This systematic review included four RCTs that evaluated prebiotics, probiotics, or synbiotics [8, 11–13]. Our meta-analysis showed that probiotic supplementation did not significantly reduce %EWL compared to placebo. The narrative synthesis demonstrated a statistically significant improvement in triglycerides and an increase in vitamin B12, as well as a decrease in the perception of gastrointestinal symptoms.
There is abundant literature supporting the relationship between the intestinal microbiota and the development of obesity and metabolic diseases [14], as well as the change in the intestinal microbiota after bariatric surgery [15–17]. An inverse relationship between the degree of obesity and the bacteroid population has been demonstrated; furthermore, an increase in the Firmicutes/Bacteroides ratio occurs after Roux-en-Y gastric bypass [17–19]. Reasons influencing intestinal colonization include (a) change in dietary habits [19]; (b) increased oxygen concentration in the distal intestine [20]; (c) decreased intestinal pH; and (d) more distal delivery of bile juices [21]. These factors would favor intestinal bacterial overgrowth, mainly responsible for the symptoms of intestinal ballooning, abdominal pain, nausea, and diarrhea [22–24].
The %EWL was not significantly altered with probiotic supplementation (L. acidophilus and B. lactis). This result contradicts other reviews reporting a significant decrease in weight [25, 26]. This difference could be due to the fact that the effect of this supplementation on body weight depends on the bacterial species and strain [27]. In addition, there is evidence that the presence of two different species modifies the mutual metabolic activity, as reported in the study by Arora et al., where the administration of lactobacillus increased the colonization of the genus Bifidobacterium [28]. Furthermore, it has been described that the survival of probiotics depends on the acidity and the presence of prebiotics, so the effect may depend on these factors [29, 30]. In our meta-analysis, only studies that have used probiotics as an intervention were synthesized, so this must be taken into account.
In obese individuals, probiotic administration positively modulates the functions of the intestinal microbiota, thus regulating glucose and fat metabolism pathways, insulin sensitivity, and reducing chronic systemic inflammation [31]. The modulatory mechanisms involved are reinforcement of intestinal epithelial barrier integrity, increased production of short-chain fatty acids, protection against metabolic endotoxemia (LPS), regulation of the host immune response, and competitive exclusion of pathogenic microbes [32, 33]. Probiotics based on Lactobacillus and/or Bifidobacterium have been shown to inhibit various pathogenic microorganisms, such as Escherichia coli, Shigella, Klebsiella, and Proteus [34]. However, in patients undergoing RYGB a shift in the taxonomic composition of the microbiome was found towards putatively pathogenic bacterial phyla such as Proteobacteria, and the Enterobacteriaceae family including genera such as Escherichia, Shigella, Klebsiella, and Pseudomonas [35–39]. Previously, Sanchez-Alcoholado et al. [39] found an inversely proportional correlation between Enterobacteriaceae and total cholesterol and LDL cholesterol in patients undergoing RYGB. The competitive inhibition mechanism of probiotics on this family of bacteria could counteract their effects on the regulation of lipid metabolism and, therefore, on %EWL. However, much remains to be elucidated about the beneficial effects of changing microbiota composition in patients undergoing RYGB.
Another effect reported by Wagner et al.[12] is the improvement of gastrointestinal symptoms caused by SIBO after Roux-en-Y gastric bypass. This is in agreement with a systematic review that reported the benefits of probiotic supplementation in patients with irritable bowel syndrome and other gastrointestinal problems [40]. The use of probiotics modulates the growth of the intestinal microbiota thanks to their facility to adhere to the intestinal mucosa, this would improve intestinal motility and acidity, thus decreasing the proliferation of pathogenic bacteria responsible for gastrointestinal symptoms [41].
On the other hand, Ramos et al. [13] report at 12 weeks a higher concentration of vitamin B12 and lower triglyceride levels in those patients taking probiotics after Roux-en-Y gastric bypass. These findings are in agreement with reviews that reported an improvement in the lipid profile as well as an increase in certain micronutrients (vitamin B12, calcium, folate, iron, and zinc) [42, 43]. These findings would be directly related to the functions of the microbiota in the absorption of vitamins and fats [44].
The certainty of evidence from our meta-analysis was very low due to imprecision and a high risk of bias in the two included studies. Therefore, we recommend that future trials have better strategies for loss-to-follow-up control or report the reasons in detail. On the other hand, our quantitative synthesis was based on two trials using L. acidophilus and B. lactis strains, so other bacterial species should be investigated to find that combination with positive effects.
Our study has some limitations that should be considered when interpreting our results. First, we performed a comprehensive literature search, however, we may have missed studies because we did not use Chinese or gray literature databases. In addition, we cannot determine the long-term effect of our intervention as all studies had a maximum follow-up of 12 weeks, so efficacy and safety after this point is unknown. Finally, this review did not look for real-world cohorts evaluating the effectiveness or patient satisfaction, so more studies of this type are needed.