Is my gut flora linked to being overweight?
Your gut microbiome is probably linked to excess weight through multiple mechanisms, but a proven causal relationship in humans is still lacking. A varied diet that supports your gut microbiome is worthwhile, but specific supplements for weight loss have not yet been proven effective.
Yes, your gut microbiome is probably linked to being overweight, but whether it is a cause or a consequence is not yet fully understood. People with obesity more often have a different gut microbiome than people with a healthy weight. In people with excess weight, bacteria from the Firmicutes group are found more frequently, for example. These are linked to greater energy extraction from food and changes in satiety hormones, which can lead to eating more and gaining weight.
Gut bacteria produce substances such as short-chain fatty acids and indole, which drive the production of gut hormones like GLP-1 and GIP. Those hormones regulate your feeling of fullness and your weight. A disrupted gut microbiome can throw that hormone production out of balance. How large that effect is in practice still needs to be established more precisely.
Another mechanism operates through the gut wall. A disrupted gut microbiome can make the gut wall 'leaky' to bacterial building blocks known as lipopolysaccharides (LPS, fragments of bacterial cell walls). These then enter your bloodstream and cause low-grade inflammation. That leads to insulin resistance, which is closely associated with excess weight and type 2 diabetes. Several review studies identify this mechanism as relevant, although the exact magnitude in humans has not yet been precisely measured.
There are also indications that the gut microbiome communicates with your brain via nerve pathways, hormones and the immune system, thereby influencing your eating behaviour. Mouse studies have demonstrated this convincingly: when the gut microbiome of people with obesity is transferred to germ-free mice, those mice gain more weight. However, that evidence cannot simply be extrapolated to humans.
Treatments such as faecal microbiota transplants, probiotics and prebiotics are being investigated as approaches to obesity. The results are promising, but the studies in humans are still too small and too short. There is currently insufficient evidence that any specific probiotic or prebiotic reliably helps with weight management. Larger and longer clinical trials are needed for that.
All claims are based on seven PMIDs (35421277, 34321848, 29667480, 26459447, 35134709, 39673174, 39040013, 38636809), including multiple reviews and overview studies. Most human evidence is associative; causal evidence comes largely from animal models.