Are the gut bacteria of lean people different from those of heavier people?
The gut bacteria of people with excess weight differ measurably from those of lean people, but whether those bacteria are the cause or the consequence of excess weight has not yet been proven. If you want to improve your gut bacteria, a varied diet high in fibre remains the best-supported starting point.
Yes, clear differences have been found in the bacterial populations of people with excess weight compared with lean people. In people with obesity, the Firmicutes group of bacteria appears to be relatively larger, and species such as Clostridium and Staphylococcus aureus are found more frequently. These differences are complex, however, and have not yet been fully mapped.
What gut bacteria actually do in the context of excess weight is reasonably well described at a biological level. A disrupted bacterial population can make the gut wall more permeable, allowing inflammatory substances from bacteria to enter the bloodstream. This contributes to insulin resistance. At the same time, healthy gut bacteria normally produce compounds that stimulate the production of the satiety hormone GLP-1. When that production becomes disrupted, the feeling of fullness decreases and you eat more. Both mechanisms have been described in humans, although the precise interplay has not yet been fully clarified.
The big question is: which came first? Do abnormal gut bacteria lead to weight gain, or does excess weight change the bacteria? Animal experiments involving bacterial transplants point to a causal relationship: when germ-free animals receive the gut flora of obese animals, they gain weight. In humans, this link has so far been much less firmly demonstrated and remains largely associative. In addition, genetic predisposition, diet and lifestyle all play a role, which makes it difficult to separate cause from effect.
Whether gut bacteria can be modified as a treatment for excess weight is a separate question. Probiotics, prebiotics and faecal transplantation (introducing stool from a lean donor) are being studied, but the evidence in humans is limited so far and effectiveness has not been proven. Bariatric surgery and GLP-1 injections ('weight-loss jabs') appear to increase bacterial diversity, but whether that is a cause or a by-product of weight loss is not yet known.
All claims are based on associative research in humans and mechanistic animal studies (PMID 29667480, 26459447, 34321848, 35134709, 38636809, 35421277, 39673174, 31703257). No large randomised trials are available that prove causality in humans.