Kombucha: clinical evidence, composition and step-by-step

Kombucha is a traditional fermented drink made from sweetened tea and a symbiotic culture of bacteria and yeast (SCOBY).

Key points:

• It has drawn growing scientific interest as a functional food, especially in the context of gut health and metabolic dysfunction.
• Its relevance grows alongside the rise of conditions like obesity and metabolic resistance.
• The gut microbiota plays a central role in immunometabolic regulation, connecting diet, inflammation, metabolism and the gut barrier.
• Clinical studies indicate that kombucha influences gut microbiota composition.
• Fermentation generates bioactive compounds, such as organic acids, transformed polyphenols and microbial metabolites with documented physiological impact.

Step-by-step: safe homemade kombucha

First fermentation

Ingredients

• 1 L filtered water
• 4 black, green or white tea bags
• 4 heaping tablespoons of sugar (~70 g)
• 1 SCOBY or 100 mL of unpasteurized kombucha
• A glass jar, a clean cloth and a rubber band

How to make it

1. Brew the tea, dissolve the sugar and let it cool to room temperature.

2. Transfer it to a clean glass jar, add the SCOBY or starter liquid, and cover with a cloth secured by a rubber band.

3. Ferment in a dark, airy spot for 7 to 10 days, between 22 °C and 26 °C. Start tasting from day 7 and stop once the flavor is lightly tart and balanced.

Second fermentation (carbonation and flavor)

Ingredients

• A clean, food-grade PET bottle, or a glass bottle rated for carbonated drinks
• Natural juice, fresh fruit or herbs (ginger, mint, hibiscus)
• Kombucha from the first fermentation

How to make it

1. Put up to 30% natural juice, fresh fruit or herbs into the bottle and top up with about 70% strained kombucha from the first fermentation. This ratio gives flavor and substrate for carbonation without excess sugar.

2. Leave free space in the neck for the gas to build up. In a PET bottle, this space also helps you monitor pressure safely.

3. Seal well and let it ferment for 2 to 5 days at room temperature, adjusting for climate, the fruit’s sugar content and bottle pressure. If using whole fruit, remove it after 24 h and let fermentation continue with just the liquid.

4. Open carefully over the sink to release the pressure.

5. Refrigerate once it’s carbonated. The fridge slows fermentation, stabilizes the flavor and helps control pressure.

6. Keep it refrigerated at all times and drink it preferably within 30 days while the bottle stays sealed. Once opened, drink within a few days and discard if you notice mold, an unpleasant smell or a very off flavor.

Safety tip:

• PET is the safest option for tracking pressure.
• Glass: only a bottle rated for fermented drinks.

Who can drink it, and who should be cautious or avoid it

Clinical evidence and microbiota modulation

Costa et al. (2025) evaluated daily consumption of black tea kombucha for eight weeks in adults with and without obesity:

• Increase in commensal bacteria (Bacteroidota, Akkermanciaceae)
• Rise in Subdoligranulum, a butyrate producer
• Reduction in genera associated with obesity (Ruminococcus, Dorea)
• Greater intestinal fungal diversity
• A more pronounced effect in the group with obesity

Fraiz et al. (2024) compared a hypocaloric diet alone to a diet with 200 mL/day of green tea kombucha for ten weeks in overweight adults:

• Attenuated rise in IL-6 (a marker of low-grade systemic inflammation)
• Reduction in the lipid accumulation product (LAP), a marker of visceral adiposity
• Greater salivary microbiota diversity

Isakov et al. (2023) evaluated 220 mL/day of pasteurized kombucha enriched with inulin and B-complex vitamins for ten days in women with constipation-predominant irritable bowel syndrome (IBS-C):

• Increase in bowel movement frequency (from 0.60 to 0.85 times/day)
• Improved stool consistency on the Bristol scale
• Reduced sensation of incomplete evacuation
• Good palatability as a vehicle for functional nutrition in IBS-C

In summary, kombucha modulates the gut microbiota, reduces inflammatory markers, and acts as a palatable adjunct in digestive conditions like IBS-C.

Bioactive compounds and potential mechanisms

During fermentation, kombucha undergoes metabolic transformations that build a complex matrix of bioactive compounds. Costa et al. (2025) identified 145 phenolic compounds in black tea kombucha, with 81% flavonoids and 19% phenolic acids, including quercetin 3-O-rutinoside, catechin 5-O-gallate, EGCG and 5-O-galloylquinic acid.

• Tea-derived polyphenols, especially flavonoids: catechins, theaflavins and thearubigins from the base tea. Fermentation releases bound polyphenols through enzymatic action (beta-glucosidases from yeast and bacteria), increasing bioavailability and antioxidant activity.
• Organic acids (acetic, gluconic and glucuronic): produced by the SCOBY during fermentation. They lower pH (microbiological stability), and modulate gut motility and the epithelial barrier. Glucuronic acid takes part in liver detoxification pathways through conjugation.
• B-complex vitamins and vitamin C: the SCOBY produces B1, B2, B6, B12, niacin and vitamin C during fermentation, adding to the base tea’s nutritional value.
• Microbial metabolites: bioactive peptides, exopolysaccharides and small amounts of short-chain fatty acids generated by microbial activity. They support the gut barrier and modulate the immune response.

Polyphenols as a prebiotic substrate

One notable contribution of kombucha is its supply of polyphenols that work as a substrate for the colonic microbiota. Hutkins et al. (2025), in an expert recommendation published in Nature Reviews Gastroenterology & Hepatology, included polyphenols among compounds with prebiotic potential - substances selectively used by the host’s microorganisms that confer a health benefit.

The mechanism: glycosylated flavonoids and polymeric polyphenol molecules cross the digestive tract without being absorbed and reach the large intestine, where microbial enzymes (glucuronidases, sulfatases) hydrolyze them and release bioactive metabolites. This process selectively feeds certain gut bacteria, contributing to the effects observed in clinical trials with kombucha (enrichment of Bifidobacterium, Akkermansia and butyrate producers).

In the clinical studies reviewed, this prebiotic effect of kombucha’s polyphenols translates into: an increase in commensal bacteria that produce short-chain fatty acids, a reduction in genera associated with obesity, and enrichment of Ellagibacter isourolithinifaciens, a species involved in the metabolism of dietary polyphenols (Ecklu-Mensah et al., 2024).

Mechanistically, these components are associated with:

• Reduced oxidative stress
• Modulation of inflammatory pathways
• Prebiotic interaction with the gut microbiota via polyphenols and organic acids
• Production of bioactive metabolites, such as short-chain fatty acids

Conclusion

Kombucha shows clinical evidence for modulating the gut microbiota, with particularly notable results in people with excess weight or metabolic disturbance. It’s a meaningful source of polyphenols with documented prebiotic function - compounds that selectively feed beneficial gut bacteria, contributing to short-chain fatty acid production and a reduction in genera associated with obesity. Organic acids from fermentation add to this bioactive profile, with an impact on metabolic and inflammatory health. It works as a direct alternative to replace ultra-processed drinks - like soda and packaged juices - within a diet based on real food.

There is extensive scientific literature and warnings from international bodies linking sugary drink consumption to obesity, type 2 diabetes and cardiovascular disease.

• World Health Organization (WHO): Guideline: Sugars intake for adults and children
• Harvard T.H. Chan School of Public Health: Sugary Drinks
• Centers for Disease Control and Prevention (CDC): Sugar-Sweetened Beverages