Kefir is a fermented milk drink with a tart, slightly fizzy taste, originally from the Caucasus region. Unlike yogurt, which relies on just a handful of bacterial strains, kefir is made using “kefir grains,” small clumps of bacteria and yeast that can contain up to 50 different microbial species working together. The result is one of the most probiotic-dense foods available, with a single cup delivering 25 to 30 billion colony-forming units of live cultures.
How Kefir Is Made
Kefir grains look like small, irregular cauliflower florets, typically 2 to 9 millimeters across. They aren’t actual grains like wheat or rice. They’re rubbery clusters of bacteria, yeast, and proteins held together by a unique carbohydrate called kefiran. This matrix forms a polyhedral network of hollow globular structures, essentially a living scaffold where dozens of microbial species coexist.
To make kefir, you simply drop the grains into milk and leave them at room temperature for 24 to 48 hours. During that time, the microbes consume lactose (milk sugar) and produce lactic acid, acetic acid, and carbon dioxide. The lactic acid gives kefir its tangy flavor. The carbon dioxide creates a light effervescence. When fermentation is done, you strain out the grains and use them again for the next batch. The grains grow slowly over time, so people often share extras.
What Lives Inside Kefir Grains
The microbial community in kefir is remarkably diverse. Lactic acid bacteria dominate, but the grains also harbor acetic acid bacteria and several yeast species. Two bacterial species are found in virtually every kefir grain studied: one that primarily produces lactic acid and another that generates lactic acid, acetic acid, and carbon dioxide together. The most common yeast is Saccharomyces cerevisiae, the same species used in bread and beer, alongside other fermentation yeasts.
This diversity is what sets kefir apart from yogurt. Yogurt is typically fermented by two to six bacterial strains and contains no yeast. Kefir can harbor as many as 50 strains, and a cup of kefir generally contains two to three times more live cultures than the same amount of yogurt. The yeast component matters too, because yeasts can survive stomach acid better than many bacteria, potentially delivering more live organisms to the gut.
Gut Health and Digestion
Kefir’s best-supported benefit is for people who struggle with lactose. During fermentation, the microbes in kefir grains produce an enzyme that breaks down lactose. A study directly measuring lactose absorption found that kefir made with active grains improved lactose digestion by about 30% compared to kefir whose grains had been heat-treated (killing the microbes). This means even people with lactose intolerance often tolerate kefir better than regular milk, because the bacteria have already done much of the digestive work for them.
In people with inflammatory bowel disease, kefir consumption reduced blood markers of inflammation and improved quality-of-life scores, alongside shifts in gut microbial composition. Another study found that kefir lowered levels of zonulin, a protein that signals increased intestinal permeability (sometimes called “leaky gut”), though broader inflammatory markers didn’t change in that trial. The overall picture suggests kefir supports gut barrier function, but the effects can vary from person to person.
Immune and Metabolic Effects
Animal and lab studies paint a broad picture of kefir’s potential. Kefir and its components have been shown to fight pathogens, reduce the production of inflammatory signaling molecules, lower blood sugar and cholesterol markers, and even slow tumor growth in cell lines. In mice infected with a common intestinal parasite, seven days of kefir reduced parasite counts and boosted levels of a protective antibody in the gut lining.
There’s also evidence that kefir can dial down allergic immune responses. Mice given a specific kefir-derived bacterium for about a month showed lower levels of IgE, the antibody most associated with allergic reactions, while immune cells shifted toward a pattern linked to balanced, rather than overreactive, immunity. These effects appeared to work both systemically and at specific sites like the airways.
It’s worth noting that most of this evidence comes from animal models and lab experiments. Human clinical trials are still limited, and the strength of effect in people likely depends on which microbial strains are present in the specific kefir you drink.
Kefiran: The Unique Carbohydrate
The matrix holding kefir grains together is made largely of kefiran, a polysaccharide not found in yogurt or other common fermented foods. When tested against several bacterial species and the yeast Candida albicans, both kefir and isolated kefiran showed antimicrobial activity. The strongest effect was against Streptococcus pyogenes, a bacterium responsible for strep throat and skin infections. Kefiran also showed wound-healing properties in the same research, which suggests it plays a role beyond just structural scaffolding.
Water Kefir vs. Milk Kefir
Milk kefir is the traditional version, but water kefir exists too. Water kefir grains look different (translucent, crystal-like) and ferment sugar water, coconut water, or fruit juice instead of milk. The two types differ significantly in their microbial composition, chemical properties, and nutritional content.
Milk kefir grains are more nutritionally dense overall, delivering protein, calcium, and B vitamins from the milk base. Water kefir grains, however, actually harbor a wider variety of microbial species. Water kefir is a practical option for vegans or anyone with a dairy allergy, and it still provides a meaningful dose of lactic acid bacteria and beneficial yeasts. The flavor is lighter, more like a mildly tart soda than a creamy drink.
Possible Side Effects
Most people tolerate kefir well, but there are a few things to watch for. Bloating and gas are common when you first start drinking it, especially if you’re not used to fermented foods. Starting with a small amount (half a cup or so) and increasing gradually gives your gut time to adjust.
A more specific concern involves histamine. Like all fermented foods, kefir contains biogenic amines, including histamine and tyramine, produced naturally during fermentation. Most people break these down without issue using enzymes in the digestive tract. But for the estimated 1 to 3% of the population with histamine intolerance, kefir can trigger headaches, skin flushing, hives, a runny nose, or digestive upset. One study found that following a low-histamine diet reduced headaches in 75% of participants with this sensitivity.
People with severely compromised immune systems should be cautious with any probiotic-rich food. In rare cases, live probiotic organisms have caused serious infections, including sepsis and liver abscesses, in immunocompromised individuals. This risk is very low for healthy people, but it’s real for those on immunosuppressive medications or with conditions like uncontrolled diabetes.
How to Choose and Use Kefir
Store-bought kefir is widely available in plain and flavored varieties. Plain is the better choice nutritionally, since flavored versions often contain added sugar that can rival a soft drink. Check the label for live, active cultures. Some commercial kefirs are pasteurized after fermentation, which kills the beneficial microbes and defeats much of the purpose.
Homemade kefir is straightforward if you can get your hands on kefir grains, which are sold online or shared within fermentation communities. You add grains to milk, wait 24 hours at room temperature, strain, and drink. The grains last indefinitely with regular feeding. Homemade kefir tends to be more potent than commercial versions because the microbial community is more diverse and hasn’t been standardized for shelf stability.
You can drink kefir on its own, blend it into smoothies, pour it over granola, or use it as a buttermilk substitute in baking (though heat destroys the live cultures). A cup a day is a common amount in research studies, and that’s a reasonable starting point for most people.