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.
Are the gut bacteria of children different from those of adults?
Yes, the gut bacteria of children differ fundamentally from those of adults, with consequences for allergy and asthma risk. If you want to positively influence a baby's gut microbiota, breastfeeding and a normal vaginal delivery are the factors with the strongest supporting evidence.
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.
Does eating a varied diet help your gut bacteria?
Fermented products have the strongest evidence base for increasing gut bacterial diversity. Eating broadly varied meals also helps, but above all avoid heavily processed products containing emulsifiers such as CMC.
Can your gut microbiome influence how long you live?
There are clear indications that the composition of your gut microbiome is associated with how long and how healthily you age, but whether modifying your microbiome extends your life has not yet been demonstrated in humans.
Does my gut microbiome change as I get older, and does it matter?
Your gut microbiome certainly changes as you get older, but whether that is harmful depends strongly on which direction that change goes. What you eat has a major influence on this, so that is a concrete area to pay attention to.
Are plant-based diets better for your gut than mixed diets?
A plant-based diet appears to be beneficial for your gut bacteria, but fermented foods perform at least as well in a direct comparison. Whether healthy people also notice a meaningful long-term benefit has not yet been demonstrated.