There has been a lot of interest and discussion surrounding the impact that intestinal microbiota, the microscopic bacteria that live inside the digestive tracts of animals and insects, might have on obesity. In fact, the idea that your gut bacteria is to blame for obesity and unwanted weight gain seems to be one of the most prevalent claims of pseudoscientific online gurus at this moment in time.
While they might speak with confidence on the matter they have little grounds to do so. Currently you won’t find an actual expert in the field of microbiology that will attest to any causal link between gut bacteria and obesity in humans; the evidence simply does not support this notion.
“In both humans and animals there are characteristic changes in the gut microbiota associated with obesity. In animals but not in humans altering the microbiota can result in weight loss and weight gain which does not occur in humans. This suggests that in humans the changes in gut microbiota are an association with rather than the cause of obesity.”  This was the conclusion of a research review of all available English language literature concerning the issue.
This means that while there have been some compelling results from animal studies demonstrating a direct relationship between changes in gut bacteria and weight gain, even despite reduced food intake , the same is not true for humans.
So far, our understanding based on the totality of available evidence suggests that a poor or imbalanced gastrointestinal microbiome does not cause weight gain or obesity in humans, although it may be associated with it. However, the cause of both is overwhelmingly likely to be a chronically high fat, high carbohydrate, calorie surplus diet.
In particular, this large-scale analysis of data from The Human Microbiome Project Consortium found no association between Bacteroidete to Firmicute (the most common microorganisms in the human gut) ratio and obesity or BMI, no association between alternative taxonomic composition and BMI, and no association between gut microbiome community diversity and BMI . These were considered possible mechanisms through which gut flora might cause weight gain, but the human research largely disproves these.
Ultimately, it’s important to be wary of anyone trying to convince you that your gut bacteria is the direct cause of your weight gain or obesity, especially if they’re also trying to sell you a solution to this problem. The experts aren’t making this claim and the evidence doesn’t support it. Furthermore, the majority of the time the “special gut health” interventions sold by these voodoo guru types are nothing particularly bespoke, but rather they are universal interventions such as eating more plant matter, fibre, and probiotics alongside regular exercise.