The importance of diet in shaping the human gut microbiota has been well-researched, but little is known about how wellness rituals contribute to gut health.
A new study published in Frontiers in Nutrition sheds light on an unlikely - but surprising good - pairing for the gut microbiome: yogurt consumption and bathing in Japanese hot springs.
Led by Kyushu University researchers, the study was focused on a randomized controlled trial designed to examine how these dietary and environmental factors interacted to modulate the gut microbiome.
A group of adults was recruited for the task and split into three groups - a control group; one that only consumed 180g of yogurt daily; and one that consumed yogurt and took baths in Japanese ‘onsen’ - as hot springs are known - every two days at the least.
The researchers then collected stool samples from the cohort for lab analysis and asked participants to self-evaluate their bowel function.
The findings suggested that accessible lifestyle interventions can positively influence gut health and may serve as practical strategies for promoting overall wellbeing.
The healing power of hot springs
Hot spring bathing has long been recognized as a traditional wellness practice, with research also supporting the practice’s therapeutic potential.
For example, there’s clinical and epidemiological evidence that suggests bathing in mineral-rich hot springs may support musculoskeletal comfort, dermatological health, and metabolic balance.
And more recent research has suggested that immersion in hot springs may modulate the composition of the gut microbiota, eg. by boosting the abundance of Bifidobacterium bifidum.
The Kyushu University researchers wanted to expand on these findings but also find out how the combination of hot springs bathing would complement yogurt consumption, itself an established way to promote gut health.
The cohort
The trial was carried out from September to December 2023, with 35 men (15) and women (20) aged 20 to 65 years old taking part. That cohort was formed exclusively of people who do not regularly consume yogurt - and were told not to eat other probiotic-rich foods during the study - nor had taken hot spring baths recently.
The onsen used in this study was classified as a sodium chloride-type spring (NaCl), the study reads; with a weakly acidic pH of 3.6 and the following concentrations of sodium ions (964.6 mg/kg), chloride ions (1,446 mg/kg), potassium (147.4 mg/kg), calcium (40.0 mg/kg), and sulfate (454.0 mg/kg). The water also contained metasilicic acid and boric acid.
Participants in the yogurt + hot spring group were instructed to bathe in a chloride hot spring at least once every two days for at least 15 minutes per session over the course of the 4-week intervention period.
The researchers studied participants’ gut microbiota composition by using 16S rRNA gene sequencing of stool samples; their gut metabolome by using gas chromatography-mass spectrometry, and used a self-reported questionnaire to examine the participants’ bowel function.
Key findings
The researchers found that yogurt significantly increased gut microbial diversity across all three diversity indices (the Shannon Diversity Index’ observed ASVs, and Faith’s Phylogenetic Diversity).
They also discovered taxonomic shifts - specifically, notable increases in beneficial genera such as Akkermansia, Eggerthella, Sellimonas, and Flavonifractor.
There were also metabolite changes, such as a modest reduction in formic acid, though other SCFAs remained stable.
As for bowel function, yogurt consumption was associated with improved defecation scores, suggesting enhanced bowel function. Hot spring bathing also yielded the highest numerical improvement in defecation scores, suggesting that the practice had a positive impact on bowel regularity.
The specific combination of yogurt and bathing had no synergistic effect on the microbiota, but taken separately, the two interventions complemented each other to boost digestive health and wellness overall.
The authors concluded: “Yogurt consumption significantly improved gut microbial diversity and increased the abundance of beneficial taxa such as Akkermansia, while also contributing to improved defecation scores. Hot spring bathing, although not associated with significant microbial or metabolic shifts, was similarly linked to enhanced defecation function.
“These findings highlight the potential of both dietary and environmental interventions to support gut health through complementary pathways, offering accessible strategies for promoting overall well-being.”
Source:
Dietary and environmental modulation for the gut environment: yogurt promotes microbial diversity while chloride hot springs improve defecation status in healthy adults
Authors: Jungmi Choi ;Jungmi Choi; Midori Takeda; Midori Takeda; Shunsuke Managi; Shunsuke Managi
Published: Front. Nutr., 30 June 2025 Sec. Nutrition and Microbes Volume 12 - 2025
