Can your gut microbiome control your appetite and hunger?

Gut bacteria have direct access to the brain via the vagus nerve, a large nerve that connects the abdomen to the brain. Specialised intestinal cells that produce hormones sense bacterial activity in the gut and send signals upward along that nerve. This means gut bacteria can help determine how much you eat and even which foods you prefer.

When bacteria break down indigestible fibres, short-chain fatty acids are produced, including acetate, propionate and butyrate. These compounds are not passive: they influence appetite, blood sugar balance and energy expenditure. However, it is important to note that most of the evidence for this comes from animal studies. In humans, very few well-designed studies have been carried out on this topic, so we do not yet know precisely how large this effect is in practice.

The composition of your gut microbiome is also associated with body weight. People with obesity have measurably different bacterial populations than people with a healthier weight. Studies using germ-free animals and microbiome transplants show that these differences are probably not simply a consequence of unhealthy eating, but also play a causal role in weight gain. This makes the gut microbiome a potentially interesting target, although we are still far from translating that knowledge into practical treatments.

Dietary fibre is the most important fuel for your gut microbiome. Western diets contain far too little of it, which impoverishes the microbiome and reduces its ability to perform that protective role. Eating more fibre-rich food is the most concrete, evidence-supported step you can take today, backed by multiple studies. Whether manipulating the gut microbiome through other means, such as microbiome transplants after weight loss, also works remains uncertain for now: only a handful of early studies have been conducted on this.

The diabetes medication metformin illustrates how indirect the influence of the gut microbiome can be: part of this drug's weight-reducing effect is thought to occur through changes in the gut microbiome, in addition to direct effects on the appetite centre in the brain. This mechanism has not yet been fully unravelled, but it illustrates that the gut-brain axis is a genuine route through which appetite can be regulated.