Radical radiotherapy for prostate cancer frequently causes debilitating side effects that can significantly diminish patients’ quality of life during and after treatment. The DIETRICH study, funded by Prostate Cancer UK, will investigate whether a simple intervention – dietary fibre supplementation – can help alleviate these challenging symptoms. Lead investigators Professors Anne Kiltie and Ananya Choudhury discuss the study’s design, its objectives and how a straightforward nutritional approach could potentially provide a relatively simple solution to a widespread, significant problem.
Men undergoing radical radiotherapy for prostate cancer frequently suffer gastrointestinal and urinary side effects during treatment and for some weeks afterwards.1
The most common bowel and bladder complications are diarrhoea, abdominal discomfort, more frequent urination and bleeding. These side effects can significantly impact the patients’ quality of life by disrupting daily routines, interrupting sleep, making travel more difficult and contributing to increased anxiety.
Although generally short-term during the second half of radiotherapy and for a few weeks after, these side effects can be followed by long-term, life-changing side effects in some men.
The potential impact of dietary fibre
In some centres, men are advised to remove fibre from their diet during radiotherapy or are prescribed the anti-diarrhoeal loperamide. In contrast, other centres give dietary fibre supplements such as psyllium mid-treatment to help reduce diarrhoeal side effects.
There is some evidence that supplementing dietary fibre improves radiotherapy side effects in pelvic cancers. Murphy et al2 found that psyllium given mid-treatment reduced the incidence and severity of diarrhoeal side effects in a randomised trial of 51 men with prostate cancer and nine women with gynaecological cancers receiving at least 40 Gy in 20 fractions to the pelvis.
Garcia-Peris et al3 found that inulin/fructo-oligosaccharide improved stool consistency when started one week before post-operative pelvic radiotherapy and continued for three weeks after treatment in a randomised trial of 38 women with gynaecological cancers.
Inulin is a soluble fibre derived from chicory root and other plants. It supports the growth of beneficial bacteria in the gut microbiota by promoting the production of anti-inflammatory metabolites. When it reaches the colon, inulin is rapidly fermented, selectively boosting the growth of specific gut bacteria such as Bifidobacterium and Faecalibacterium species. These bacteria play a role in reducing inflammation both in the gut and systemically, primarily through the secretion of short-chain fatty acids such as butyrate, among other metabolites.
As inflammation is a common cause of both bowel and urinary symptoms, inulin may help improve stool consistency. However, there is currently limited evidence to suggest it has a similar effect on urinary symptoms.
The DIETRICH study: dietary fibre hypothesis
In early studies conducted in mice, our team4 and others5 have found that inulin not only reduces bowel toxicity but also enhances tumour control in several types of tumours, with the potential for similar effects in humans.
The DIETRICH study aims to explore whether men who supplement their diet with inulin experience fewer gut-related side effects compared with those who do not take additional fibre. We hypothesise that men taking inulin will have improved gastrointestinal symptoms and quality of life and a reduced need for rescue medication, such as psyllium, mid-treatment.
DIETRICH is a large, phase 2, double-blind, placebo-controlled trial of inulin in 220 men across eight centres over 27 months. It will include men having radiotherapy to the prostate over four weeks for intermediate-risk disease and those having radiotherapy to the prostate and pelvic nodes for high-risk, locally advanced or node-positive disease.
Based on feedback from previous patient discussion groups, we believe men will be enthusiastic about participating in this study. Those with prostate cancer are often highly motivated to make lifestyle and dietary changes that could enhance their chances of a cure and improve their quality of life.
Patients will be randomised to receive either a supply of inulin to take for two weeks before radiotherapy, four weeks during radiotherapy, and three weeks after, or the placebo, maltodextrin. It is possible that patients may wish to self-treat with a dietary fibre supplement, so it will be necessary to ensure that the study is not contaminated in this way.
Patient-reported outcomes will be used to determine increased symptoms from baseline in bowel and urinary domains. Participants will provide faecal samples before the supplement, before radiotherapy, and after treatment, and complete dietary questionnaires and diaries to determine whether their baseline gut microbiota, associated metabolites and habitual diet predict their responses to fibre and radiotherapy.
The DIETRICH study: projected outcomes
Preliminary and ongoing studies conducted in Aberdeen have demonstrated that men receiving radiotherapy for prostate cancer are generally very willing to provide samples at the outset of their treatment. Consequently, we anticipate participants to be similarly cooperative in this aspect of the study.
We also anticipate good compliance, as the inulin supplement is easy to take. It is a simple scoop of the supplement twice daily mixed into liquids, with no bitter aftertaste. Participants will regularly complete diet diaries and report any side effects, which should help maintain motivation and engagement through the study. We also expect that many will choose the convenient option of entering their data electronically via smartphone or computer.
Multidisciplinary collaboration will be important. Regular meetings between participating centres will provide opportunities to share insights, address challenges and refine the study as it progresses, and a dedicated trial steering committee will oversee the project to ensure it remains on course. Equally important is the valuable input received from patient partners, which will help to ensure that the demands placed on participants are reasonable and proportionate.
Conclusion
Looking ahead, we are eager to expand our research to include other cancers treated with pelvic radiotherapy and head and neck cancers. In the latter, radiotherapy often causes mucositis, which can severely impact eating, swallowing, weight and quality of life. Our ongoing interest lies in dietary fibre interventions, not only inulin but also other promising fibres that might offer a range of unique benefits across different cancer types and treatment side effects.
If the DIETRICH study is successful, the next step would, of course, be a larger-scale phase 3 evaluation. This would allow us to evaluate the benefits of inulin further and potentially explore other types of dietary fibre, recognising that not all men may respond to inulin alone.
Should a larger trial confirm the positive outcomes, we envisage inulin becoming a highly cost-effective addition to standard clinical care, improving patients’ quality of life and cancer outcomes, while adding minimal financial burden to the NHS.
The DIETRICH study is funded by Prostate Cancer UK, grant reference number RIA23-ST2-006.
Authors
Anne E Kiltie MA DM DSc MRCP(UK) FRCR
Friends of ANCHOR clinical chair in oncology, The Rowett Institute, University of Aberdeen, UK
Ananya Choudhury MA PhD MRCP FRCR
Chair and honorary consultant in clinical oncology, The Christie NHS Foundation Trust, UK
References
- Cancer Research UK. Side effects of prostate cancer radiotherapy. Updated 16 Jun 2022. [Accessed May 2025].
- Murphy J et al. Testing control of radiation-induced diarrhea with a psyllium bulking agent: A pilot study. Can Oncol Nurs J 2025;10(3):96–100.
- Garcia-Peris P et al. Effect of inulin and fructo-oligosaccharide on the prevention of acute radiation enteritis in patients with gynecological cancer and impact on quality-of-life: a randomized, double-blind, placebo-controlled trial. Eur J Clin Nutr 2016;70:170–74.
- Then CK et al. Association of Bacteroides acidifaciens relative abundance with high-fibre diet-associated radiosensitisation. BMC Biol 2020;18:102.
- Taper HS, Roberfroid MB. Nontoxic Potentiation of Cancer Chemotherapy by Dietary Oligofructose or Inulin. Nutrition and Cancer 2000;38:1–5.
