Rossi, M. et al. Synbiotics easing renal failure by improving gut microbiology (SYNERGY): a randomized trial. Clin. J. Am. Soc. Nephrol. 11, 223–231 (2016). Liu, L., Li, P., Liu, Y. & Zhang, Y. Efficacy of probiotics and synbiotics in patients with nonalcoholic fatty liver disease: a meta-analysis. Dig. Dis. Sci. 64, 3402–3412 (2019). This meta-analysis highlights the benefits of synbiotics for the hallmarks of non-alcoholic fatty liver disease, liver steatosis, liver enzymes, lipid profiles and liver stiffness. Klurfeld, D. M. et al. Considerations for best practices in studies of fiber or other dietary components and the intestinal microbiome. Am. J. Physiol. Endocrinol. Metab. 315, E1087–E1097 (2018). A statistician’s involvement in the design phase of a study will provide confidence that the sample size is sufficient for the defined outcomes using intention-to-treat analysis. Mediation analyses might be useful to assess whether microbiota effects are contributing to the health benefit. Ancillary analyses, which could include population subgroups, responders versus non-responders or exploratory microbiota analyses should be pre-specified.

Pisano, A., D’Arrigo, G., Coppolino, G. & Bolignano, D. Biotic supplements for renal patients: a systematic review and meta-analysis. Nutrients 10, 1224 (2018). Rowe, S. et al. Funding food science and nutrition research: financial conflicts and scientific integrity. J. Nutr. 139, 1051–1053 (2009). Shafaghi, A. et al. The effect of probiotic plus prebiotic supplementation on the tolerance and efficacy of Helicobacter pylori eradication quadruple therapy: a randomized prospective double blind controlled trial. Middle East J. Dig. Dis. 8, 179–188 (2016). Dethlefsen, L. & Relman, D. A. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. Proc. Natl Acad. Sci. USA 108, 4554–4561 (2011).A two-arm parallel or crossover study would be sufficient to test a complementary synbiotic. The aim is to demonstrate that the combination is better than the placebo group with a relevant health end point. As a demonstrated prebiotic is used to formulate a complementary synbiotic, we do not require that selective utilization by the indigenous microbiota be reconfirmed in this clinical trial. Measurement of selective utilization Alshawaqfeh, M., Wajid, B., Minamoto, Y., Markel, M., Lidbury, J., Steiner, J., Serpedin, E., Suchodolski, J., 2017. A dysbiosis index to assess microbial changes in fecal samples of dogs with chronic inflammatory enteropathy. FEMS Microbiology Ecology.. doi:10.1093/femsec/fix136 Ustundag, G. H., Altuntas, H., Soysal, Y. D. & Kokturk, F. The effects of synbiotic “ Bifidobacterium lactis B94 plus inulin” addition on standard triple therapy of Helicobacter pylori eradication in children. Can. J. Gastroenterol. Hepatol. 2017, 8130596 (2017). There are many probiotic options to choose from on the market. It is recommended to use probiotics that are at least third party tested and inspected for quality and potency.

Malaguarnera, M. et al. Bifidobacterium longum with fructo-oligosaccharides in patients with non alcoholic steatohepatitis. Dig. Dis. Sci. 57, 545–553 (2012). ISAPP is a non-profit collaboration of scientists dedicated to advancing scientific excellence and providing objective, science-based information on probiotics and prebiotics. The organization’s activities are funded by companies involved in the sale of probiotics and prebiotics, but ISAPP is guided by an international, all-volunteer academic board that functions independently. This ISAPP-organized panel was composed of experts in microbiology, nutrition and gastrointestinal physiology, including many who were involved in the latest updates of the probiotic 12 and prebiotic 13 definitions according to ISAPP. Panellists were charged with accomplishing the following goals: consider what a synbiotic is and provide a clear, concise and testable definition; suggest the appropriate experimental conditions necessary to establish synbiotic activity; describe the evidence required to demonstrate the health benefits and establish safety; and provide guidance for stakeholders, including researchers, industry, public health professionals and regulatory agencies. The most promising applications for probiotics are treatment of acute uncomplicated diarrhea, prevention of stress diarrhea, prevention of antibiotic-associated GI signs, and adjunct therapy for chronic enteropathies. Moher, D. et al. CONSORT 2010 explanation and elaboration: updated guidelines for reporting parallel group randomised trials. Int. J. Surg. 10, 28–55 (2012).Closa-Monasterolo, R. et al. Safety and efficacy of inulin and oligofructose supplementation in infant formula: results from a randomized clinical trial. Clin. Nutr. 32, 918–927 (2013). As for probiotics, prebiotics and synbiotics, it might be possible a priori to stratify study participants to enhance either the magnitude of the response or the number of responders to a given synbiotic treatment. The baseline composition of gut microbiota of study participants could be one useful stratification factor 91, 92. This stratification might improve mechanistic understanding of observed effects and could be useful for characterizing responders and non-responders. Ultimately, this approach could enable better translation of clinical trial outcomes to those who are likely to benefit. Pilla R, Suchodolski JS. The role of the canine gut microbiome and metabolome in health and gastrointestinal disease. Front Vet Sci 2020;6:498. When choosing the study population, many parameters need to be defined, including the target host (including non-human species), target life stage (for example, pregnant women, infants, children, adults or elderly) and health status (for example, healthy, at-risk, or with a diagnosed disease or condition). Furthermore, microbiome-related factors, such as recent probiotic, prebiotic, synbiotic or antibiotic use, microbiota-disrupting medications and diet can be considered as (in)eligibility criteria, depending on primary health and microbiota outcomes. A 2-week wash-in period is generally adequate for probiotics; however, certain populations might require longer wash-in periods (such as 4 weeks for those with slow transit time) 56. The time since last antibiotic exposure will vary depending on a range of factors, including type of antibiotic, dose, duration and whether antibiotic use has occurred on multiple occasions in fairly short succession 57, 58. A general recommendation is difficult to make, although a common exclusion is individuals who have taken antibiotics 3 months prior to a study. However, at a minimum, 4 weeks since cessation of antibiotics is recommended. Non-antibiotic prescription medications should also be considered if they can influence bowel function and the microbiome 59.

Ferrarese, R., Ceresola, E. R., Preti, A. & Canducci, F. Probiotics, prebiotics and synbiotics for weight loss and metabolic syndrome in the microbiome era. Eur. Rev. Med. Pharmacol. Sci. 22, 7588–7605 (2018). Gerardo, N. & Hurst, G. Q&A: friends (but sometimes foes) within: the complex evolutionary ecology of symbioses between host and microbes. BMC Biol. 15, 126 (2017). The gastrointestinal (GI) tract of animals harbors various microorganisms along with their collective genomes. 2,3 The estimated total microbial load in the intestine is approximately 100 trillion microbial cells, representing approximately 100 times more microbial genes than the host genome.Agencia Nacional de Vigilancia Sanitaria. Guia para instrucao processual de peticao de avaliacao de probiotics para uso em alimentos [Portugese]. http://portal.anvisa.gov.br/documents/10181/5280930/21.pdf/1c99eeb1-7143-469a-93ff-7b2b0f9187c0 (2019). Symbiotic 180-S 's Probiotics and Prebiotics also assist in the maintenance of a strong & functional immune system. Reddy, B. S. et al. Randomized clinical trial of effect of synbiotics, neomycin and mechanical bowel preparation on intestinal barrier function in patients undergoing colectomy. Br. J. Surg. 94, 546–554 (2007).