Microbiome science

Kefir

Also called: milk kefir, water kefir, fermented milk

Kefir is a fermented milk drink originally from the Caucasus region. It is made by adding kefir grains (a SCOBY-like cluster of bacteria and yeast) to milk and letting it ferment for 24 hours. The result is a tart, slightly fizzy drink with up to 30 different bacterial and yeast strains, far more diverse than typical yoghurt. Daily kefir is one of the easiest ways to add microbial diversity to your gut.

What's in kefir

Kefir grains are a symbiotic culture of bacteria and yeast (SCOBY) held together by polysaccharides. They contain Lactobacillus, Lactococcus, Streptococcus, Acetobacter, and Kluyveromyces yeast among others. When added to milk, they ferment lactose into lactic acid, produce small amounts of CO2 (the slight fizz) and ethanol (typically under 1 percent), and create bioactive peptides that may have additional benefits.

Kefir vs yoghurt

  • Strain diversity: kefir typically has 20-30+ strains, yoghurt has 2-7.
  • Fermentation: kefir ferments at room temperature with grains; yoghurt at 40-45°C with starter cultures.
  • Texture: kefir is drinkable (yoghurt is spooned).
  • Lactose: kefir has less than yoghurt because more is fermented away. Many lactose-intolerant people tolerate it.
  • Yeast: kefir contains beneficial yeasts (Saccharomyces, Kluyveromyces); yoghurt does not.

What it can do

  • Increase gut bacterial diversity in 4-12 week trials.
  • Modestly improve lactose tolerance because of in-vitro lactase activity from kefir microbes.
  • Reduce some markers of inflammation in adults with metabolic syndrome.
  • Mild blood pressure-lowering effect in some studies, possibly via bioactive peptides.

How to use it

  1. Start with 100-150 ml a day. Build up as tolerated. Higher doses can cause initial bloating as your gut adjusts.
  2. Buy plain, unsweetened versions. Flavoured kefirs often have as much sugar as ice cream and undo most of the gut benefit.
  3. Look for 'live cultures' on the label, not pasteurised after fermentation. Many supermarket kefirs are pasteurised, killing the bacteria.
  4. Brew your own with kefir grains for the highest live-strain count and lowest cost. About 5 minutes a day, 24-hour fermentation.
  5. Use it like yoghurt: in smoothies, salad dressings, on cereal, as a marinade tenderiser.

Water kefir

Water kefir uses different grains and ferments sugar water (often with fruit or coconut water) instead of milk. It's dairy-free and lower in protein but still produces fermentation byproducts and live microbes. Less studied than milk kefir. Useful for people who can't or don't want dairy, but milk kefir has the stronger evidence base.

Common questions

Is kefir alcoholic?
Trace amounts. Most kefir is under 1 percent alcohol, less than the residual alcohol in ripe fruit or some breads. Long fermentation can push it slightly higher. Not enough to affect adults, but pregnant women and people avoiding alcohol entirely sometimes prefer kefir that has been fermented for shorter periods.
Can kefir replace a probiotic supplement?
For general gut diversity, often yes. For specific clinical effects (treating C. difficile, infant colic, IBS bloating with B. infantis 35624), the strain-specific dose in kefir is too unreliable. Use kefir for maintenance, supplements for targeted situations.
Why does my kefir give me gas the first week?
Adjustment. Your existing bacteria are reacting to the new arrivals and the fermentable lactose. Most people stabilise within 7 to 10 days. Start with a small daily amount and build up rather than going from zero to 500 ml on day one.
Is supermarket kefir as good as homemade?
Less diverse but still useful. Commercial kefir typically uses defined starter cultures (often 6-10 strains) for consistency. Real grain-fermented kefir has 20-30+ strains. Both improve on no kefir; the gap matters more for people with specific gut issues than for general use.

Sources