BACKGROUND: Although many studies have reported the therapeutic effects of hot spring bathing on various diseases, its influence on healthy individuals is not well understood. Myoban Onsen, a sulfur-rich hot spring in Beppu City, Japan, is traditionally believed to improve skin conditions, relieve fatigue, and promote relaxation. However, scientific verification of these effects, particularly their impact on gut microbiota and related metabolic outcomes in healthy individuals, remains scarce. This study aimed to evaluate the effects of Myoban hot spring bathing on gut microbiota composition and SCFA concentrations in healthy individuals. METHODS: In this study, 16 healthy adult males (n = 16) participated in Myoban hot spring bathing four times over two weeks. Fecal samples were collected before and after the intervention, and 16S rRNA sequencing and gas chromatography-mass spectrometry (GC-MS) were performed to analyze gut microbiota composition and organic acid concentrations. The effects of hot spring bathing were evaluated using the Wilcoxon matched-pair signed-rank test to compare pre- and post-intervention. RESULTS: After Myoban hot spring bathing, there was a significant increase in beneficial gut bacteria, Bifidobacterium, Blautia, and Anaerostipes, compared to pre-bathing (p = 0.0012, p = 0.0103, and p = 0.0017, respectively). Conversely, significant decreases were observed in Parabacteroides, Alistipes, and Oscillibacter (p = 0.0125, p = 0.0215, and p = 0.0125, respectively). Significant increases in SCFAs, including acetic acid, propionic acid, and butyric acid, were observed after Myoban hot spring bathing (p = 0.0067, p = 0.0125, and p = 0.0302, respectively). These findings suggest that Myoban hot spring bathing may benefit healthy adult males. CONCLUSIONS: This study suggests that Myoban hot spring bathing may improve gut health in healthy males. The observed increases in beneficial bacteria and SCFAs indicate a potential contribution to improved health status through modulation of the gut environment. TRIAL REGISTRATION Registration number: UMIN000055229, retrospectively registered.
Humans ; Gastrointestinal Microbiome ; Male ; Hot Springs ; Pilot Projects ; Fatty Acids, Volatile/analysis* ; Adult ; Japan ; Feces/chemistry* ; Bacteria/genetics* ; Young Adult ; Baths ; RNA, Ribosomal, 16S/analysis* ; Middle Aged