How do tumours outside the gastrointestinal tract respond to dietary fibre supplementation?

Cancer remains one of the leading causes of death worldwide, despite advances in treatments such as surgery, chemotherapy, radiotherapy and immunotherapy. The role of the gut microbiota in human health and disease, particularly in relation to cancer incidence and treatment response, has gained increasing attention. Emerging evidence suggests that dietary fibre, including prebiotics, can modulate the gut microbiota and influence antitumour effects. In this review, we provide an overview of how dietary fibre impacts the gut-tumour axis through immune and non-immune mechanisms. Preclinical evidence shows that β-glucan or inulin effectively suppress extraintestinal tumour growth via immunomodulation. Other fibres such as resistant starch, modified citrus pectin and rye bran may confer antitumour effects through metabolic regulation, production of metabolites or downregulation of the insulin/insulin-like growth factor 1 axis. Additionally, we highlight the potential for dietary fibre to modify the response to immunotherapy, chemotherapy and radiotherapy, as shown by inulin increasing the abundance of beneficial gut bacteria, such as Bifidobacterium, Akkermansia, Lactobacillus and Faecalibacterium prausnitzii, which have been associated with enhanced immunotherapy outcomes, particularly in melanoma-bearing mice. Furthermore, certain types of dietary fibre, such as psyllium, partially hydrolysed guar gum, hydrolysed rice bran and inulin plus fructooligosaccharide, have been shown to mitigate gastrointestinal toxicities in patients with cancer undergoing pelvic radiotherapy. Despite the proven benefits, it is noteworthy that most adults do not consume enough dietary fibre, underscoring the importance of promoting dietary fibre supplementation in patients with cancer to optimise their treatment responses.