The development of metabolic syndrome is usually associated with changes in the gut microbiota. In recent times, one of the most essential aspects of obesity has been considered to be a modification in bacterial aches in the human gut
[143,144][41][42]. Metagenomic studies and an analysis of 16S ribosomal DNA revealed significant differences in the composition of gut microbiota, and the number of genes when the feces of obese subjects and people of a healthy weight were compared
[145][43]. Without diminishing the role of heredity and environmental factors, the gut microbiota makes a significant contribution to the development of metabolic disorders and obesity, modulating the cascade of host enzymatic reactions, interacting with receptors directly and/or using its own metabolites and signaling molecules
[146][44].
It is obvious that maintaining homeostasis and normal metabolism is impossible without restoring the diversity of normal associations of gut microbiota. Despite the proven effect of diet, pre- and probiotics, further research is needed in order to develop differentiated regimens for the impact on gut microbiota, and thus achieve an improvement in metabolism and lose weight
[147,148][45][46]. FMT may be considered as a potential therapeutic strategy for the treatment of obesity in the future. An early pilot study split 18 patients into two groups: patients who received allogeneic FMT from lean donors (
n = 9) and obese patients who received autologous FMT (
n = 9). The group receiving allogeneic FMT displayed improved insulin sensitivity after 6 weeks
[149][47]. However, a subsequent larger study (
n = 38) showed that allogeneic FMT (
n = 26) failed to reduce insulin resistance, compared with autologous FMT (
n = 12) after 18 weeks, and correlated to a lack of overall change in the composition of the intestinal microbiota
[150][48]. FMT with oral capsules from lean donors to obese patients was tested in double-blind, placebo-controlled studies by Allegretti et al. It was shown that FMT did not affect the patient’s body mass index (BMI) and area under the curve (AUC) of GLP1, but helped to reduce the concentration of taurocholic acid. Patients who received FMT had sustained shifts in microbiomes associated with obesity toward those of the donor
[151][49]. The same group reported the secondary analysis of a previous RCT, with the analysis of post-prandial glucose and insulin levels. There was a significant change in glucose AUC at week 12, compared with the baseline, and in the insulin AUC at week 6 compared with the baseline in the FMT group versus placebo
[152][50]. Weekly administration of FMT capsules in a double-blind randomized placebo-controlled study for adults with obesity resulted in gut microbiota engraftment in most recipients for at least 12 weeks. Despite the lack of metabolic parameters, changes such as insulin sensitivity, HbA1c, body weight, and body composition by DXA were assessed
[153][51]. Zhang et al. have analyzed and compared data on the use of FMT in systemic review. Studies reported mixed results about improvement in metabolic parameters. Two studies reported improved peripheral insulin sensitivity at 6 weeks in patients receiving donor FMT versus patients receiving the placebo
[154][52]. No differences in fasting plasma glucose, hepatic insulin sensitivity, BMI, or cholesterol markers were observed between the two groups across all included studies
[154][52]. FMT has significantly increased the number of species such as
Roseburia intestinalis,
Akkermansia muciniphila, and
Clostridium species.
[154][52]. The most recent meta-analysis, with the inclusion of 6 RCTs and a total of 154 patients, evaluated the role of FMT from the lean donor(s) compared with any form of placebo (sham, saline, autologous FMT, or placebo capsules) for the treatment of obesity and metabolic syndrome. It was found that 6 weeks post-FMT, the level of HbA1c was significantly reduced. However, no difference was found for anthropometric parameters that characterize obesity at 6–12 weeks after the procedure
[155][53].
It is believed that the responses of patients with metabolic syndrome to modification of the gut microbiome may depend on the microbiota’s initial state and diet. Guirro et al. evaluated the effect of a hypercaloric diet on gut microbiota, and this was combined with antibiotic treatment to deplete the microbiota before FMT to verify its effects on gut microbiota-host homeostasis in rats. An HFD affected the gut microbiome and after the antibiotic therapy and subsequent use of FMT, the number of
Bacteroidetes,
Firmicutes was increased to the level that was before the antibiotic therapy
[156][54]. The largest recent RCT included 90 participants to evaluate the efficacy and safety of diet-modulated autologous FMT for the treatment of weight regain after the weight-loss phase (DIRECT PLUS trial)
[157][55]. The participants were randomly assigned to groups that received 100 capsules containing their own frozen fecal microbiota or placebo, until month 14. It was found that FMT administrated in the regain phase might preserve weight loss and glycemic control, and was associated with specific microbiome signatures
[158][56]. A high-polyphenols, green plant-based or Mankai diet better optimized the microbiome for an autologous FMT procedure
[158][56].