Microbiome science

Kombucha

Also called: fermented tea, SCOBY tea

Kombucha is fermented sweetened tea, made by adding a SCOBY (symbiotic culture of bacteria and yeast) to brewed tea with sugar. The microbes ferment most of the sugar into acetic acid, producing a tart, slightly fizzy drink with live bacteria, organic acids, and trace alcohol. It's a useful low-effort way to add some microbial diversity but the evidence base for specific health benefits is weaker than for kefir.

How it's made

Black or green tea is brewed strong, sweetened with sugar, cooled, and inoculated with a SCOBY (a thick floating mat of bacteria and yeast). Fermentation runs 7 to 14 days at room temperature. The yeast converts sugar into ethanol; the bacteria convert ethanol into acetic acid (the same vinegar acid). Most of the sugar is consumed; what's left is the residual sweetness.

What's in finished kombucha

  • Live bacteria (Acetobacter, Gluconacetobacter, Lactobacillus) and yeast (Brettanomyces, Saccharomyces).
  • Organic acids, dominated by acetic acid, plus gluconic acid, glucuronic acid and lactic acid.
  • Polyphenols from the tea, modified by fermentation.
  • Trace ethanol: under 0.5 percent in commercial bottles, sometimes higher in homebrew.
  • Residual sugar: 2-8 g per 250 ml depending on brand and fermentation length.
  • B vitamins produced by yeast.

What it can do

  • Adds microbial diversity (smaller effect than kefir, larger than nothing).
  • Acetic acid may modestly slow gastric emptying, improving post-meal glucose response.
  • Glucuronic acid plays a role in the liver's detoxification pathway, but the human relevance is unproven and the mechanisms need further verification.
  • Polyphenols from green tea kombucha may have antioxidant effects.
  • Honest answer: most claims (cancer prevention, immunity, weight loss) are extrapolated from cell or animal studies, not human trials.

Watch the sugar

  • Some commercial kombucha brands have 8-12 g of added sugar per bottle. That's not just residual fermentation sugar.
  • Aim for under 4 g of sugar per 250 ml.
  • Flavoured kombuchas often have post-fermentation fruit juice added.
  • The tarter the kombucha, the lower the residual sugar.
  • If a brand markets itself as 'sweet' or 'mild', sugar is usually higher.

Kombucha vs kefir

  • Strain diversity: both are complex cultures of bacteria and yeasts; reported genus counts vary widely between studies for each drink.
  • Evidence base: kefir has more human metabolic trials, though human evidence for both drinks remains limited.
  • Sugar: kefir negligible, kombucha 2-8 g per serving.
  • Lactose: kombucha is naturally dairy-free.
  • Acidity: kombucha is much tarter from the acetic acid.
  • Practical use: kefir for daily microbiome support, kombucha for an alternative to soda.

Common questions

Is kombucha alcoholic?
Trace amounts. Commercial kombucha is required to be under 0.5 percent alcohol in most markets. Homebrew can drift higher, especially in warm weather or long fermentation. Pregnant women and people avoiding alcohol entirely should check labels carefully.
Can I drink kombucha every day?
Most healthy adults can. Start with 100-150 ml a day to gauge tolerance. Higher doses can cause acidity issues, dental erosion (rinse with water after) and rare cases of GI upset. Watch sugar content if drinking multiple bottles.
Will kombucha give me a stomach ache?
Sometimes, especially the first few days as your gut adjusts to the live cultures and acidity. Mild bloating and gas is common and usually settles in a week. If pain persists or is severe, stop and try a smaller dose or kefir instead.
Should I make my own?
Optional. Homebrew is much cheaper but requires care: clean equipment, monitored fermentation time, and pH testing if you brew large batches. Commercial kombucha gives most of the benefits with less risk and less effort. The main downside of commercial is cost and added sugar.

Sources

The Book of Suna is general information, not medical advice. It is not a substitute for talking to a qualified healthcare professional about your own situation.