Exploring Mechanism of Action of Dietary Fibre on the Gut Microbiota and Metabolites (MERLIN)

February 10, 2026 updated by: University of Aberdeen

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?

Cancer patients receiving pelvic radiotherapy suffer side effects to the intestine, including diarrhoea and occasionally rectal bleeding, often worsened by chemoradiation. Many pelvic radiotherapy patients receive a form of dietary fibre, psyllium husk, to manage their diarrhoeal symptoms. There is evidence in mice that dietary fibre supplementation can improve tumour response and reduce normal tissue side effects caused by radiation. If this were to be confirmed in humans, it would be a major advance in patient treatment. The investigators will conduct a study in healthy subjects to assess mechanisms and response to dietary fibre.

The investigators will conduct a within-subject diet intervention study on healthy older adults (>60 years old) to determine if a subset of the population are responders to dietary fibre manipulation, using inulin with or without psyllium. This will allow assessment of the physiological impact of dietary fibre. Participants will receive placebo, inulin, or psyllium plus inulin in random order (6 possible combinations) for two weeks each with two-week washout periods.

Baseline faecal microbial activity, short chain fatty acid (SCFA) concentrations, and habitual fibre intake, determined by dietary questionnaires/food diaries will be used to identify individuals in whom dietary fibre manipulation might have a positive impact. Individuals who respond to dietary fibre supplementation may be found to have elevated levels of some SCFAs and significantly increased relative abundance of certain microbiota. Dietary parameters, bacterial relative abundance and SCFA levels will be correlated with plasma cytokine levels.

Study Overview

Status

Completed

Conditions

Intervention / Treatment

Detailed Description

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.

The investigators found promising preliminary data in our mouse studies, indicating that psyllium with inulin enhances tumour control. Immunodeficient mice fed a 10% inulin diet had an improved tumour control following 6 gray (Gy) ionising radiation. Immunocompetent mice showed a delayed tumour growth when receiving psyllium plus inulin when compared to a normal or low fibre diet, accompanied by significantly increased tumour infiltration of cytotoxic T cells. Psyllium plus inulin also significantly increased the percentage of intestinal remaining after 14 Gy compared to a low fibre diet or psyllium alone. A psyllium plus inulin diet also modified intestinal immunity, particularly B cells. Furthermore, in mouse bladder cancer cell UPPL 1591-derived allografts, CD8+ (cytotoxic) T cells scored higher for psyllium and psyllium plus inulin than psyllium plus resistant starch. However, compared to humans, inbred mice are genetically more similar and are held in controlled environmental conditions, so humans are likely to demonstrate more variability in response to dietary fibre intervention.

The investigators will study a group of human participants to assess tolerance and responses to inulin and psyllium plus inulin, in terms of intestinal side effects, significant alterations in the microbiota, levels of SCFAs in plasma and faeces, and plasma inflammatory cytokines. The investigators will also investigate whether responders and non-responders within the study can be identified from baseline dietary information and/or microbial activity (based on faecal SCFA measurements), as an individual's current diet will influence their response to dietary fibre modification.

Study Type

Interventional

Enrollment (Actual)

40

Phase

  • Not Applicable

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Adult
  • Older Adult

Accepts Healthy Volunteers

Yes

Description

Inclusion Criteria:

  • >60 years old

Exclusion Criteria:

Medication exclusion criteria:

  • Oral antibiotic use (within the past 3 months due to impact on gut microbiota).
  • Anti-coagulants (Warfarin).
  • Carbamazepine (epilepsy)
  • Digoxin (heart conditions)
  • Mesalazine (gut problems)
  • Regular use of anti-constipation medication/laxatives.

Medical exclusion criteria:

  • Food allergies, self-reported food sensitivity or intolerance.
  • Type 1 or Type 2 diabetes.
  • Coeliac disease or gluten intolerance.
  • Metformin intake.
  • Bowel obstruction
  • Muscle weakness of the bowel
  • Phenylketonuria.

Other exclusion criteria:

  • Anyone with unsuitable veins for blood sampling.
  • Anyone with chronic constipation (>3 days without defaecation).
  • Anyone who is unable to fluently speak, read and understand English.

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Basic Science
  • Allocation: Randomized
  • Interventional Model: Crossover Assignment
  • Masking: Double

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Inulin
The participant will be asked to administer the provided doses of 8g inulin twice daily.
Dietary supplement: Inulin
The participant will take inulin twice daily over a 14 day period.
Experimental: Inulin + psyllium
The participant will be asked to administer the provided doses of 8g inulin + 3.5g psyllium twice daily.
Dietary supplement: Inulin
The participant will take inulin twice daily over a 14 day period.
Dietary supplement: Psyllium
The participant will take psyllium twice daily over a 14 day period.
Other Names:
  • Ispaghula husk
Placebo Comparator: Maltodextrin
The participant will be asked to administer the provided doses of 8g maltodextrin twice daily.
Dietary supplement: Maltodextrin
The participant will take maltodextrin twice daily over a 14 day period.

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Microbial composition using 16S sequencing
Time Frame: 11 weeks per participant
To explore the effect of inulin and psyllium plus inulin on microbial composition at baseline and after a 14-day intervention.
11 weeks per participant
Short Chain Fatty Acid (SCFA) levels
Time Frame: 11 weeks per participant
To explore the effect of inulin and psyllium plus inulin on short chain fatty acid levels at baseline and after a 14-day intervention.
11 weeks per participant

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Tolerability of Inulin defined by questionnaire 'quality of life' and 'gastrointestinal wellbeing'
Time Frame: 11 weeks per participant
To determine if 16 g inulin with or without 7 g psyllium dietary interventions are tolerable in older adults (>60 years old) assessed by completing gastrointestinal wellbeing questionnaires and quality of life questionnaires (SF36).
11 weeks per participant
Tolerability of psyllium defined by questionnaire 'quality of life SF36' and 'gastrointestinal wellbeing'
Time Frame: 11 weeks per participant
To determine if 16 g inulin with or without 7 g psyllium dietary interventions are tolerable in older adults (>60 years old) assessed by completing gastrointestinal wellbeing questionnaires and quality of life questionnaires (SF36).
11 weeks per participant
Rate of habitual fibre intake before and during the intervention
Time Frame: 11 weeks per participant
To determine if Food Frequency Questionnaires/3- day weighed food diaries (to assess habitual fibre intake before and during the intervention) can identify individuals in whom dietary fibre supplementation would have a positive impact on microbiota composition and SCFA levels.
11 weeks per participant
Faecal SCFA concentrations
Time Frame: 11 weeks per participant
Identify individuals in whom dietary fibre supplementation would have a positive impact using baseline faecal microbial activity (determined by faecal SCFA concentrations).
11 weeks per participant
Elevated plasma SCFAs
Time Frame: 11 weeks per participant
Change in SCFA levels in plasma from baseline in individuals that respond to dietary fibre supplementation after 14 days.
11 weeks per participant
A 0.5 log10 change in health-associated Bifidobacterium spp. from baseline
Time Frame: 11 weeks per participant
Detecting a 0.5 log10 change in health-associated Bifidobacterium spp. from baseline in individuals given psyllium plus inulin using faecal quantitative polymerase chain reaction.
11 weeks per participant
Plasma cytokine IL-6
Time Frame: 11 weeks per participant
To determine if dietary parameters, bacterial relative abundances and SCFA levels are correlated with plasma cytokine IL-6 level (cytokine level will only be measured at the end of each intervention), which may influence radiotherapy outcomes in terms of tumour response and normal tissue effects.
11 weeks per participant
Plasma cytokine IL-8
Time Frame: 11 weeks per participant
To determine if dietary parameters, bacterial relative abundances and SCFA levels are correlated with plasma cytokine IL-8 level (cytokine level will only be measured at the end of each intervention), which may influence radiotherapy outcomes in terms of tumour response and normal tissue effects.
11 weeks per participant
Immunoglobulin A (IgA) and calprotectin levels
Time Frame: 11 weeks per participant
To determine if the interventions alter gut health and inflammation, measured by faecal IgA and calprotectin at the end of each of the three interventions.
11 weeks per participant

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

University of Aberdeen

Collaborators

Friends of ANCHOR

Investigators

  • Principal Investigator: Anne E Kiltie, Prof, University of Aberdeen

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

General Publications

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

September 7, 2023

Primary Completion (Actual)

July 10, 2025

Study Completion (Actual)

July 10, 2025

Study Registration Dates

First Submitted

May 19, 2023

First Submitted That Met QC Criteria

June 13, 2023

First Posted (Actual)

June 18, 2023

Study Record Updates

Last Update Posted (Actual)

February 12, 2026

Last Update Submitted That Met QC Criteria

February 10, 2026

Last Verified

February 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

  • 501

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

NO

IPD Plan Description

No.

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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