The answer
Kefir is milk fermented with kefir grains: small clumps of bacteria and yeasts held together by protein and a gel the microbes make.1 The Codex food standard for kefir sets at least 10 million live starter microbes and 10,000 yeasts per gram.2 It carries live microbes but is not a probiotic in the strict sense: ISAPP says few microbes in fermented foods have been named and tested for health benefits.3
In short
- Kefir grains are not grains: they are clusters of bacteria and yeasts, named for how they look.4UF/IFAS Extension
- The Codex standard names the kefir starter as Lactobacillus kefiri with Leuconostoc, Lactococcus and Acetobacter, plus yeasts that do and do not ferment lactose.2Codex Alimentarius
- Water kefir is a separate drink: its own grains ferment sugar water, usually with dried fruit, and milk kefir grains cannot make it.5,6Lynch et al., 2021; CSU Extension
- Trials in healthy adults gave 150 to 250 mL of kefir a day for 2 to 6 weeks.7,8,9Three trials, 2025 and 2026
- Kefir keeps most of milk's lactose: USDA records list about 3.7 g per 100 g for a plain kefir, against 5.2 g for 1% milk.10,11USDA FoodData Central
What is kefir made of?
Milk and kefir grains. The grains are a living starter: lactic acid bacteria and yeasts held in a mass of protein and polysaccharide, much of it a microbe-made sugar chain called kefiran, and after each batch they are strained out and moved to fresh milk. Codex Alimentarius, the international food code, names the starter's core microbes and sets minimum counts that must hold to the product's date of minimum durability.1,6,2
| What it covers | What the Codex standard says for kefir |
|---|---|
| Starter culture | Kefir grains, Lactobacillus kefiri, and species of Leuconostoc, Lactococcus and Acetobacter growing in a strong specific relationship |
| Yeasts in the grains | Lactose-fermenting Kluyveromyces marxianus, and non-lactose-fermenting Saccharomyces unisporus, Saccharomyces cerevisiae and Saccharomyces exiguus |
| Live starter microbes | At least 10 million CFU per gram, in total |
| Yeasts | At least 10,000 CFU per gram |
| Milk protein | At least 2.7% |
| Acidity | At least 0.6%, as lactic acid |
| When the counts apply | Up to the date of minimum durability; the live-microbe rule does not apply to a product heat-treated after fermentation |
| Alcohol | No level set for kefir; kumys, a related fermented milk, must have at least 0.5% ethanol |
Source 2, linked below: the Codex standard for fermented milks, adopted 2003 and revised in 2008 and 2010. CFU means colony-forming units, a count of live microbes.
Most store kefir is not made with grains. A 2016 review says industrial-scale production rarely uses kefir grains, and instead uses starter cultures of microbes isolated from kefir or its grains, to make the product more consistent.1 At home, CSU Extension's method is 2 tablespoons of grains in 2 cups of pasteurized milk, left at 65 to 85 °F for 18 to 24 hours.6 Its starter is what sets kefir apart from yogurt, which is made with two bacteria where kefir grains carry yeasts too; the yogurt side, culture by culture, is in is yogurt a probiotic?4
What is the difference between milk kefir and water kefir?
The grains and the liquid. Water kefir is a sparkling, slightly acidic drink made by fermenting sugar water, to which dried fruit has been added, with water kefir grains of bacteria and yeast in a polysaccharide matrix, a 2021 review says. CSU Extension says milk kefir grains cannot be used to make water kefir, or the other way round, and that water kefir is much lighter and tends to taste sweeter.5,6
Which microbes lead in a water kefir grain depends on where the grains came from and on the sugar and conditions used, the 2021 review adds, and defined starter cultures are not common in water kefir making. Lactic acid bacteria, yeasts and acetic acid bacteria are its main members.5 CSU Extension lists water kefir and non-dairy kefir as ways to have kefir without a lactose digestion challenge.6
Is kefir a probiotic?
Not in the strict sense, though it carries live microbes. A probiotic is a named live strain tested for a benefit; ISAPP says few microorganisms in fermented foods meet that bar because they have not been specifically named and tested, and files them as live dietary microbes instead. A 2016 review of kefir adds that because grains vary so much, health claims have to be checked grain by grain and drink by drink.3,1
That variation is measurable. In the sequencing studies that review gathered, a kefir grain held 24 to 56 types of bacteria and the fermented milk 22 to 61, with one or two genera, Lactobacillus and sometimes Acetobacter, leading each grain.1 How fermented foods and probiotic supplements compare, with Stanford's 10-week fermented food trial, is in fermented foods vs probiotic supplements.
The trial evidence on kefir itself is limited. A 2023 systematic review found 16 randomized trials of milk kefir in people, reported in 18 papers; 12 of the trials had a high risk of bias, and only 5 of the papers assessed safety. Several looked at specific health conditions, which this page does not cover. The authors judged kefir safe for generally healthy people to drink, and asked for better trials before any recommendation.12
How much kefir should you drink a day?
No agency sets an amount. Trials in healthy adults gave 150 to 250 mL a day, about two-thirds of a cup to one cup, for 2 to 6 weeks, and CSU Extension suggests starting with 4 to 6 oz a day and increasing over time if you want. Cleveland Clinic says gas and bloating are common when you first eat foods high in probiotics, and dietitian Amber Sommer advises adding kefir slowly.7,8,9,6,13
| Study | Who took part | Kefir a day, and for how long | What was reported | Source |
|---|---|---|---|---|
| 2025 trial, South Korea | 28 healthy adults aged 18 to 30: 13 drank kefir, 9 unfermented milk and 6 yogurt | 150 mL a day for 2 weeks | More lactate-producing gut bacteria after kefir, including Bifidobacterium breve, and more Blautia | Choi et al. 7 |
| 2026 trial, Sweden | 40 healthy adults | 200 mL of homemade water kefir a day for 14 days | Gut bacteria shifted; 32% reported more gas and 24% bloating, 28% less belly pain, and 66% no change; the authors say causality cannot be established | de Mel et al. 8 |
| 2026 trial, Türkiye | 65 healthy young adults | 250 mL of lactose-free kefir a day for 6 weeks | Gut symptom scores fell; immune and inflammation markers did not change significantly | Bakırhan et al. 9 |
| 2003 study, United States | 15 healthy adults who digest lactose poorly | Single servings holding 20 g of lactose, after a 12-hour fast | Less breath hydrogen than after milk, and gas rated 54% to 71% less severe | Hertzler and Clancy 14 |
Sources 7 to 9 and 14, linked below. All four are small and short, and none sets an amount anyone should drink.
How you drink it matters for the microbes. Cleveland Clinic says heating kefir kills the good bacteria that make it gut-friendly, and lists smoothies among its cold uses.13 On sugar, Sommer's advice is direct: "Pick options that are lower in sugar, or just stick to plain kefir."13
Does kefir have lactose?
Yes, less than milk but far from none. USDA records list about 3.7 g of lactose per 100 g of a plain kefir, against 5.2 g in 1% milk, or roughly 9 g a cup against 12.7 g. Kefir's microbes use up part of the lactose as it ferments, and in a 2003 study, adults who digest lactose poorly handled kefir better than milk.10,11,6,14
| Food | Lactose per 100 g | Galactose per 100 g | Lactose per cup | Source |
|---|---|---|---|---|
| 1% milk | 5.2 g | 0 g | 12.7 g (244 g cup) | USDA FDC 170872 11 |
| Plain kefir | 3.7 g | 0.91 g | 9.0 g (243 g cup) | USDA FDC 170904 10 |
Sources 10 and 11, linked below. Per-cup figures are our arithmetic from each record's per-100 g value and the grams it lists for a cup. Galactose is one of the two sugars lactose splits into.
In that 2003 study at Ohio State University, 15 healthy adults who digest lactose poorly drank milk, kefir or yogurt servings that each held 20 g of lactose. Their breath-hydrogen score, the measure a lactose breath test uses, was 224 after milk and 87 after plain kefir, with flavored kefir in between at 156, and the kefirs and yogurts all cut how severe people rated their gas by 54% to 71% compared with milk.14
Cleveland Clinic's Sommer calls kefir an extremely low-lactose product; the USDA figures above put a plain kefir at about 70% of milk's lactose per gram.13,10 CSU Extension's advice for anyone who has trouble digesting lactose is to talk with a physician before eating dairy foods.6
Who should be careful with kefir?
People with a milk allergy, anyone making it at home from raw milk, people with a weakened immune system, and anyone avoiding alcohol. UF/IFAS Extension says people allergic to cow's milk need to avoid kefir, that kefir should be made with pasteurized milk and refrigerated after fermenting, and that kefir may contain alcohol from fermentation, which would make it unsuitable for children and pregnant women.4
Raw milk is the bigger risk. CDC says products made from raw milk can expose people to germs such as Campylobacter, E. coli, Listeria and Salmonella, and that children under 5, adults over 65, pregnant women and people with weakened immune systems are at higher risk of serious illness from them.15 UF/IFAS adds that fermented kefir with a pH below 4.5 suppresses many pathogens, but E. coli, Listeria and Salmonella can survive.4
For live microbes in general, NCCIH says people with severe illnesses or weakened immune systems face a greater risk of harm from probiotics, which it notes are found in yogurt and other fermented foods.16 On alcohol, the 2016 kefir review describes the drink as having low levels of it.1
Where Physician's Choice fits
Physician's Choice, a maker of gut health supplements, publishes this site. No product is named in this read: kefir is a food, and the sources above studied the drink itself.
The short version
Kefir is milk fermented with grains of bacteria and yeasts, and the Codex standard sets at least 10 million live starter microbes and 10,000 yeasts per gram. It carries live microbes but is not a defined probiotic, and trials in healthy adults used 150 to 250 mL a day. It keeps most of milk's lactose, and homemade kefir should start from pasteurized milk.
Notes on this page
- Where the question came from
- Of 1,145 public Reddit posts we read (250 communities, Sep 12 to Oct 12, 2026), 1 mentioned kefir: a question about taking probiotic pills alongside homemade kefir. It is not quoted here.
- What engines were asked
- What is kefir, on Oct 22, 2026.
- What was left out
- Brand names, products, nutrient claims, and trial results tied to a named condition.
- Sources
- 16 sources, each opened on Oct 22, 2026.
- Products
- None. This site names no product.
- Checks
- Last checked Oct 22, 2026. Each later check is dated here.
Kefir questions
Is kefir good for you?
It is a fermented milk that carries live microbes, and a 2023 systematic review judged it safe for generally healthy people to drink. The evidence for specific benefits is limited: of 16 randomized trials in that review, 12 had a high risk of bias, and the authors asked for better trials before any recommendation.
How much kefir should you drink a day?
No agency sets an amount. Trials in healthy adults used 150 to 250 mL a day, about two-thirds of a cup to one cup, for 2 to 6 weeks. CSU Extension suggests starting with 4 to 6 oz a day and increasing over time if you want, since cramps, nausea or gas can come at first.
Is kefir a probiotic?
Not in the strict sense. Kefir carries live bacteria and yeasts, but ISAPP says few microorganisms in fermented foods have been specifically named and tested for health benefits, the bar for a probiotic. A 2016 review adds that kefir's microbes vary so much from grain to grain that each kefir's claims need checking on their own.
Does kefir have alcohol?
It can. UF/IFAS Extension says kefir may contain alcohol produced during fermentation, which would make it unsuitable for children and pregnant women, and a 2016 review describes low levels of alcohol in the drink. The Codex standard sets no alcohol level for kefir; it sets a 0.5% minimum only for kumys, a related drink.
Does kefir have less lactose than milk?
Yes, but not much less. USDA records list about 3.7 g of lactose per 100 g of a plain kefir, against 5.2 g for 1% milk. In a 2003 study, 15 adults who digest lactose poorly had less breath hydrogen after plain kefir or yogurt than after milk, and rated their gas 54% to 71% less severe after any of the kefirs or yogurts.
More reads
- Is yogurt a probiotic? Live cultures, labels and yogurt types compared
Live cultures in the dairy case.
- Fermented foods vs probiotic supplements: what each one gives your gut
Live cultures in food, and the Stanford trial.
- Do probiotics help with weight loss? What the trials measured
What the trials measured, strain by strain.
- Do probiotics work? A research note on what studies show
What the probiotic trials found.
- How to reset your gut: what studies show about how fast gut bacteria change
How fast gut bacteria shift with food.
- High-fiber foods for gut health: grams of fiber per serving
Fiber, food by food.
- A 30-day gut habits checklist: fiber, fluids, meals and sleep
The daily habits, one source each.
- Gut health supplements doctors recommend: what the guidelines actually say
What guidelines name, and what they do not.
- Signs of an unhealthy gut: what is common, and what needs a doctor
Which signs need a doctor.
- Digestive enzymes vs probiotics: what each can and cannot change
Two different things, side by side.
- Is bone broth good for gut health? What the evidence shows
A popular gut claim, checked.
- What helps bloating fast, naturally? Habits first, then the supplements with evidence
When bloating is the problem today.
Sources
- Bourrie BC, Willing BP, Cotter PD. The microbiota and health promoting characteristics of the fermented beverage kefir. Front Microbiol. 2016;7:647. pmc.ncbi.nlm.nih.gov. Full text opened Oct 22, 2026.
- Codex Alimentarius Commission. Codex standard for fermented milks, CODEX STAN 243-2003. Adopted 2003, revised 2008 and 2010. fao.org. Opened Oct 22, 2026.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Fermented foods. isappscience.org. Opened Oct 22, 2026.
- Skinner JR. Fermented foods: kefir. UF/IFAS Extension publication FSHN24-2. July 2024. ask.ifas.ufl.edu. Opened Oct 22, 2026.
- Lynch KM, Wilkinson S, Daenen L, Arendt EK. An update on water kefir: microbiology, composition and production. Int J Food Microbiol. 2021;345:109128. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- Colorado State University Extension, FoodSmart Colorado. Understanding and making kefir. Reviewed October 2025. extension.colostate.edu. Opened Oct 22, 2026.
- Choi Y, Keum GB, Kang J, et al. Evaluation of kefir consumption on gut microbial diversity in a healthy young population using full-length 16S rRNA sequencing. Front Microbiol. 2025;16:1587831. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- de Mel R, Al Khafaji AH, Muthusamy S, Xu J, Håkansson Å. Changes in gut microbiota composition following water kefir consumption in healthy adults. Sci Rep. 2026;16. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- Bakırhan H, Ozyurek Arpa F, Pehlivan M, Kalkan I. Effects of kefir consumption on gastrointestinal health, biochemical parameters, sleep, and mental well-being in healthy young adults: a randomized controlled trial. J Nutr. 2026;156(1):101247. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- USDA FoodData Central, SR Legacy record 170904: plain kefir. Published Apr 1, 2019. fdc.nal.usda.gov. Opened Oct 22, 2026.
- USDA FoodData Central, SR Legacy record 170872: 1% milk with added vitamins A and D. Published Apr 1, 2019. fdc.nal.usda.gov. Opened Oct 22, 2026.
- Kairey L, Leech B, El-Assaad F, Bugarcic A, Dawson D, Lauche R. The effects of kefir consumption on human health: a systematic review of randomized controlled trials. Nutr Rev. 2023;81(3):267-286. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- Cleveland Clinic. Is kefir good for you? Health Essentials, with registered dietitian Amber Sommer, RD, LD. July 16, 2026. health.clevelandclinic.org. Opened Oct 22, 2026.
- Hertzler SR, Clancy SM. Kefir improves lactose digestion and tolerance in adults with lactose maldigestion. J Am Diet Assoc. 2003;103(5):582-7. pubmed.ncbi.nlm.nih.gov. Abstract read Oct 22, 2026.
- CDC. Raw milk. January 31, 2025. cdc.gov. Opened Oct 22, 2026 in a headless browser.
- NCCIH. Probiotics: usefulness and safety. Last updated August 2019. nccih.nih.gov. Opened Oct 22, 2026.