Abstract
<title>Abstract</title> <p> Background Drinking hot spring water has been reported to be beneficial for individuals with type 2 diabetes. Objectives A proposed mechanism involves modulation of the gut microbiota. Therefore, this study aimed to assess the effects of hot spring water consumption on glycemic parameters and gut microbiota composition. Methods We conducted a multicenter, open-label, randomized, controlled, 2-sequence, 2-period crossover trial. The primary endpoint was fasting blood glucose level, and the secondary endpoints included lipid profiles and gut microbiota relative abundance. Eligible participants were adults residing in Tarumizu City, Kagoshima, Japan, with borderline HbA1c levels. Twenty participants were randomly assigned to two groups. Each consumed either Onsensui 99 or emergency bottled water for 1 month, followed by a 1-month washout period, and then consumed the alternate beverage. Participants were instructed to consume 500 mL/day of the assigned water, divided before meals. Results The primary endpoint did not differ significantly before and after Onsensui 99 consumption (p = 0.255). Additionally, no carryover effect was observed in the fasting blood glucose level (p = 0.451). In contrast, non-high density lipoprotein (non-HDL) cholesterol significantly decreased during Onsensui 99 consumption from 153 (121–167) to 144 (115–165) mg/dL (p = 0.014), with significant between-group differences (p = 0.019). Regarding gut microbiota, <italic>Romboutsia</italic> increased significantly, while <italic>Sutterella</italic> decreased significantly following Onsensui 99 consumption compared with emergency bottled water. Conclusions: Consuming Onsensui 99 for 1 month did not significantly improve fasting blood glucose levels; However, it significantly reduced non-HDL cholesterol. Trial registration: The study was registered with the Japan Registry of Clinical Trials (jRCT1070220018; first published on June 21, 2022). </p>