Protocol for a pilot single-centre, parallel-arm, randomised controlled trial of dietary inulin to improve gut health in solid organ transplantation: the DIGEST study

Dietary Inulin to Improve SARS-CoV-2 Vaccine Response in Kidney Transplant Recipients: The RIVASTIM-Inulin Randomised Controlled Trial

Kidney transplant recipients are at an increased risk of hospitalisation and death from SARS-CoV-2 infection, and standard two-dose vaccination schedules are typically inadequate to generate protective immunity. Gut dysbiosis, which is common among kidney transplant recipients and known to effect systemic immunity, may be a contributing factor to a lack of vaccine immunogenicity in this at-risk cohort. The gut microbiota modulates vaccine responses, with the production of immunomodulatory short-chain fatty acids by bacteria such as Bifidobacterium associated with heightened vaccine responses in both observational and experimental studies. As SCFA-producing populations in the gut microbiota are enhanced by diets rich in non-digestible fibre, dietary supplementation with prebiotic fibre emerges as a potential adjuvant strategy to correct dysbiosis and improve vaccine-induced immunity. In a randomised, double-bind, placebo-controlled trial of 72 kidney transplant recipients, we found dietary supplementation with prebiotic inulin for 4 weeks before and after a third SARS-CoV2 mRNA vaccine to be feasible, tolerable, and safe. Inulin supplementation resulted in an increase in gut Bifidobacterium, as determined by 16S RNA sequencing, but did not increase in vitro neutralisation of live SARS-CoV-2 virus at 4 weeks following a third vaccination. Dietary fibre supplementation is a feasible strategy with the potential to enhance vaccine-induced immunity and warrants further investigation.

Nutrition in the Management of Kidney Transplant Recipients

Kidney transplantation offers patients with end stage kidney disease the best outcomes. Concentration on nutrition is pivotal throughout the transplant life course. Nutritional requirements change during each phase of transplantation, from pretransplant evaluation and wait-time, acute transplantation, maintenance and ultimately declining graft function, and care should be taken to consider each stage. In this article we concentrate on addressing each phase, with additional focus on current hot topics of dysglycaemia management and on the impact of diet on gut microbiome.

Synbiotics, prebiotics and probiotics for solid organ transplant recipients

BACKGROUND

Solid organ transplantation has seen improvements in both surgical techniques and immunosuppression, achieving prolonged survival. Essential to graft acceptance and post-transplant recovery, immunosuppressive medications are often accompanied by a high prevalence of gastrointestinal (GI) symptoms and side effects. Apart from GI side effects, long-term exposure to immunosuppressive medications has seen an increase in drug-related morbidities such as diabetes mellitus, hyperlipidaemia, hypertension, and malignancy. Non-adherence to immunosuppression can lead to an increased risk of graft failure. Recent research has indicated that any microbial imbalances (otherwise known as gut dysbiosis or leaky gut) may be associated with cardiometabolic diseases in the long term. Current evidence suggests a link between the gut microbiome and the production of putative uraemic toxins, increased gut permeability, and transmural movement of bacteria and endotoxins and inflammation. Early observational and intervention studies have been investigating food-intake patterns, various synbiotic interventions (antibiotics, prebiotics, or probiotics), and faecal transplants to measure their effects on microbiota in treating cardiometabolic diseases. It is believed high doses of synbiotics, prebiotics and probiotics are able to modify and improve dysbiosis of gut micro-organisms by altering the population of the micro-organisms. With the right balance in the gut flora, a primary benefit is believed to be the suppression of pathogens through immunostimulation and gut barrier enhancement (less permeability of the gut).

OBJECTIVES

To assess the benefits and harms of synbiotics, prebiotics, and probiotics for recipients of solid organ transplantation.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Specialised Register up to 9 March 2022 through contact with the Information Specialist using search terms relevant to this review. Studies in the Register are identified through searches of CENTRAL, MEDLINE, and EMBASE, conference proceedings, the International Clinical Trials Register (ICTRP) Search Portal and ClinicalTrials.gov.

SELECTION CRITERIA

We included randomised controlled trials measuring and reporting the effects of synbiotics, prebiotics, or probiotics, in any combination and any formulation given to solid organ transplant recipients (any age and setting). Two authors independently assessed the retrieved titles and abstracts and, where necessary, the full text to determine which satisfied the inclusion criteria.

DATA COLLECTION AND ANALYSIS

Data extraction was independently carried out by two authors using a standard data extraction form. The methodological quality of included studies was assessed using the Cochrane risk of bias tool. Data entry was carried out by one author and cross-checked by another. Confidence in the evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach.

MAIN RESULTS

Five studies (250 participants) were included in this review. Study participants were adults with a kidney (one study) or liver (four studies) transplant. One study compared a synbiotic to placebo, two studies compared a probiotic to placebo, and two studies compared a synbiotic to a prebiotic. Overall, the quality of the evidence is poor. Most studies were judged to have unclear (or high) risk of bias across most domains. Of the available evidence, meta-analyses undertaken were of limited data from small studies. Across all comparisons, GRADE evaluations for all outcomes were judged to be very low certainty evidence. Very low certainty evidence implies that we are very uncertain about results (not estimable due to lack of data or poor quality). Synbiotics had uncertain effects on the change in microbiota composition (total plasma p-cresol), faecal characteristics, adverse events, kidney function or albumin concentration (1 study, 34 participants) compared to placebo. Probiotics had uncertain effects on GI side effects, infection rates immediately post-transplant, liver function, blood pressure, change in fatty liver, and lipids (1 study, 30 participants) compared to placebo. Synbiotics had uncertain effects on graft health (acute liver rejection) (2 studies, 129 participants: RR 0.73, 95% CI 0.43 to 1.25; 2 studies, 129 participants; I² = 0%), the use of immunosuppression, infection (2 studies, 129 participants: RR 0.18, 95% CI 0.03 to 1.17; I² = 66%), GI function (time to first bowel movement), adverse events (2 studies, 129 participants: RR 0.79, 95% CI 0.40 to 1.59; I² = 20%), serious adverse events (2 studies, 129 participants: RR 1.49, 95% CI 0.42 to 5.36; I² = 81%), death (2 studies, 129 participants), and organ function measures (2 studies; 129 participants) compared to prebiotics.

AUTHORS' CONCLUSIONS

This review highlights the severe lack of high-quality RCTs testing the efficacy of synbiotics, prebiotics or probiotics in solid organ transplant recipients. We have identified significant gaps in the evidence. Despite GI symptoms and postoperative infection being the most common reasons for high antibiotic use in this patient population, along with increased morbidity and the growing antimicrobial resistance, we found very few studies that adequately tested these as alternative treatments. There is currently no evidence to support or refute the use of synbiotics, prebiotics, or probiotics in solid organ transplant recipients, and findings should be viewed with caution. We have identified an area of significant uncertainty about the efficacy of synbiotics, prebiotics, or probiotics in solid organ transplant recipients. Future research in this field requires adequately powered RCTs comparing synbiotics, prebiotics, and probiotics separately and with placebo measuring a standard set of core transplant outcomes. Six studies are currently ongoing (822 proposed participants); therefore, it is possible that findings may change with their inclusion in future updates.