The coupled disorders of morbid obesity and type 2 diabetes (T2D) are a major and growing public health problem in Canada. The Public Health Agency of Canada 2011 report, Obesity in Canada, showed that 5.1% and 2.7% of Canadians had obesity class II (body mass index (BMI) 35.0-39.9) and III (BMI \>40 kg/m2), respectively. Morbid obesity is associated with not only T2D (up to 42% of morbidly obese patients), but also cardiovascular complications, non-alcoholic fatty liver disease and sleep apnea, and the prevalence of psychiatric disorders, particularly depression and anxiety, is high in this population. As morbid obesity is very difficult to treat, drastic measures are required, and bariatric surgery is often the only viable treatment option. Bariatric surgery is nowadays a frequently performed procedure (about 400 surgeries/year at the University Health Network (UHN)), and there are different techniques, including gastric banding, sleeve gastrectomy, and Roux-en-Y gastric bypass. Of these procedures, the Roux-en-Y gastric bypass surgery has become the gold standard, and it is highly effective in inducing long-term weight loss and improving or even resolving all of the obesity associated comorbidities. However, bariatric surgery is invasive, costly, has a risk of complications, and requires life-long commitment to a restricted diet. Therefore, a significant proportion of patients with morbid obesity is not willing to undergo the procedure, and a small proportion is excluded from the procedure due to other physical or mental health problems.In addition, depending on the health care system, bariatric surgery might not be available to a large proportion of patients. Regarding the growing rates of obesity and morbid obesity world-wide, less expensive and less invasive alternatives to bariatric surgery are urgently needed.

Even though obesity largely results from an imbalance between energy intake and energy expenditure, it has been shown that several factors, including the genetic background can render individuals susceptible to obesity. Most recently, the role of the intestinal microbiome has been under investigation. It has been shown that obese people have an intestinal microbiome and metagenome that is significantly different from lean controls, and this is even true in identical twins discordant for obesity. In addition, patients with T2D have a different intestinal microbiome than controls. Work done in rodents showed that fecal microbiota transplant (FMT) from lean to obese animals (and vice versa) can affect fat mass and parameters of the metabolic syndrome. It is thus possible that FMT may benefit human obesity with related metabolic abnormalities. FMT is becoming standard therapy for patients with refractory Clostridium difficile colitis and may become a treatment option in other gastrointestinal disorders. Only one human study (n=9) investigated FMT from lean subjects to obese patients (\~BMI 35 kg/m2) with metabolic syndrome and showed improvement in insulin sensitivity. Taken altogether, these very exciting results led us to hypothesize that that FMT from healthy lean donors could effectively induce metabolic improvement (i.e. insulin resistance (IR)) and weight loss) by distinct microbe-specific mediated mechanisms. The investigators will examine this in a single-center, double-blind, randomized controlled parallel-group trial (RCT).

Furthermore, emerging evidence shows that the intestinal microbiome, through the gut-brain axis, can influence mood disorders. First animal studies suggest that FMT can transfer depression and anxiety and therefore, FMT from healthy individuals may provide some benefits. Therefore, the investigators will also assess, whether FMT influences depression and anxiety, which are highly prevalent in obese patients. Finally, our Canadian Institutes for Health Research (CIHR) Team Grant, which supports this RCT, is also studying the potential relationship between obesity, T2D and the oral microbiome (OM). Considering that microbes present in the saliva are swallowed in significant numbers-about 10\^12 oral bacteria per day-and that this may influence the composition of the intestinal microbiome, the investigators are also exploring the potential relationship between oral and intestinal microbiome and their associations with obesity and T2D. Having a FMT protocol gives us the unique opportunity to further assess this potential relationship and determine if FMT may change OM by improving obesity and metabolic parameters. This has not been previously studied in animals or humans.

The aims of this RCT are to assess whether FMT from healthy lean individuals into morbidly obese patients with IR who decline bariatric surgery, leads to 1) improvement in metabolic parameters: IR, BMI, and other obesity related parameters; 2) improvement in mood disorders: depression and anxiety scores; 3) changes in the intestinal microbiome and metabolome. 4) In addition, by assessing the OM through the FMT protocol and by combining these results with the results of our other protocols in a similar patient population going through bariatric surgery, the investigators will explore the relationship between oral/intestinal microbiome and obesity/metabolic parameters. 5) Furthermore, as planned in our CIHR Team Grant, the investigators will use the FMT from the lean donors and transfer into obese mice to assess the effect of FMT on mechanisms related to glucose metabolism. These additional experiments are not part of this protocol but are mentioned briefly, as stool samples and data from the patients and donors participating in the FMT trial will be used for the other studies in our CIHR Team Grant.

Significance. The number of obese patients is growing world-wide. The investigators are examining here a'medical bypass' solutions to treat the 40-50% of obese patients that meet the criteria set by the National Institutes for Health (NIH) for bariatric surgery (12, 36) but decline (13), or the much larger population of obese patients (BMI 30-40 kg/m2) who may not be considered for surgical treatment.