Gut Health, Immunity, and the Gut-Brain Axis

A clinically studied probiotic strain with evidence for reducing diarrhea, inhibiting H. pylori, supporting gum health, and stimulating oxytocin through the gut-brain axis

Lactobacillus reuteri (now formally reclassified as Limosilactobacillus reuteri) is one of the most extensively studied probiotic bacteria in humans. Unlike many gut bacteria, it colonizes multiple body sites — the intestinal tract, mouth, urinary tract, and breast milk — and it co-evolved with humans over millennia. Modern life has sharply reduced its prevalence: widespread antibiotic use, formula feeding, and processed-food diets have depleted it from many people's microbiomes [1]. Clinical trials show it meaningfully shortens infectious diarrhea, reduces infant colic, helps suppress H. pylori, and reduces periodontal pathogens in the mouth. Emerging research on the gut-brain axis suggests it stimulates oxytocin production in the gut — a finding that may help explain some of its broader health effects [7].

A Bacterium That Once Lived in Most of Us

L. reuteri is what researchers call a "human-adapted" commensal — a bacterium that has evolved alongside humans and colonizes multiple body sites. It produces a natural antimicrobial compound called reuterin (3-hydroxypropionaldehyde), which selectively inhibits competing pathogens without harming beneficial bacteria. It also produces short-chain fatty acids, lactic acid, and certain B vitamins, including traces of B12 [1].

Epidemiological surveys suggest that L. reuteri was present in the vast majority of humans as recently as a few generations ago. Today, some estimates put colonization rates as low as 10–20% of the population in industrialized countries. The timing of its decline tracks with the rise in inflammatory diseases — though this correlation does not prove causation, it has motivated substantial research interest [1].

Two Strains, Two Roles

Most of the human clinical research focuses on two strains:

DSM 17938 — the strain most used in gastrointestinal trials. It is the primary strain in BioGaia Gastrus and similar products and has the strongest evidence base for diarrhea, colic, constipation, functional abdominal pain, and H. pylori adjunct therapy.

ATCC 6475 — studied primarily for immune modulation, oxytocin signaling, and bone health in preclinical work. It is often paired with DSM 17938 in oral health products. In animal research, this strain is the one most consistently linked to elevated oxytocin and testosterone levels; human data on these effects are still limited.

Gut Health: Diarrhea, Colic, and Functional Pain

The strongest clinical evidence for L. reuteri covers gastrointestinal conditions. A systematic review and meta-analysis of 12 randomized controlled trials involving L. reuteri DSM 17938 in children with acute diarrhea found it significantly reduced diarrhea duration by approximately one day compared to placebo (weighted mean difference: −1.01 day, 95% CI: −1.33 to −0.70), and reduced stool frequency on day 2 [2].

In infantile colic — a condition that causes otherwise healthy babies to cry for more than three hours a day — a double-blind RCT showed that breastfed infants given 10⁸ CFU/day of L. reuteri DSM 17938 had a median of 35 minutes of crying per day by day 21, compared to 90 minutes in the simethicone (placebo) group. The difference was statistically significant from day 7 onward [3].

L. reuteri also appears helpful for functional constipation and recurrent abdominal pain in children, with multiple double-blind trials showing benefit versus placebo.

Helicobacter Pylori: An Adjunct That Improves Outcomes

H. pylori infects roughly half the global population and is the leading cause of peptic ulcers and a major risk factor for gastric cancer. Standard triple therapy (two antibiotics plus a proton pump inhibitor) achieves eradication in 70–80% of cases, but antibiotic resistance is rising and side effects are common.

Adding L. reuteri to eradication regimens has been studied in multiple trials. A 2024 meta-analysis of eight RCTs including over 1,000 patients found that the L. reuteri group achieved significantly higher eradication rates in both intention-to-treat (80.0% vs. 72.6%, p = 0.005) and per-protocol analysis. Side effect rates — particularly diarrhea and nausea — were also lower in the probiotic group [4]. Individual trials confirm this pattern, with one Egyptian RCT finding improved eradication and reduced GI symptoms over a 4-week supplementation period following standard triple therapy [5].

Oral Health: Targeting Periodontal Pathogens

L. reuteri colonizes the oral cavity naturally, and its reuterin production has specific activity against oral pathogens including Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans — two key species in gum disease. In a randomized clinical trial of 40 gingivitis patients, L. reuteri tablets (DSM 17938 + ATCC PTA 5289, 2×10⁸ CFU/day) taken for 8 weeks significantly reduced the number of periodontal pathogens in subgingival plaque compared to placebo [6].

Clinical trials in periodontitis patients — often using L. reuteri as an adjunct to professional scaling and root planing — have generally shown reductions in gingival bleeding and plaque index. The evidence is moderate strength and consistent with the bacterium's known antimicrobial properties [6].

Oxytocin and the Gut-Brain Axis

One of the more striking findings in L. reuteri research is its apparent ability to stimulate oxytocin production. Oxytocin is known as a bonding hormone, but it has broad roles throughout the body: it accelerates wound healing, reduces inflammation, regulates appetite, and supports social behavior.

L. reuteri appears to activate oxytocin-producing neurons in the intestinal lining through vagus nerve signaling. Studies in rodents show that oral administration of L. reuteri ATCC 6475 increases circulating oxytocin levels, and that the behavioral and metabolic benefits of the bacterium are blocked when the vagus nerve is severed or when oxytocin receptors are pharmacologically blocked [7]. Whether this mechanism translates meaningfully to humans has not been established in controlled trials, but the mechanistic pathway is biologically coherent.

Immune Modulation

L. reuteri exerts anti-inflammatory effects by promoting regulatory T-cell activity and increasing production of IL-10 (an anti-inflammatory cytokine) while reducing IL-6 and other pro-inflammatory signals. These effects have been documented in gut mucosa, skin, and systemic circulation in animal models. The anti-inflammatory profile may underlie its benefits across diverse conditions — from gut inflammation to colic to periodontal disease [1].

How to Supplement

L. reuteri DSM 17938 is the strain with the broadest human trial base. Standard doses in trials range from 10⁸ to 10⁹ CFU per day. Liquid drops (e.g., BioGaia Gastrus) are typically used in infants; chewable tablets are used in older children and adults.

L. reuteri is naturally found in some fermented dairy foods, but levels are generally low and variable. Targeted supplementation is the most reliable way to reach clinically relevant doses.

It tolerates broad-spectrum antibiotics poorly — if taking antibiotics, wait 2–3 hours before taking L. reuteri and continue supplementation for at least 1–2 weeks after completing the course.

See our probiotics page for broader context on cultivating a healthy microbiome, and our gut-brain axis page for more on microbial effects on mood and behavior.

Evidence Review

Systematic Review: Diarrhea in Children — Urbańska & Szajewska (2016)

Urbańska and Szajewska conducted a systematic review and meta-analysis of 12 RCTs examining L. reuteri DSM 17938 for acute diarrhea in children [2]. Pooled analysis showed a statistically significant reduction in diarrhea duration (weighted mean difference: −1.01 day; 95% CI: −1.33 to −0.70 days) and a significant reduction in stool frequency on day 2. Trials were conducted across diverse settings including Poland, Italy, Turkey, India, and Egypt. Effect sizes were consistent across populations and settings. Heterogeneity was moderate (I² = 55%). No serious adverse events were reported across trials. The authors concluded the evidence supports use of L. reuteri DSM 17938 as an adjunct treatment for acute diarrhea in children, with a modest but clinically meaningful effect on duration.

Infantile Colic RCT: Savino et al. (2010)

Savino et al. published a double-blind, placebo-controlled trial in Pediatrics testing L. reuteri DSM 17938 in 50 exclusively breastfed infants with colic [3]. Infants received 10⁸ CFU/day (5 drops of BioGaia) or simethicone (a commonly used but evidence-weak colic treatment) for 21 days. The primary endpoint was daily crying time measured by parental diary.

By day 7, the L. reuteri group had a median crying time of 51 min/day vs. 71 min/day for simethicone (p < 0.05). By day 21, the difference was striking: 35 min/day vs. 90 min/day (p < 0.01). Response rate (defined as ≥50% reduction in crying) was 95% in the L. reuteri group vs. 7% in the simethicone group. This was a landmark study that established L. reuteri as a first-line treatment for colic in breastfed infants. The mechanism may involve normalization of gut motility, reduction of gas-producing coliform bacteria, and possibly effects on pain perception via gut-brain signaling.

H. Pylori Meta-Analysis: Gao et al. (2024)

Gao et al. performed a meta-analysis of eight randomized controlled trials with 1,087 patients evaluating L. reuteri as an adjunct to standard H. pylori eradication therapy [4]. The analysis showed:

Intention-to-treat eradication rate: 80.0% (L. reuteri) vs. 72.6% (placebo); p = 0.005
Per-protocol eradication rate: significantly higher in the L. reuteri group across all trials
Side effect rates, including diarrhea, nausea, and taste disturbance, were lower in the L. reuteri group

Trials included patient populations from China, Italy, Egypt, and Taiwan. Most used L. reuteri DSM 17938 or DSM 17648 (a postbiotic form). The magnitude of benefit is modest but consistent — L. reuteri appears to improve eradication rates by roughly 7–10 percentage points, likely through its reuterin-mediated direct inhibition of H. pylori and its ability to reduce antibiotic-associated GI side effects that might otherwise lead to treatment non-compliance.

H. Pylori RCT: Emara et al. (2014)

Emara and colleagues conducted a double-blind placebo-controlled trial in 70 treatment-naive H. pylori-positive adult patients with dyspepsia [5]. After 2 weeks of standard triple therapy, patients received either L. reuteri or placebo for 4 weeks. Eradication rates were 74.3% in the L. reuteri group vs. 65.7% in the placebo group — a modest but consistent advantage. GI symptom scores (bloating, nausea, epigastric pain) improved more in the probiotic group throughout the supplementation period. This trial is notable for testing a period of probiotic supplementation after antibiotic therapy, suggesting a role not just during eradication but in mucosal recovery.

Oral Health: Iniesta et al. (2012)

Iniesta and colleagues conducted an 8-week, double-blind, parallel-group RCT in 40 patients with gingivitis [6]. Participants took one daily tablet containing L. reuteri DSM 17938 and ATCC PTA 5289 (1×10⁸ CFU each) or placebo, without changes to their usual oral hygiene. Subgingival plaque samples were collected at baseline and at 4 and 8 weeks and analyzed for key periodontal pathogens.

The L. reuteri group showed significant reductions in counts of Porphyromonas gingivalis and other anaerobic pathogens in subgingival plaque at both 4 and 8 weeks. However, clinical measures of gingival inflammation (bleeding on probing, plaque index) did not significantly differ between groups, suggesting that 8 weeks may be insufficient for microbiological changes to translate into measurable clinical improvement. Salivary bacterial counts also shifted favorably in the treatment group. No adverse events were reported. This study supports a role for L. reuteri in oral microbiome remodeling, with clinical benefits likely requiring longer supplementation or combination with professional cleaning.

Oxytocin: Poutahidis et al. (2016)

Poutahidis and colleagues reviewed evidence linking L. reuteri and microbial communities more broadly to oxytocin production, primarily from rodent models [7]. L. reuteri ATCC 6475 administration increased circulating oxytocin in mice and rats. Behaviorally, treated animals showed increased social grooming, reduced anxiety, and accelerated wound healing — effects that were abolished by cutting the vagus nerve, demonstrating gut-to-brain signaling rather than direct systemic hormone release. The proposed mechanism involves L. reuteri activating enteric neurons that send signals via the vagus nerve to hypothalamic oxytocin-producing nuclei.

The oxytocin-wound healing connection was particularly striking: rodents supplemented with L. reuteri showed significantly faster skin wound closure. The authors attributed this to systemic oxytocin increasing immune regulatory cell activity at wound sites.

This work is mechanistically compelling but remains largely preclinical. Human trials measuring oxytocin levels in response to L. reuteri supplementation have not yet been published in peer-reviewed literature, making this an active and promising area of research rather than an established clinical benefit.

Testosterone: Poutahidis et al. (2014)

Poutahidis et al. reported that male mice supplemented with L. reuteri ATCC 6475 in drinking water maintained higher serum testosterone levels and larger testicular size into old age compared to control mice — including those on a high-fat diet [8]. The treated mice had more Leydig cells (testosterone-producing cells) per testis and improved seminiferous tubule histology. Blocking the pro-inflammatory cytokine IL-17A with antibodies produced similar testicular benefits, suggesting that L. reuteri's anti-inflammatory effects — specifically its suppression of age-associated interleukin-17 — may protect Leydig cell function.

This is mouse data only. No controlled human trials have examined whether L. reuteri supplementation affects testosterone or testicular function in men. The mechanistic pathway (gut inflammation → IL-17 → Leydig cell damage) is plausible, but the translation to humans remains speculative at this stage.

Strength of Evidence

The clinical evidence for L. reuteri is strongest in three areas: acute diarrhea in children (multiple RCTs, consistent effect), infantile colic in breastfed infants (compelling RCT data, high response rates), and H. pylori adjunct therapy (consistent modest benefit across meta-analysis). Oral health evidence is mechanistically sound but clinical trial data are limited. The oxytocin and testosterone findings are intriguing preclinical signals that have not yet been validated in human trials. Overall, L. reuteri has one of the more robust evidence profiles among commercially available probiotic strains, with genuine clinical utility in specific, well-defined conditions.

References

Role of Lactobacillus reuteri in Human Health and DiseasesMu Q, Tavella VJ, Luo XM. Frontiers in Microbiology, 2018. PubMed 29725324 →
Systematic review with meta-analysis: Lactobacillus reuteri DSM 17938 for diarrhoeal diseases in childrenUrbańska M, Szajewska H. European Journal of Pediatrics, 2016. PubMed 26991503 →
Lactobacillus reuteri DSM 17938 in infantile colic: a randomized, double-blind, placebo-controlled trialSavino F, Cordisco L, Tarasco V, Palumeri E, Calabrese R, Oggero R, Roos S, Matteuzzi D. Pediatrics, 2010. PubMed 20713478 →
Lactobacillus reuteri compared with placebo as an adjuvant in Helicobacter pylori eradication therapy: a meta-analysis of randomized controlled trialsGao F, Zhang Y, Zhou H, Wang Y, Zhao X, Liu J. Helicobacter, 2024. PubMed 38846173 →
Lactobacillus reuteri in management of Helicobacter pylori infection in dyspeptic patients: a double-blind placebo-controlled randomized clinical trialEmara MH, Mohamed SY, Abdel-Aziz HR. Therapeutic Advances in Gastroenterology, 2014. PubMed 24381643 →
Probiotic effects of orally administered Lactobacillus reuteri-containing tablets on the subgingival and salivary microbiota in patients with gingivitis: a randomized clinical trialIniesta M, Herrera D, Montero E, Zurbriggen M, Matos AR, Marín MJ, Sánchez-Beltrán ML, Llama-Palacio A, Sanz M. Journal of Clinical Periodontology, 2012. PubMed 22694350 →
Microbes and Oxytocin: Benefits for Host Physiology and BehaviorPoutahidis T, Smillie K, Bhela S, Shiu L, Swartz E, Economopoulos KP, Fox JG, Bhowmick NA, Singh J, Bhanu NV. International Review of Neurobiology, 2016. PubMed 27793228 →
Probiotic microbes sustain youthful serum testosterone levels and testicular size in aging micePoutahidis T, Springer A, Bhela S, Shiu L, Swartz E, Economopoulos KP, Fox JG, Bhowmick NA, Singh J. PLOS ONE, 2014. PubMed 24392159 →