compulsive disorder [25]. Reduced cerebrospinal fluid levels of 5-HIAA have been observed in Alzheimer's patients [37]. Moreover, tryptophan metabolites such as serotonin correlate with cognition, and neurological and psychiatric disorders have been related to reduced levels of 5-HIAA [37]. The current study results revealed that the supplementation of _L.__paracasei_ HII01 significantly improved urine 5-HIAA levels in children (Table 2). An increased QA/5-HIAA ratio indicates inflammation and immune stimulation. The host-microbiota regulates serotonin synthesis in intestinal enterochromaffin cells and serotonergic signaling. Serotonin influences mucosal inflammation, immune responses, and brain activity in the host. Host microbiota-mediated serotonergic signaling is involved in intestinal diseases and disorders [38]. Accordingly, _L.__paracasei_ HII01 supplementation reduced the QA/5-HIAA ratio (Table 2) in healthy children, possibly due to improved gut microbiota after the probiotic supplementation. We observed significant changes in both go accuracy and go error. However, the changes in both no-go accuracy and no-go error levels were not significant. The go response time increased, but this was not statistically significant. Beta wave activity was reduced, while theta and alpha wave activity increased significantly after 12 weeks of _L.__paracasei_ HII01 supplementation (Figure 3). There was no concrete evidence for the influence of probiotic intervention on go/no-go task responses and EEG/ERP. [PERSON] et al. reported that the supplementation of long-chain polyunsaturated fatty acids significantly improved behavior and ERP while children performed a go/no-go task [26]. Similarly, the n-back/no-go combination paradigm and ERP were used to study working memory and response inhibition in ADHD subjects. The n-back/no-go combination paradigm indicated that ADHD patients have problems with their working memory and reaction inhibition, higher omission errors, [slower/faster] reaction times, and decreased n-back and no-go-P3 amplitudes [39]. [PERSON] et al.