Yogurt and Hot Springs: How Food and Environment Together Change the Gut Ecosystem and Stool in Healthy Adults

Gut microbiota respond to both your plate and your surroundings. We know a lot about the effects of fermented foods, but how habits “outside the kitchen” — like regular baths in mineral springs — affect microbes and bowel movements is poorly understood. Japanese researchers conducted a randomized controlled trial and compared three scenarios: no changes, nightly yogurt, and yogurt plus bathing in a chloride hot spring. The bottom line: yogurt increased the diversity of the microbiota and enriched a number of “beneficial” taxa, while the addition of a hot spring did not shift the microbiota significantly, but gave the greatest numerical improvement in bowel movements (albeit without strict statistical significance).

Background of the study

The intestinal microbiota is a dynamic ecosystem that is simultaneously influenced by diet and environment. There is a lot of data on fermented foods, particularly yogurt: regular consumption can increase microbial diversity, support the barrier, modulate immune responses, and generally improve gastrointestinal well-being. However, “outside the kitchen” factors have been studied much less. In Japan, hot mineral springs are a common habit, of which chloride springs (with ≥1 g/kg chloride ions) are among the most accessible, and are credited with benefits for blood circulation and overall tone. However, there is almost no evidence of the microbiota effects of such bathing, and isolated observations (for example, the growth of Bifidobacterium bifidum after a week of “bicarbonate” onsen) still look preliminary. Against this background, the idea of combining a “dietary” intervention (yogurt) and an “environmental” one (chloride source) and seeing how they individually and together affect the microbiota and stool in healthy individuals seems like a logical next step.

The authors of the work proceed from a simple hypothesis: yoghurt as a fermented product with Lactobacillus/S. thermophilus is an understandable driver of microbial shifts, while a hot spring can affect intestinal physiology by "bypass routes" (through relaxation, hydration, peripheral hemodynamics), which 16S sequencing catches worse. Together, these arms can be complementary: one is "about the composition of microbes", the other is "about the function of the intestine". Therefore, the design included an assessment of both the microbial profile and the well-being of defecation (a questionnaire on frequency/consistency/feeling of incomplete emptying, etc.).

Another motive is practicality. If accessible daily habits can improve the “gut ecology”, this is a potentially scalable public health tool. But for a fair assessment, randomization, control and comparable protocols are needed. Here they were provided: healthy adults, not “fed” with probiotics and without recent onsen, were divided into three branches (control; evening yogurt 180 g; yogurt + bathing in a sodium chloride spring at least once every two days, ≥15 minutes), observed for 4 weeks, before/after collecting feces for 16S (V1-V2) and SCFA (GC-MS) and filling out a questionnaire. This “two-headed” design allows us to separate the microbial effects of food from the functional shifts associated with balneology.

Finally, the authors are honest about the limits of their knowledge: although “heat therapy” has been linked in studies to musculoskeletal, dermatological, and metabolic benefits, there is no systematic evidence that chloride springs alter the composition of gut microbiota; moreover, early work has hinted that the onsen itself may not produce significant taxonomic shifts. Hence the key question of the paper: would a randomized comparison reveal the opposing but beneficial effects of “yogurt → microbial diversity,” “onsen → subjective defecation” and whether we should expect synergy from their combination in real life.

Design and what exactly they did

This is a randomized controlled trial (September-December 2023). Forty-seven adults aged 20-65 years who did not regularly consume yogurt or visit an onsen for 2 weeks before the study intervention were recruited. After dropouts, 35 participants were included in the analysis: control (n=10), yogurt (n=14), and yogurt+onsen (n=9). For 4 weeks, the intervention groups ate 180 g Meiji Bulgaria Yogurt LB81 (L. bulgaricus 2038, S. thermophilus 1131) every evening, and the yogurt+onsen group additionally bathed in a sodium chloride spring at least once every other day (≥15 minutes; pH 3.6; ~1,446 mg/kg chloride ion). Before and after, stool was collected, 16S rRNA (V1-V2) was sequenced, SCFA was counted (GC-MS), and defecation questionnaires were completed (14 items; lower scores indicate worse condition). Participants were asked not to change their lifestyle and to avoid other probiotics/sources.

What was found in the microbiota: yogurt "expands" diversity

After 4 weeks, only yogurt showed significant growth in all three alpha diversity metrics: Shannon (p=0.0031; q=0.0062), Observed ASVs (p=0.0007; q=0.0015), and Faith's PD (p=0.0001; q=0.0002). There were no changes in the control group; in "yogurt+onsen" there was only a non-significant trend. According to the taxonomy, several genera grew within "yogurt" (Sellimonas, Eggerthella, Flavonifractor, Ruminiclostridium 9 - passed FDR), and in the intergroup comparison, "yogurt" had more Akkermansia, Eggerthella, Ruminiclostridium 9, and Sellimonas and less Megasphaera than the control. In the yogurt+onsen group, Lachnoclostridium and Holdemania were lower than in the pure yogurt.

Metabolites in feces: no major changes

Short-chain fatty acids (acetate, propionate, butyrate, etc.) did not change significantly in any group. Only a nominal decrease in formic acid was noted against the background of yogurt (p=0.028), which disappeared after correction for multiple tests (q=0.364). There were no significant differences between the groups for SCFA. The authors' conclusion: over a 4-week period, yogurt can slightly "correct" individual organic acids, but the overall SCFA profile is stable.

Defecation: The biggest "plus" of combining with onsen

The total score of defecation increased in the "yogurt" and "yogurt + onsen" groups, and in the control group - without improvement. According to the change from the initial value, the average (±SD) was as follows: yogurt + onsen +2.89 ± 3.79, yogurt +1.00 ± 4.30, control -1.25 ± 3.67. Formally, the intergroup differences did not reach significance (small sample), but the trend towards the benefits of bathing is obvious. The authors cautiously conclude: yogurt helps stool, and chloride onsen can add a little more effect - this should be tested in larger cohorts.

How to understand this in simple words

The picture is as follows: the diet (evening yogurt) quite quickly increases the "diversity" of the intestinal ecosystem and shifts the composition towards several taxa associated with healthy metabolism (including Akkermansia). The environmental factor (mineral baths) does not noticeably change the microbiota over 4 weeks, but it may improve the sensations during defecation - due to hydration, peripheral blood flow, relaxation or other mechanisms not "caught" by 16S sequencing and the SCFA panel. In total, these are two independent shoulders of influence on intestinal well-being: one "about microbes", the other - "about the well-being of the intestines".

Useful details for practitioners and the curious

• What kind of yogurt is this: 180 g Meiji Bulgaria LB81 (Low Sugar), strains Lactobacillus bulgaricus 2038 + Streptococcus thermophilus 1131, taken every evening after dinner.
• What kind of source: sodium chloride (NaCl type), pH 3.6, ~1,446 mg/kg Cl-, ≥15 min every 1-2 days. This is a mineral water immersion, not a "spa sauna".
• Who was included: healthy adults, no antibiotics, no regular probiotics and no onsen at the start. Final analysis: 35 people (microbiota/metabolites) and 33 by defecation questionnaire.

What it means (and what it doesn't mean)

• Yes: a simple “nighttime yogurt” habit can increase microbial diversity in healthy individuals within 4 weeks and shift a range of taxa associated with ecosystem resilience.
• Possibly: Regular bathing in chloride springs improves bowel movements according to self-reports, especially when combined with yogurt - but so far without strict statistics.
• No: it is too early to talk about “treatment of constipation/SIBO/GI tract infection” etc. - the participants were healthy, the time frame was short, the indicators were microbiota, SCFA and a questionnaire, not clinical diagnoses.

Restrictions

Small sample and short duration; open-label nature of the behavioral intervention; self-report on defecation; microbiota analysis at the 16S level (without metagenomics of functions); SCFA - only in feces, not in the lumen/blood; study in healthy people - tolerability to people with complaints requires separate testing. The authors honestly admit all this and call for larger and longer RCTs.

What to check next

• Longer and more: 8-12 weeks, n≥100, stratified by baseline stool frequency/diet and by "onsen lovers".
• Functions and mechanism: shotgun metagenomics, serum metabolomics, gut hormones, water-electrolyte status, stress axes.
• Clinic: pilots in people with functional disorders (mild constipation/IBS-C): "yogurt vs. yogurt+onsen" versus standard recommendations.

The main thing in three points

• Yogurt increased microbial diversity over 4 weeks and enriched several "beneficial" genera; Akkermansia was higher than in the control.
• Chloride hot springs did not significantly alter microbiota, but produced the greatest numerical improvement in bowel movements when combined with yogurt (not strictly significant).
• Food and environment act complementarily: the “microbial arm” (yogurt) + the “functional arm” (bathing) - together can support intestinal well-being.

Study source: Choi J., Takeda M., Managi S. Dietary and environmental modulation for the gut environment: yogurt promotes microbial diversity while chloride hot springs improve defecation status in healthy adults. Frontiers in Nutrition, June 30, 2025; doi:10.3389/fnut.2025.1609102.