Kombucha, a fermented beverage made from Camellia sinensis tea (black, oolong, or green) with sugar and a symbiotic culture of bacteria and yeast (SCOBY), has gained global attention for its potential health benefits. Factors like the type and amount of sugar substrate, fermentation time, and temperature significantly influence its organic compounds, total phenolics, vitamin content, and alcohol levels.

In a previous study, kombucha's impact on glucose tolerance, insulin sensitivity, body composition, and liver function was tested in male prediabetic mice with diet-induced obesity. Daily supplementation (200 µL per mouse) improved glucose tolerance after nine days (equivalent to one year in humans) and reduced liver steatosis, despite no changes in body composition.

Although kombucha has been associated with antioxidant, antimicrobial, probiotic, antidiabetic, and anticancer activities, strong scientific evidence in humans remains limited. Further clinical studies are needed to substantiate kombucha's health benefits in humans.

The objectives of this clinical study aim to explore the effects of kombucha on the health of individuals with overweight and class 1 obesity, while also determining whether the kombucha microbiota plays a role in the observed effects. Specifically, by investigating metabolic parameters such as glucose and insulin levels and lipid profile, as well as the composition and diversity of the gut microbiota and liver function, the study will contribute to a deeper understanding of the potential benefits and mechanisms of action of kombucha consumption in humans. The study aims to recruit at least 30 individuals with overweight and class 1 obesity, aged between 18 and 60 years, randomly distributed into 3 arms (each arm should have about 10 participants). The first arm receives a daily amount of 33 cl of kombucha (live drink) for 4 weeks, the second arm receives a daily amount of 33 cl of kombucha (pasteurized drink) for 4 weeks. The control group receives 33 cl of sparkling water for 4 weeks.

Inclusion Criteria:

Individuals with a Body Mass Index (BMI) between 25 kg/m² and 34.9 kg/m², of both biological sexes, aged between 18 and 60 years, available to comply with the study protocol (described in this document) and sign informed consent.

Exclusion Criteria:

Volunteers will be excluded from the study if they present one or more of the following conditions:

1. Subjects with sensitivity to kombucha;
2. Consumption of kombucha, kefir, kimchi, cheese, raw vinegar, sauerkraut, kvass, and other fermented products during the study and in the 3 weeks before the study.
3. Use of antibiotics in the 6 months prior to the start of the study;
4. Use of pro/prebiotics or fibers as dietary supplements or any food/molecule that modifies intestinal transit time 6 weeks before recruitment; use of laxatives 6 weeks before recruitment;
5. Specific dietary regimen (e.g., vegan); specific dietary treatment (e.g., high protein);
6. Excessive consumption of substances and alcohol; smokers;
7. Diagnosis of gastrointestinal disorders, hormonal or thyroid diseases, autoimmune diseases, and/or chronic use of corticosteroids; psychiatric disease; Type 1 or 2 diabetes;
8. Use of proton pump inhibitors; antidiabetic drugs or insulin and statins;
9. Subjects with insulin sensitivity;
10. Pregnant or lactating women;
11. Subjects with tooth sensitivity
12. Participation in another clinical trial in the last 3 months.

Partial double blinding (participants will not know if they are consuming live or pasteurized kombucha; the nutritionist will not know which arm the participant belongs to; the technician responsible for microbiota sequencing will receive coded samples without information regarding the intervention/control arm the sample belongs to).