Exploring Mechanism of Action of Dietary Fibre on the Gut Microbiota and Metabolites: Can we Identify Responders and Non-responders to Psyllium and Inulin Dietary Fibres?

Of all patients receiving radiotherapy for cancer in 2018 and 2019, 28.6% were treated for a pelvic malignancy. The dose of radiotherapy necessary to cure the tumour often causes side effects in the surrounding healthy tissues, which can significantly impact the patients quality of life. Chemoradiation, the addition of chemotherapy to radiotherapy, improves tumour cure rates by radiosensitising the tumour, but at the expense of worsened normal tissue effects. Most patients over 80 years are unable to tolerate chemoradiation. With an aging population, there is an urgent clinical need to find alternatives to chemoradiation.

A difference in the composition and diversity of the gut microbiota has been found in responders versus non-responders to immunotherapy and in patients experiencing different levels of toxicities following radiotherapy. This indicates that the microbiota could be used to predict, prevent, or treat radiation enteropathy. An often-found side effect in pelvic radiotherapy patients is diarrhoea. Many pelvic radiotherapy patients receive a form of dietary fibre, known as psyllium husk (ispaghula husk or FybogelTM) to manage their diarrhoeal symptoms. Dietary fibres like psyllium and inulin are non-digestible carbohydrates found in fruits, vegetables, whole grains and legumes. They are anaerobically fermented by the gut bacteria, producing SCFAs, including acetate, propionate, and butyrate. SCFAs are the main performers in the interplay between diet, microbiota and health, and have anti-inflammatory and also anti-tumorigenic properties.

A high intake of dietary fibre is proven to decrease the risk of type 2 diabetes, colon cancer and cardiovascular disease by reducing the digestion and absorption of macronutrients and decreasing the contact time of carcinogens with the intestinal lumen. There is emerging evidence that the inclusion of dietary fibre supplements in the diet could modify the tumour response to radiotherapy. If confirmed in humans, this would be a major advance in patient treatment.

In a mouse model of colitis, exhibiting similar acute symptoms to those due to radiotherapy, the addition of psyllium in the diet reduced colonic inflammation and protected intestinal tight junctions. This prevented penetration of luminal inflammatory molecules through the gut wall. In a similar model system, psyllium reduced the severity of colitis in C57BL/6 specific pathogen-free and germ-free mice, via microbiota-dependent and independent mechanisms.

Inulin, another dietary fibre, is rapidly fermented in the proximal large intestine. Here, it selectively increases the growth of the butyrate producer Faecalibacterium prausnitzii and Bifidobacterium spp.. Bifidobacteria are associated with anti-tumour effects and augmented dendritic cell function leading to increased Cytotoxic T-cell (CD8+) priming and accumulation in the tumour microenvironment.