The available evidence consistently points in the direction that probiotics, particularly specific strains such as Saccharomyces boulardii, have more demonstrable effects than prebiotics alone, although those effects are not always clinically large enough to recommend supplementation broadly. Prebiotics on their own do not outperform placebo or standard treatment in the most researched applications. This is based on multiple randomised studies and meta-analyses, but the effect sizes are modest and context-dependent.
Prebiotics and probiotics are related but do not do the same thing. Probiotics are living micro-organisms; prebiotics are the fibres that feed these bacteria. The question is whether prebiotics deliver as much benefit as probiotics. Based on multiple studies, the answer is largely no: for the most researched applications, prebiotics on their own perform worse than probiotics.
In acute diarrhoea in children, the difference is clearest. Probiotics, especially the yeast Saccharomyces boulardii (sometimes combined with zinc), shorten the duration of diarrhoea by 17 to 40 hours. The combination of Saccharomyces boulardii with zinc produced the best results, particularly in lower-income countries. Prebiotics alone showed no demonstrable benefit over standard treatment or placebo in the same research. Synbiotics, which combine probiotics and prebiotics, do work.
In people with type 2 diabetes the picture is similar, but even more nuanced. A large network meta-analysis of 68 randomised studies showed that prebiotics did not demonstrably improve blood sugar values (HbA1c and fasting glucose). Probiotics did, but the measured effect, an average reduction in HbA1c of 0.25 percentage points, is statistically detectable but clinically unimportant. The researchers therefore do not recommend supplementation for blood sugar regulation.
In athletes there is reasonably good evidence that certain probiotic strains can reduce the number, severity and duration of upper respiratory tract infections. Specific strains also appear to support the gut barrier, which becomes more vulnerable during intensive exercise. Prebiotics in athletes were not examined in the available studies, so a comparison simply cannot be made here.
In two other situations, organ transplantation and the prevention of colorectal cancer in high-risk groups, the evidence is too limited or of too low a quality to draw conclusions. A Cochrane review in kidney and liver transplant patients found no clear difference between synbiotics and prebiotics alone, but the studies were small and the quality was very low. On colorectal cancer prevention, the available research data contain insufficient results for a reliable conclusion.
Based on two network meta-analyses and randomised studies (PMID 30517196, 38134725, 31864419, 36126902, 38551082). The total number of participants is difficult to calculate precisely due to the use of network meta-analyses; conservatively estimated at well over 5,000 across all studies combined.