Is kefir good for you? What the trials show, and what they do not

Two trials sum up where kefir research stands. In the first, 125 children aged one to five took antibiotics and drank either kefir or a heat-killed lookalike. Diarrhea hit 18% of the kefir group and 21.9% of the placebo group, and the authors found no difference between the groups.⁴ In the second, 15 adults who struggle to digest lactose drank kefir, and their breath hydrogen, a rough gauge of undigested lactose reaching the colon, after plain kefir came to roughly 40% of what milk produced.³

So kefir does something measurable, and it is not a cure-all. The rest of this piece sorts out which is which.

What kefir is, and why it is not just thin yogurt

Kefir starts with kefir grains: rubbery clumps that hold a community of bacteria and yeasts. Put them in milk and the bacteria turn lactose into lactic acid, which sours and thickens it, while the yeasts make carbon dioxide and a little alcohol.⁶ That yeast step is the big difference from yogurt. It makes kefir thinner, tangier and sometimes lightly fizzy, and it means the drink carries yeasts as well as bacteria.

Yogurt is fermented by a smaller, more standardized set of bacteria. Whether kefir’s wider cast is actually better for you is not shown by anything above. The trials covered here never set the two against each other on gut health.

What the trials found

The Stanford trial. In a 2021 study in Cell, 36 healthy adults (18 per group) were randomly given either a high-fiber diet or a high-fermented-food diet. The fermented group went from 0.4 servings a day to an average of 6.3 by the end of the 10-week diet phase of a 17-week protocol. The paper sorted the fermented foods into eight categories: yogurt, kefir, fermented cottage cheese, fermented vegetables, vegetable brine drinks, kombucha, other fermented non-alcoholic drinks and other foods. Their microbiome diversity rose steadily and a set of inflammatory markers fell.¹ The catch for anyone searching for kefir benefits: kefir was one category of eight, so the result belongs to the whole pattern, not to kefir. The authors even noted that servings of yogurt and vegetable brine drinks tracked diversity most strongly, not kefir.

The review of kefir trials. A 2023 systematic review in Nutrition Reviews pulled together 18 publications covering 16 randomized studies. Its authors saw possible promise for lowering mouth bacteria linked to cavities and for helping H. pylori treatment, and called the evidence for cholesterol and blood pressure very limited. Safety was assessed in only 5 of the 18 publications, and 12 of the studies had an overall high risk of bias. The authors’ own conclusion: kefir appears safe for generally healthy people, and high-quality trials are needed before any recommendation for conditions.²

Antibiotics. The children’s trial above is the cleanest test of the popular idea that kefir protects the gut during antibiotics. In that one, it did not.⁴

None of this means kefir is useless. It means the claims people attach to it, from immunity to weight, run ahead of anything these trials can back.

Lactose: where kefir has its best case

Hertzler and Clancy gave 15 adults with lactose maldigestion a series of test meals, each with 20 g of lactose: milk, plain and raspberry kefir, plain and raspberry yogurt. Breath hydrogen over the next eight hours averaged 224 ppm × h after milk, 87 after plain kefir and 76 after plain yogurt. The flavored kefir landed in between at 156. Perceived flatulence fell by 54% to 71% with the yogurts and kefirs compared with milk.³

Kefir clearly beat milk, and it did not beat yogurt. Also, 15 people is a small group. If dairy gives you trouble, a small glass of kefir is a reasonable thing to try, with the understanding that some lactose is still in there.⁶

Live or not: the label decides

Kefir’s whole pitch rests on living microbes, and heat kills them. A kefir that was pasteurized after fermenting, or a long-life product from the shelf, can still taste right and still be fine to drink, but it is no longer a live food. Look for kefir in the chilled aisle, a short ingredient list (milk and cultures, nothing else) and wording like “live cultures”. It should smell fresh and sour, a little yeasty, never rancid, and a slight fizz is normal.⁶ In the fridge it keeps fermenting slowly, getting more sour and slightly more alcoholic over time.⁶

Water kefir is a different drink

Water kefir grains ferment a sugar solution, often with dried fruit, instead of milk. The microbes are a different mix of lactic acid bacteria, acetic acid bacteria and yeasts, and a 2024 review notes the result is chemically quite different from milk kefir.⁵ The same review reports a final pH of roughly 3.5 to 4.5 and alcohol content usually below 1%, though in one comparison with the same grains, water kefir reached up to 2.14%.⁵

It is a nice dairy-free ferment. As a health food, the case is mostly potential: the review lists possible immune and anti-inflammatory properties without pointing to human trials of water kefir. Treat it as a tasty sparkling drink with some live cultures, not as a proven gut remedy, and keep in mind that it is made from sugar.

Making it at home

If you ferment your own, a German consumer-health source gives the basics: milk kefir ferments for roughly 24 to 48 hours at room temperature, shorter for a milder taste, longer for a stronger one with more alcohol.⁶ Use clean jars, strain out the grains, and refrigerate the finished drink. If it smells rotten rather than pleasantly sour, throw it out.

Where it fits in your week

Kefir’s job is narrow: it is a cheap, easy source of live microbes you can pour over oats or blend with berries. What it does not do is feed the microbes. In the Stanford trial, the fiber group and the fermented-food group moved the microbiome in different ways, which is a decent argument for doing both. A glass of kefir over oats and blueberries is a tidy way to do both at breakfast.