Can you strengthen your immune system by eating more fibre?

Eating more fibre does not automatically lead to less inflammation or a more diverse gut microbiome in healthy adults. In a 17-week randomised study (36 participants), the cytokine response, a measure of inflammatory activity, did not change measurably in the fibre group. The diversity of gut bacteria also remained the same, although the bacteria that were already present became more active in processing fibre.

What does seem to matter is which gut bacteria you already have before you start eating more fibre. The same study found three distinct immune trajectories, depending on the baseline composition of a person's gut microbiome. Fibre therefore played out differently for one participant than for another. That makes simple recommendations difficult.

Fermented foods performed clearly better in that same study: yoghurt, kefir or kimchi increased the variety of gut bacteria and lowered inflammatory markers. That was the strongest immune effect in the entire study. If you want to do something for your gut microbiome, fermented foods currently appear to be better supported by evidence than extra fibre.

In people with melanoma receiving immunotherapy, researchers observed that a higher fibre intake was associated with better survival odds. This is, however, an association study in cancer patients undergoing a specific treatment, not evidence for healthy people. Also notable was that commercial probiotic supplements in that same context appeared to actually worsen the therapy response, both in patients and in animal studies. This is a relevant safety signal if you are undergoing cancer treatment: consult your doctor before taking probiotics.

Review articles describe that fibre, as a prebiotic, can stimulate beneficial gut bacteria and influence the immune system, but concrete effect sizes from large clinical studies are still lacking. The broader picture, then, is that fibre is certainly not harmful, but that the direct immune effect in healthy people turns out to be smaller and more variable than is often assumed.