**4. Discussion**

This review was aimed to evaluate the efficacy of probiotics supplements as a therapeutic strategy for treatment-related side effects in adult oncology patients. After analyzing 20 randomized controlled trials, our findings showed the beneficial effects that probiotic may have in a range of common treatment-related side effects, which have a direct impact of the oncology patients' quality of life. In this manner, 11 of 20 studies (55%) observed positive outcomes among gastrointestinal adverse effects managemen<sup>t</sup> such as diarrhea, abdominal pain, nausea, and vomiting among others. Similarly, another six studies (30%) reported promising results in the control of immune and inflammatory responses, as well as other side effects related to their overall well-being and daily life activities. These findings further support the idea of previous reviews [44,45], suggesting that microbiota plays a key

role in the pathogenesis of some treatment-related side effects, although further evidence is needed to determine their safety and accuracy [46,47].

The studies included in this review were heterogenous in the use of probiotic strains, where *Lactobacillus acidophilus* (LA-5, BMC12130, LAC-361, and NCDO1748) was the most widely used strain among other 15 different strains, both in single strain [29,37] and multiple strain trials [25–28,31,34,42]. This heterogeneity added to the number of cancers included may explain some of the between-studies variability of the results [47]. Another possible explanation may be the interindividual diversity of the microbiota composition, where personalized medicine might well contribute to predicting the most suitable probiotic strain for the individual [48]. In this vein, strong evidence suggests that the efficacy of probiotics is strain-specific as well as disease-specific, and therefore, these factors should be considered when recommending the best probiotic for the patient [49]. Furthermore, the duration of treatment may also have to be considered to demonstrate probiotic clinical position in the oncology of treatment-related side effects, 4 weeks being the most common duration of treatment among the studies included. These results are consistent with the findings of De Sanctis and collaborators (2019) [39], who stated that a probiotic treatment period of less than 4 weeks may not be sufficient to observe and confirm their beneficial effects. However, to date, there are not standardized procedures available on the minimum treatment duration for the selected probiotic strain in order to observe positive outcomes, as it requires time to promote gu<sup>t</sup> microbiota re-shaping and, as a result, the beneficial effect [50].

In reference to the treatment-related side effects, authors such as Delia and collaborators (2007) [34], Golkhalkhali and collaborators (2018) [26], as well as Osterlund and collaborators (2007) [33] among others, concur that the use of probiotics and microbial cell preparations improves the intestinal immune barrier, particularly intestinal IgA responses. In line with the results of other studies, these probiotic strains are able to stabilize the intestinal microbial environment and improve the permeability of the intestinal barrier, leading to a reduction in inflammatory response and promoting changes in the intestinal flora [51,52]. This promotes an ideal environment for the growth of non-pathogenic bacteria, helping to protect epithelial cells, the process of apoptosis, and some cytoprotective processes [53]. Interestingly, similar results were found using probiotic strains such as *Lactobacillus*, *Bifidobacterium,* or *Streptococcus* along with other eco-nutrients such as glutamine and fish oil [38]. These results match those observed in recent preclinical studies [54,55], where the colonization of this bacteria genera enhanced the immune and anti-inflammatory response to radiation, forming an enteric–intestinal barrier that increased the thickness of the intestinal flora. Moreover, the optimization of the medium promotes the life of living microorganisms, which can restore the balance of a radiation-damaged microecosystem by repairing the intestinal membrane, inhibiting the growth of intestinal pathogens, and reducing endotoxin production [56]. These probiotic strains are antioxidant agents that act by eliminating free radicals produced by ionization and preventing lipid oxidation, thereby prioritizing the repair and regeneration of the cell membrane, DNA, and proteins, resulting in their high efficacy in reducing abdominal pain, flatulence, and diarrhea, as these authors highlight in their findings [38,56].

In accordance with these findings, Holma and collaborators (2013) [30] underline the importance of fecal pH and methane production in this type of patient, where intestinal microbiota plays a central role in the incidence of unpleasant side effects such as diarrhea and constipation, bloating, or abdominal inflammation. These results confirm the association between the higher production of elements such as methane and microbiota, where a higher production of methane is associated with a lower incidence of diarrhea and a methane deficiency is associated with a higher incidence of abdominal discomfort [57]. In this context, the results showed that the *L. rhamnosus* GG ATCC 53,103 strain did not alter the production of pH or methane, as opposed to studies such as Salminen and collaborators (1988) [37], in which *L. acidophilus* NCDO1748 was administered and increased flatulence was observed, pointing directly to the lactulose content as a non-absorbable substrate, a

mechanism favoring the production of methane and probiotic absorption. In this sense, Osterlund and collaborators (2004) [35] provide information on lactose intolerance caused by low intestinal villus height in relation to its depth of treatment, resulting in malabsorption syndrome and therefore hindering the production of diarrhea, flatulence, and abdominal pain [58]. In line with the overall evidence, it is worth noting how adverse effects can be managed by modifying gu<sup>t</sup> microbiota and methane production mechanisms [30,59]. Replacing lactulose with another non-absorbable substrate would not cause diarrhea and would, in turn, allow the amount of methane to be controlled to achieve balance in intestinal transit, vary the amount of substrate administered, and greatly improve or even reduce the number of treatment doses administered to patients [35,57].

On the other hand, oral mucositis and oral health stand as one of the most treated side effects as they significantly reduce the patients' quality of life [60]. In that matter, probiotics such as *B. longum* (BCMC02120, BB-536), *L. lactis* BCMC12451, *E. faecium*, and *L. brevis* CD2 have shown to reduce the incidence of severe oral mucositis by promoting the growth and protection of the bacterial flora and, as a result, decreasing the number of adverse effects, severity, and incidence of mucositis [20,40–42]. These findings are in agreemen<sup>t</sup> with those of Vesty and collaborators (2020) [41], who identified that using *S. salivarius* M18 improved patients' quality of life by reducing the number of oral infections (candidiasis) and adverse effects (mucositis, diarrhea) that these patients experienced after their treatments. However, recent research found that the effects of *L. brevis* CD2 were unable to confirm its beneficial impact for severe oral mucositis, though one possible explanation for these findings could be the premature closure of patient accrual [39]. Lastly, it is also interesting to note the effect of probiotics on other side effects of these patients such as pleural effusion, which can severely affect their performance status and even shorten their life expectancy. Only Masuno and collaborators (1991) [43] evaluated the use of the *L. casei* LC9018 strain against this complication, demonstrating promising results in controlling pleural effusions by reducing the number of malignant cells at the pleural level, which are supported by preclinical models [61,62].
