Can you strengthen your immune system by eating more fibre?

Gut bacteria convert fibre into short-chain fatty acids, such as butyrate. Those fatty acids play a proven role in regulating the immune system, both in the gut itself and elsewhere in the body. This mechanism is well documented and forms the biological basis for the expectation that fibre could strengthen your immune defences.

Whether that mechanism also leads to a genuinely better immune response in practice is far less certain. In a 17-week randomised study in healthy adults, a high-fibre diet did not produce a measurable change in the primary immune outcome: the antibody response to external stimuli. Notably, three distinct response patterns did emerge, depending on how much bacterial diversity a person already had before the diet. People who already had a rich gut microbiome responded differently from people with a less varied microbiome. Fibre does not act here like a switch you simply flip, but rather as something whose effect varies considerably from person to person.

Fermented foods, such as yoghurt, kefir or sauerkraut, performed more consistently in that same study. They increased bacterial diversity in the gut and reduced inflammatory markers, while the high-fibre diet did not do so in a clear-cut way.

In people with melanoma receiving immunotherapy, adequate fibre intake was associated with longer progression-free survival. This is an observational finding, not proof of cause and effect, but the signal is striking. In precisely that context there is also a clear warning: commercial probiotic supplements taken alongside immunotherapy worsened the effectiveness of treatment, both in patient data and in animal experiments. If you are receiving immunotherapy and want to take probiotics, discuss this with your treating physician first.