Can you strengthen your immune system by eating more fibre?

Fibre does not automatically strengthen your immune system. Gut bacteria convert fibre into short-chain fatty acids, such as butyrate. These fatty acids have a proven role in regulating your immune defences, both in the gut and elsewhere in the body. That mechanism is well documented, but whether it leads in practice to a measurably better immune system is a different question.

In a 17-week study in healthy adults, compared with a placebo, a high-fibre diet did not change the primary immune outcome. 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 population responded differently from people with a less varied microbiome. Fibre does not work here like a switch you simply flip: the effect varies considerably from person to person. If you want to try it for your gut flora, choose dietary fibre through varied foods such as wholegrains, vegetables, fruit and legumes. On the label, look for 'dietary fibre' or 'prebiotic fibre'. Which specific type of fibre contributes most to microbiome benefits is unclear, so the brand or exact composition matters less.

Fermented foods, such as yoghurt, kefir or sauerkraut, performed more consistently in that same study: they increased bacterial diversity and reduced inflammatory markers, while the high-fibre diet did not do so unambiguously. If you want to do something for your gut flora and immune defences, fermented foods are the better choice at this point.

In people with melanoma receiving immunotherapy, those with a higher fibre intake had on average a longer progression-free survival, but it has not been proven that the fibre itself was the cause. What is clear is that commercial probiotic supplements taken alongside immunotherapy worsened the effectiveness of the treatment, both in patients and in animal models. If you are on immunotherapy and are considering probiotics, discuss this with your doctor first.