Prebiotics

1. What Prebiotics Are

Prebiotics are indigestible food components that are utilised by bacteria in the large intestine and thereby favour certain microorganisms. The term encompasses an entire class of substances; in the works presented, among other things, a galacto-oligosaccharide mixture (B-GOS), soluble maize fibre and prebiotic dietary fibres are investigated. The definition and delimitation is itself the subject of consensus papers: The ISAPP has set out the definition and scope of synbiotics in a much-cited consensus paper, that is, the combination of prebiotic and probiotic. Review articles classify types, sources, mechanisms and clinical applications. Prebiotics are tested partly as a single substance, partly as a mixture, partly in synbiotic combination with strains such as Lactobacillus rhamnosus GG.

For understanding all studies it is crucial that prebiotics are not a uniform active substance. The works differ in the substance, in the preparation and in the endpoint investigated. One part measures the composition of the gut flora in the stool, one part immunological parameters, one part metabolic variables such as metabonomics, one part questionnaires on appetite or mood. These endpoints are not the same: an altered marker is not a disease course. In addition, many works combine prebiotics with probiotics, so that the effect of the individual substance cannot be derived from such mixture studies. Anyone reading the literature must therefore first ask which substance in which form was tested at which endpoint.

2. What the Studies Show in Detail

The human studies, ordered by application and the controlled ones first — with participants, duration, preparation, measure and result, as the original works report them. Numbers appear only where the work states them.

Distribution of controlled human studies by field of application — the numbers are not additive, as one work may be assigned to several fields:

Prebiotics Taken: Human Studies

The evaluation of a randomised, double-blind study examined whether the body-mass index before pregnancy modifies the effect of prebiotic administration on the risk of gestational diabetes. Pregnant women before the 21st week of pregnancy received daily prebiotics (14.2 g galacto-oligosaccharides and fructo-oligosaccharides) or placebo (8.7 g maltodextrin). Between June 2016 and November 2021, 329 women were allocated to the prebiotic group (50.4 %) and 323 (49.5 %) to the placebo group; 288 of 652 women (44.2 %) were overweight or obese before pregnancy (BMI ≥ 25 kg/m2), evenly distributed (146/329, 44.4 % vs. 142/323, 44.0 %). The BMI before pregnancy modified the effect: in women with a BMI ≥ 25 kg/m2 the intervention lowered the GDM rates 33. What is missing for normal-weight women and for the size of the effect is the complete results report.

The pooled evaluation of the microbiota therapies showed changes in liver and fibrosis values (hepatic fibrosis, SMD = -0.31; 95 % CI: -0.53, -0.09; aspartate aminotransferase, SMD = -0.35; 95 % CI: -0.55, -0.15; alanine aminotransferase, SMD = -0.48; 95 % CI: -0.71, -0.25; alkaline phosphatase, SMD = -0.81; 95 % CI: -1.55, -0.08), in the lipid profile (triglycerides, SMD = -0.22; 95 % CI: -0.43, -0.02; high-density lipoprotein, SMD = -0.47; 95 % CI: -0.88, -0.06) and in the inflammatory marker (tumour necrosis factor alpha, SMD = -0.86; 95 % CI: -1.56, -0.56) 34. Because the microbiota therapies were pooled together, the effect of prebiotics alone cannot be read from this work.

A network meta-analysis compared prebiotics, probiotics and synbiotics in non-alcoholic fatty liver disease in 1921 patients from 37 randomised studies. Only 4 studies examined prebiotics, 23 probiotics, 10 synbiotics, 3 faecal microbiota transplantation and one antibiotics. Probiotics and synbiotics were associated with a significant reduction in alanine aminotransferase (MD: -5.09; 95 % CI not fully stated in the abstract) 6. For prebiotics alone the network meta-analysis showed no advantage; the small number of 4 studies limits the statement.

A systematic review and meta-analysis of 55 randomised studies in 8868 infants and children tested infant formula with probiotics, prebiotics and synbiotics. The fortified formula did not significantly improve growth (weight, length, BMI, head circumference). The frequency of mild adverse events (OR 0.88; 95 % CI 0.70-1.11; P = 0.28) and serious adverse events (OR 0.92; 95 % CI 0.62-1.36; P = 0.67) was comparable to the control group. The intestinal microbial diversity of the infants who received the fortified formula was lower than in the comparison group 8. The combined results for prebiotics, probiotics and synbiotics do not permit conclusions about prebiotics alone.

A systematic review and meta-analysis of 90 randomised studies in 5207 children and adults compared synbiotics and prebiotics on inflammatory and anti-inflammatory biomarkers. Prebiotics lowered CRP (SMD = -0.81; 95 % CI: -1.51, -0.10; P = 0.026), hs-CRP (SMD = -0.50; 95 % CI: -0.86, -0.15; P = 0.006), TNF-α (SMD = -0.57; 95 % CI: -0.93, -0.21; P = 0.002) and IL-1β (SMD = -0.44; 95 % CI: -0.76, -0.12; P = 0.0). Synbiotics outperformed prebiotics on several markers 35. Evidence quality was assessed with GRADE; heterogeneity between the studies remains to be clarified.

Synbiotics significantly lowered the risk of gastrointestinal symptoms (RR 0.52; 95 % CI not fully stated in the abstract). The ranks of the treatments were exploratory and the certainty of evidence was not formally assessed 19. For prebiotics alone the work reports no significant advantage for gastrointestinal symptoms.

One work linked NHANES data from 2001 to 2018 with a meta-analysis of randomised studies. 5014 adults were included; the weighted prevalence of metabolically associated fatty liver disease (MASLD) was 24.47 %. Adults with MASLD who took prebiotics or probiotics differed in the analysis; the meta-analysis comprised randomised studies on liver function in MASLD or non-alcoholic steatohepatitis under probiotics, prebiotics or yoghurt for at least 6 months 36. The observational data cannot establish causation; the intervention data concern several substances together.

A systematic review and meta-analysis of 19 randomised studies in 4188 children tested prebiotics, probiotics and synbiotics on haemoglobin and anaemia. Compared with placebo or standard care they showed no important effects on haemoglobin, haematocrit or ferritin (certainty: high to very low). Probiotics versus placebo might reduce anaemia (1 RCT; RD: -6 per 100; 95 % CI: -15 to +6; low certainty). Serious adverse events were not reported; mainly mild gastrointestinal symptoms were observed 41. For prebiotics alone a separate statement on anaemia is missing.

Prebiotics and synbiotics increased stool frequency per week. Versus placebo, lactulose (MD = 3.39; 95 % CI [1.13, 5.65]; moderate certainty), mix2 of lactulose and Bacillus coagulans (MD = 3.63; 95 % CI [1.37, 5.89]; moderate certainty), mix6 of lactulose and Bifidobacterium coagulans (MD = 4.30; 95 % CI [1.04, 7.54]; low certainty) and mix7 of lactulose, Bifidobacterium subtilis and Enterococcus faecium (MD not fully stated in the abstract) showed an effect 5. Most effective combinations contained lactulose, so the contribution of the prebiotic component remains open.

A systematic review and meta-analysis of 55 randomised studies in 2 (participant number not fully stated in the abstract) adults examined inulin-type fructans on cardiovascular risk factors. Included were studies with administration for at least 2 weeks that reported lipids, apolipoproteins, triglycerides, fasting blood glucose, BMI, body weight, waist circumference, waist-hip ratio, blood pressure or HbA1c. The data were pooled with a random-effects model and the certainty of evidence was assessed with GRADE 7. Which endpoints changed significantly cannot be determined from the present extract.

Anxiety, depression and sleep values were calculated as standardised mean difference with 95 % confidence interval 42. The results for prebiotics alone cannot be separated from the joint evaluation.

Primary endpoints were pain on the visual analogue scale, WOMAC pain, WOMAC function and WOMAC total score; secondary outcomes included WOMAC stiffness, body weight, knee flexion angle, cartilage oligomeric matrix protein, hs-CRP, erythrocyte sedimentation rate, IL-1β and IL-6. Effects were pooled as standardised mean differences, risk of bias was assessed with RoB 2.0 and certainty was graded with GRADE 17. Prebiotics were not evaluated separately.

A systematic review and meta-analysis of 20 randomised studies in 1979 adults examined probiotics and prebiotics on the immune response after influenza vaccination. Nine studies with 623 participants could be pooled. Intake improved the seroprotection rate against the H1N1 strain (OR 1,83; 95 % CI 1,19-2,82; p = 0,006; I² = 0 %), against the H3N2 strain (OR 2,85; 95 % CI 1,59-5,10; p < 0,001; I² = 0 %) and the seroconversion rate against the B strain (OR 2,11; 95 % CI 1,38-3,21; p < 0,001; I² = 0 %) 1. Because probiotics and prebiotics were pooled jointly, the contribution of prebiotics remains open.

The evaluation showed a significant reduction in nasal symptom scores (SMD = -1,40; 95 % CI: -2,26 to -0,54; I² = 91,3 %) and ocular symptom scores (SMD = -3,59; 95 % CI: -5,84 to -1,33; I² = 94,3 %). Quality of life (RQLQ) also improved significantly (SMD = -2,98; 95 % CI: -4,85 to incomplete in the abstract) 16. High heterogeneity and the joint administration of several substances limit the statement on prebiotics.

Prebiotics markedly increased Bifidobacterium abundance (SMD = 1,09), probiotics less so (SMD = 0,40). Synbiotics showed no overall effect but increased specific strains (B. angulatum, B. longum, B. breve). Probiotics increased microbial diversity (Shannon index: SMD = 0,76), synbiotics increased the abundance of Lactobacillus casei (SMD = 0,75) and lowered Pseudomonas (SMD = -0,55) 43. The result for inflammatory markers is not complete in the extract.

Probiotics lowered serum creatinine (MD -0,21; 95 % CI -0,34, -0,09) and triglycerides (MD -9,98; 95 % CI -19,47, -0,49) more than placebo and achieved the largest area under the cumulative ranking curve; prebiotics and synbiotics showed no advantages. Probiotics also lowered malondialdehyde (MD -0,54; 95 % CI -0,96, -0,13) and raised glutathione 44. The work reports no advantage for prebiotics alone on renal function or metabolic profile.

A meta-analysis of 18 randomised studies in 1322 preterm infants examined prebiotics on sepsis, necrotising enterocolitis, mortality, feeding intolerance, time to full enteral feeding, length of hospital stay and stool frequency. Prebiotics lowered sepsis incidence (RR 0,64; 95 % CI: 0,51, 0,78), mortality (RR 0,58; 95 % CI: 0,36, 0,94), length of hospital stay (MD: -5,18; 95 % CI: -8,94, -1,11) and time to full enteral feeding (MD: -0,99; 95 % CI: -1,15, 0,83). No significant differences were found for necrotising enterocolitis morbidity (RR 0,79; 95 % CI: 0,44, 1,44) or feeding intolerance 2. The work concerns prebiotics exclusively and therefore provides the clearest single-substance findings for this application.

A systematic review and meta-analysis of 20 randomised studies in 1271 women with polycystic ovary syndrome examined prebiotics alone or as part of synbiotics on cardiometabolic parameters. High-quality evidence supported reductions in body-mass index (n = 853; WMD: -0,510; 95 % CI: -0,669, -0,351 kg/m2) and diastolic blood pressure (WMD: -2,218; 95 % CI: -4,425, -0,010 mmHg). Moderate evidence was found for weight, waist-hip ratio and triglycerides; low or very low evidence for waist circumference, fat mass, fasting plasma glucose, fasting insulin, LDL, total cholesterol, hs-CRP and total testosterone 12. Joint administration with synbiotics complicates attribution to the prebiotic component.

A follow-up of a multicentre randomised placebo-controlled study in seven centres across five Western European countries examined infant formula with a prebiotic mixture of neutral short- and long-chain oligosaccharides and pectin-derived acidic oligosaccharides. Of 1130 term infants, 672 participated in the follow-up: 232 (56 %) from the prebiotic group, 243 (58 %) from the control group and 197 (66 %) from the non-randomised breastfed group. The frequency of febrile episodes at three to five years of age was 1,17 (interquartile range 0,50-2,08) per year in the prebiotic group versus 1,20 (0,52-2,57) in the control group and 1,48 (0,65-2,60) in the breastfed group. The mixture during infancy did not reduce febrile episodes 45. The follow-up captured only 60 % of the original population.

A systematic review and network meta-analysis of 15 randomised studies in 1087 children examined prebiotics, probiotics and synbiotics in functional constipation. At low or very low certainty of evidence, no significant differences were found in the number of bowel movements per week between intervention and placebo. At very low certainty, the mixture showed positive effects on stool consistency versus S. boulardii (SMD = 0,86; 95 % CI [0,48, 1,23]; P < 0,001), and synbiotics were supported by low evidence. Versus S. boulardii, lactulose showed an effect (SMD = 0,62; 95 % CI [0,25, 0,99]; P < 0,001; low certainty) 15. For prebiotics alone, no proven benefit for stool frequency exists.

A systematic review and meta-analysis of 5 randomised studies in 199 patients with functional constipation examined prebiotics and in 8 studies in 825 patients synbiotics. Prebiotics increased weekly stool frequency (MD: 1,01 bowel movements/week; 95 % CI: 0,04-1,99) and improved stool consistency (SMD: -0,59; 95 % CI: -1,16 to -0,02). A subgroup analysis showed specific effects for galacto-oligosaccharides on stool frequency, consistency, ease of defaecation and abdominal pain. Synbiotics significantly improved stool frequency (MD: 1,15 bowel movements/week; 95 % CI incomplete in the abstract) 46. The number of prebiotic studies is small at 5, which limits the robustness of the subgroup findings.

No supplement improved gastrointestinal permeability, gastrointestinal symptoms or systemic endotoxaemia at rest. Only modest positive changes in inflammatory cytokine profiles were seen in 3 of 15 studies at rest. Prebiotic studies (4 of 5) reported significantly increased faecal bifidobacteria at rest, without consistent differences in other microbes 47. The review excludes people with gastrointestinal diseases, so the statement is limited to healthy individuals.

An observational study of 110 participants at a hospital in Eskisehir, Turkey, between 15 February 2019 and 15 February 2020 examined synbiotics in obesity. Persons with obesity who followed a weight-loss diet were randomly assigned to two groups: one group followed the diet only, the other received additional synbiotic tablets morning and evening. In the synbiotic group, vitamin B12 and vitamin D rose significantly from baseline. Both groups lost similar amounts of weight 48. Because both groups followed the diet, the contribution of synbiotics to weight loss cannot be isolated.

The jointly evaluated interventions lowered incidence significantly (RR = 0,74; 95 % CI: 0,70-0,79), confirmed by subgroup analyses, and reduced the SCORAD score (WMD = -3,75; 95 % CI: -5,08 to -2,42). In the subgroup analysis, multi-strain probiotics and Lactobacillus were effective 49. Prebiotics were not evaluated separately, so their own contribution remains open.

During a mean follow-up of 10,6 years, 9117 deaths were recorded, including 2364 from heart disease, 1964 from cancer and 4700 from other causes. Compared with non-users, users of prebiotics, probiotics or synbiotics had a 59 % (HR 0,41; 95 % CI 0,30-0,56), 56 % (HR 0,44; 95 % CI 0,26-0,74), 49 % (HR 0,51; 95 % CI 0,31-0,83) and 64 % (HR 0,36; 95 % CI 0,23-0,59) lower risk of all-cause, cancer, cardiovascular and other mortality 50. The observational study cannot establish causality; healthier lifestyle habits of users cannot be excluded as an explanation.

Probiotic intake correlated positively with the oxidative balance score, suggesting healthier dietary habits and lifestyles among users. A mediation analysis showed associations with depression, sleep disorders and cognitive function 51. The work reports no separate results for prebiotics.

A systematic review and meta-analysis of 38 nutritional, 13 prebiotic, 11 probiotic and 1 synbiotic study in a total of 4842 participants (59 % women) examined interventions on sarcopenia or its defining parameters. The qualitative analysis included these studies; the quantitative analysis included 49 studies. Probiotics improved muscle strength by 1,90 kg and gait speed by 0,08 m/s. Fibre-rich whole-food diets improved muscle strength by 1,25 kg; energy-restricted diets for weight loss improved muscle mass in participants with a mean age under 60 years and intervention duration up to 12 weeks. Protein-rich diets improved muscle mass in women and with a duration of at least 12 weeks. The work reports no separate effect of prebiotics on muscle strength or gait speed.

Prebiotics on Skin and Mucous Membranes: Human Studies

A combined intervention significantly improved HPV clearance. The evaluation considered subgroups by strain, route of administration, duration and comparator; pregnant and breastfeeding women as well as immunocompromised patients were excluded from the HPV studies, while pregnant women were included and analysed separately in the vaginosis studies 18. The contribution of the prebiotic component to HPV clearance cannot be derived from the joint evaluation.

A systematic review of 33 studies in 2112 patients with acne vulgaris examined oral and topical prebiotics, probiotics and postbiotics over treatment durations of 4 to 25 weeks. Included were 5 prebiotic studies, 24 probiotic studies and 7 postbiotic studies; 2 studies compared prebiotics and probiotics, one study compared prebiotics and postbiotics directly. The work reports safety and efficacy of microbiome-modulating therapies via the gut-skin axis, inflammation and barrier function 37. The number of prebiotic studies is small and results are not reported separately by substance group.

A randomised placebo-controlled intervention study examined the prebiotic UG1601 from inulin, lactitol and aloe-vera gel in adults with a stool frequency of fewer than three times per week. Participants received either the prebiotic or a placebo for 4 weeks. At the beginning and end of the intervention, stool and blood samples were collected, gastrointestinal symptoms and stool frequency were recorded, and serum endotoxaemia markers and faecal short-chain fatty acids were determined. The relative abundance of short-chain-fatty-acid-producing bacteria and the gut microbiome community were compared between responders and non-responders in the prebiotic group 38. The study is small and the preparation is a mixture of several components.

An open-label, single-centre clinical study over 12 weeks examined probiotics (Bacillus indicus HU36, Bacillus subtilis HU58, Bacillus coagulans SC208, Bacillus licheniformis SL307 and Bacillus clausii SC109) and precision prebiotics (fructo-oligosaccharides, xylo-oligosaccharides and galacto-oligosaccharides) in 63 patients with psoriasis under topical therapy. The first 42 enrolled patients were assigned to the intervention group, the next 21 to the control group (ratio 2:1; not randomised). At baseline, differences existed between the groups in several characteristics, including age, psoriasis severity (severe psoriasis more frequent in the intervention group), nail psoriasis and psoriatic arthritis 39. The open-label, non-randomised design and baseline differences limit the statement.

A narrative review summarises the clinical evidence on a vaginal gel based on Trametes versicolor that contains prebiotics, moisturising agents and anti-inflammatory accompanying substances and is intended to support natural HPV clearance as well as healing or regression of lesions. The structured literature search in MEDLINE, Embase and Scopus was conducted without language or date restrictions; additional entries were identified via ClinicalTrials.gov and Google Scholar. Included were peer-reviewed publications and conference abstracts on clinical outcomes of the gel 40. The review is narrative and the preparation is a mixture of several components.

A randomised, double-blind clinical study examined an antioxidant serum with 30 % tetrahexyldecyl ascorbate, a patent-pending mixture of antioxidants and prebiotics, in photoaged and hyperpigmented skin. In an in-vitro tissue model under blue light, the formation of reactive oxygen species decreased by 88 % after 30 minutes, by 87 % after 60 minutes and by 82 % after 120 minutes compared with the blue-light-exposed control. Melanin production fell by 24 % in the in-vitro tissue co-culture. In the in-vitro tissue model, the serum improved the structure of the epidermis, dermo-epidermal junction zone and dermis 25. The preclinical findings derive from cell models; a corresponding structural effect in humans was not examined. The contribution of the prebiotics to the clinical outcome was not isolated.

Reviews and Mechanisms of Action

The ISAPP consensus conference of May 2019 revised the definition of synbiotics to a mixture of live microorganisms and a substrate that is selectively utilised by host microorganisms and confers a health benefit on the host. The panel noted that defining it as a mere mixture of probiotics and prebiotics could suppress the innovation of cooperatively designed synbiotics and that the requirement for each component to individually meet the evidence and dose requirements could be an obstacle. A complementary synbiotic must consist of a probiotic plus a prebiotic, a synergistic synbiotic does not 52. The consensus statement concerns the definition, not clinical efficacy.

A review describes prebiotics as nutrients that are degraded by the gut microbiota. Their degradation products are short-chain fatty acids that enter the bloodstream and influence not only the gastrointestinal tract but also distant organs. Fructo-oligosaccharides and galacto-oligosaccharides are the two major groups with health effects. As both occur only in small amounts naturally in foods, attempts are being made to produce prebiotics industrially. Owing to their benefits, safety and production and storage advantages over probiotics, they are regarded as candidates for promoting health, as a replacement for or in combination with probiotics 53. The work is a review without its own data collection.

A review describes the immunomodulatory role of probiotics and prebiotics on cells, molecules and immune responses in the intestinal mucosa, from the epithelial barrier to the priming of adaptive responses by antigen-presenting cells. It discusses the modulation of normal homeostatic mechanisms and findings from pathological studies and places the role of these substances in dysbiosis in the context of immunopathological conditions such as inflammatory bowel diseases, colorectal cancer and hypersensitivity 54. The work provides no clinical endpoints of its own.

A review summarises findings on the role of the gut microbiota and the immune system in maintaining brain function and the development of neurological disorders. It describes the bidirectional connection between the gut and the central nervous system and names endocrine, neuronal, Toll-like receptor and metabolite-dependent pathways. Alterations in this relationship are linked to the pathogenesis of gastrointestinal and neurological disorders; the gut–brain axis is regarded as an emerging and widely accepted concept 55. The work is a review without its own intervention data.

A review examines the role of the gut microbiota in metabolic syndrome, which comprises central obesity, elevated plasma glucose, hyperlipidaemia and hypertension. Manipulation of the microbiota by prebiotics or probiotics could aid weight loss and lower plasma glucose and serum lipid levels. Short-chain fatty acids, bile salt hydrolase, metabolic endotoxaemia and the endocannabinoid system are described as essential for the regulation of adipogenesis, insulin secretion, fat accumulation, energy homeostasis and plasma cholesterol 56. The work calls for further studies on microbiota manipulation.

A review examined the association between obesity and the gut microbiome and the role of probiotics, prebiotics and synbiotics in the prevention and treatment of obesity. The authors show that the microbiome influences nutrient metabolism and energy expenditure and that various forms of obesity treatment alter the diversity and composition of the microbiome. Supplementation with probiotics, prebiotics and synbiotics could alter the secretion of hormones, neurotransmitters and inflammatory factors and thereby attenuate triggers of food intake 57. The work calls for further clinical studies on doses, compositions and regimens.

Overall Picture

3. How Robust the Research Is

Research is distributed very unevenly. The gut is by far the most common application area for controlled human studies, followed by inflammation, metabolism, the immune system and safety. The fields of cognition, cardiovascular, oncology, fatigue and antioxidant questions are markedly smaller. Alongside the controlled studies stands a large number of reviews and mechanistic papers. Notably, the most cited works in the entire field are reviews and consensus papers, not the controlled studies. The most frequently cited controlled study from 2009 examined the fermentation of prebiotics in the gut and its consequences for satiety hormones, appetite and the glucose response after a meal.

The methodological basis is broad but inconsistent. There are controlled studies, observational studies, meta-analyses and guidelines — for example the GLAD-P guideline of the World Allergy Organization and McMaster University on the prevention of allergic diseases. Proven: A meta-analysis of controlled studies exists for the antibody response after influenza vaccination. Such summaries carry more weight than individual studies. At the same time, many controlled studies remain at the level of markers: gut flora, immune parameters, metabolites. Whether a clinical benefit follows from this is a separate question. The heterogeneity of the substances and endpoints makes it difficult to generalise findings; a result for one galacto-oligosaccharide mixture does not automatically apply to soluble maize fibre.

4. What Is Frequently Claimed — and What the Data Provide

5. Profile: Molecule and Charge

Prebiotics are not a single molecule but a mixture — there is no molecular formula and no fixed molecular mass. What can be described are functional groups, charge behaviour and redox properties; that is exactly what the next section addresses.

6. Charge, Electrons, Radicals

Prebiotic, non-digestible oligosaccharides (NDOs) can neutralise free radicals directly or indirectly. Proven: is that the review presents this mechanism as a testable approach; it contains no measurement of its own. Free radicals damage proteins, lipids and nucleic acids, and this oxidative damage is associated with neurological disorders (26). Whether neutralisation occurs via electron transfer, via charge interactions or indirectly via redox systems remains open in the cited sentences.

In humans it has not yet been measured how prebiotic NDOs behave electrochemically. Charge, cation exchange, electron transfer and redox behaviour in humans remain an open question; this measurement is still outstanding. Hypothesis: It is conceivable that the described radical neutralisation (26) is coupled to surface charges or to redox pairs; this cannot be deduced from the sentences available. What would need to be clarified is whether charge and electron transfer of prebiotic NDOs can be measured in controlled human studies and whether such a measurement is linked to the observed radical neutralisation.

7. Interactions with Other Substances

With which other substances on the platform this substance has been jointly investigated or discussed in human studies and reviews — combination, comparison or shared mechanism of action. The number indicates studies in which both occur.

Controlled studies in humans describe prebiotics primarily in combination with probiotics. In cases of food intolerance the same combination achieved RR 0.45 (95 % CI 0.29 to 0.67) 24. No effect of the individual substance can be derived from combination studies.

Inulin was tested over eight weeks in 62 women with obesity and depression against 10 g/day maltodextrin, both groups with calorie restriction 28. In seasonal allergic rhinitis a randomised, double-blind study tested probiotics plus prebiotics against a placebo over 90 days 2929. For L. rhamnosus GG together with SCF, IL-6 decreased, which was not observed with the synbiotics 27. Whether prebiotics alter the absorption of medicinal substances or influence CYP enzymes is an open, testable question; studies with plasma level measurements would be the next step.

8. Overdosage, Tolerability, Limits

For FOS the highest proven amount is 10 g/d: in patients after allogeneic stem cell transplantation (allo-HCT) FOS at this amount was described as safe and well tolerated, without significant adverse effects 31 31. An upper limit or toxicological thresholds do not emerge from this; a dose at which adverse effects begin is not stated. Preterm infants with very low birth weight also tolerated prebiotic oligosaccharides well 30, as did infants receiving a formula enriched with the investigated oligosaccharides and adolescents with type 2 diabetes during metformin treatment.

Adverse effects concern the gut: with one formulation (OF-IN) flatulence and fullness occurred more frequently than in controls, while other adverse events with the remaining prebiotics were similar to controls 32. Infants who received a formula with the investigated oligosaccharides had more frequent yellow and more frequent loose stools than the comparison group. Contraindications are not mentioned. It would need to be examined whether a reliable limit for FOS above 10 g/d can be determined and whether flatulence under OF-IN is dose-dependent 32.

9. The Most Robust Studies

Ordered by strength of evidence, not by familiarity: reviews and studies with many participants first. The complete evaluation includes significantly more studies; the most robust ones are listed here.

Controlled Studies in Humans

• 1 Effect of Probiotics and Prebiotics on Immune Response to Influenza Vaccination in Adults: A Systematic Review and Meta-Analysis of Randomized … (2017) — Review, 1,979 participants
• 2 Effects of prebiotics on sepsis, necrotizing enterocolitis, mortality, feeding intolerance, time to full enteral feeding, length of hospital stay … (2019) — Review, 1,322 participants
• 3 Effects of Microbiota-Driven Therapy on Circulating Indoxyl Sulfate and P-Cresyl Sulfate in Patients with Chronic Kidney Disease: A Systematic Review … (2022) — Review, 513 participants
• 4 Effectiveness of Probiotic, Prebiotic, and Synbiotic Supplementation to Improve Perinatal Mental Health in Mothers: A Systematic Review and … (2021) — Review, 713 participants
• 5 Network meta-analysis of probiotics, prebiotics, and synbiotics for the treatment of chronic constipation in adults (2024) — Review article, 3.903 participants
• 6 The effect of gut microbiome-targeted therapies in nonalcoholic fatty liver disease: a systematic review and network meta-analysis (2024) — Review article, 1.921 participants
• 7 The effects of inulin-type fructans on cardiovascular disease risk factors: systematic review and meta-analysis of randomized controlled trials (2024) — Review article, 2.518 participants
• 8 Effectiveness and safety study of formula containing probiotics, prebiotics, synbiotics on fullterm infants' growth - a systematic review and … (2025) — Review article, 8.868 participants
• 9 Effects of probiotics, prebiotics, synbiotics and postbiotics on pediatric asthma: a systematic review (2025) — Review article, 2.419 participants
• 10 Efficacy and tolerability of probiotics, prebiotics, and symbiotics consumption on oral complications of patients with thyroid and head and neck … (2025) — Review article, 885 participants
• 11 Effects of probiotics, prebiotics, and synbiotics on cardiometabolic risk factors in children and adolescents with overweight or obesity: a … (2025) — Review article, 820 participants
• 12 The Effect of Prebiotics, Alone or as Part of Synbiotics, on Cardiometabolic Parameters in Women with Polycystic Ovary Syndrome: A Systematic Review … (2025) — Review article, 1.271 participants
• 13 Comparative Effects of Probiotics, Prebiotics, and Synbiotics on Clinical Outcomes in Coronary Artery Disease: A Systematic Review and Meta-Analysis (2026) — Review article, 793 participants
• 14 Prebiotic Supplementation in Celiac Disease: A Systematic Review of Clinical Efficacy, Safety, and Nutritional Outcomes (2026) — Review article, 1.066 participants
• 15 Efficacy and Safety of Probiotics, Prebiotics and Synbiotics for the Treatment of Functional Constipation in Children: Systematic Review and Network … (2026) — Review article, 1.087 participants
• 16 Effect of probiotics on allergic rhinitis in children; A systematic review and meta-analysis (2026) — Review article, 1.739 participants
• 17 Effects of gut microbiota-targeted interventions on pain, physical function, and inflammatory biomarkers in osteoarthritis: a systematic review and … (2026) — Review article, 2.080 participants
• 18 The efficacy of prebiotics and probiotics on HPV clearance and bacterial vaginosis: a systematic review and meta-analysis (2025) — Review article, 2.997 participants
• 19 Prebiotics, probiotics, and synbiotics therapy in critically ill patients: a systematic review and network meta-analysis (2026) — Review article, 5.073 participants
• 20 Probiotics for Mild Cognitive Impairment and Alzheimer's Disease: A Systematic Review and Meta-Analysis (2021) — Review article, 174 participants
• 21 Effects of Prebiotic Therapy on Gastrointestinal Microbiome of Individuals with Different Inflammatory Conditions: A Systematic Review of Randomized … (2024) — Review article, 135 participants
• 22 Effect of the Gut Microbiome on the reduction of uremic toxins in patients with chronic kidney disease: A systematic review & Network meta-analysis (2025) — Review article, 331 participants
• 23 The efficacy of probiotics, prebiotics or synbiotics in scleroderma: A systematic review (2025) — Review article, 103 participants
• 24 Probiotics, Prebiotics, Lactoferrin, and Combination Products for Prevention of Mortality and Morbidity in Preterm Infants: A Systematic Review and … (2023) — Review article

10. What Remains Open

• Clinical endpoint: Which controlled study examines disease progression rather than a marker — for example infection frequency, weight or depression severity — over a longer period?
• Single substance: Can the effect of a single prebiotic be isolated from the synbiotic mixture studies, or are separate studies required for each substance?
• Transferability: Do findings on a galacto-oligosaccharide mixture also apply to other prebiotics, or must each substance be examined separately?

11. Further Paths

Topics, pathways of action and chains in which this substance occurs — for all who want to look beyond the substance.

• Interaction network