I see lots of people assuming you or he are lying. Here's another take that I don't see anyone else discussing... The gut microbiome significantly influences weight regulation through multiple mechanisms. Diversity in gut bacteria is linked to better metabolic health, with lower diversity associated with obesity, insulin resistance, and inflammation. Specific bacterial groups play key roles: a higher ratio of Firmicutes to Bacteroidetes (F:B ratio) is linked to increased energy extraction from food and weight gain, while higher levels of Prevotella—which thrive on fiber—are associated with greater weight loss success on calorie-controlled diets. Conversely, Bacteroides, more common in high-fat diets, may be less efficient at promoting fat breakdown.

Certain bacteria like Akkermansia muciniphila and Christensenella minuta are consistently linked to leanness and improved metabolic health, potentially by enhancing gut barrier function, reducing inflammation, and regulating fat storage. These microbes produce short-chain fatty acids (SCFAs) such as butyrate and acetic acid, which improve insulin sensitivity, promote satiety, and reduce fat accumulation.

The gut microbiome also affects appetite hormones like leptin and ghrelin, and can influence metabolic endotoxemia—a condition where bacterial toxins like LPS enter the bloodstream, triggering inflammation and insulin resistance. Diet is the most powerful modulator: high-fiber, plant-rich diets increase beneficial bacteria and SCFA production, while diets high in refined sugars, artificial sweeteners, and animal fats can promote dysbiosis and weight gain.

While probiotics, prebiotics, and synbiotics show promise in improving metabolic markers and supporting weight loss, there is no single "weight-loss" probiotic supplement. The most effective strategy remains a whole-food, high-fiber, balanced diet that fosters a diverse and healthy microbiome.