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Traditionally, kefir is made by fermenting mammalian milk (usually cow, sheep, goat, or camel) at room temperature with a symbiotic culture of bacteria and yeast (SCOBY). The kefir SCOBY forms “kefir grains,” which are small yellow- to white-colored clumps that look a bit like cauliflower florets. These grains are made of bacteria and yeast in a matrix of proteins and polysaccharides. During fermentation, the bacteria break much of the lactose down into lactic acid, giving kefir its sour taste; later in the fermentation, the slow-acting yeasts ferment some of the remaining lactose into ethanol and carbon dioxide, resulting in a carbonated, slightly-alcoholic beverage.
Commercial kefir is often prepared not from kefir grains, but from purified cultures of bacteria and yeasts derived from those grains; this is sometimes referred to as "Russian kefir". Some commercial kefirs are fermented only with a handful of bacterial cultures, omitting the yeasts, and aren’t carbonated.
There is no generally accepted standard culture for kefir; the term refers to a wide range of active-culture lactic-acid-bacteria fermented milks.
Kefir’s amino acid and mineral profiles are similar to the milk it was fermented from (the fermentation substrate). The finished kefir is 30% lower in lactose than the substrate, and higher in ammonia, serine, lysine, alanine, threonine, tryptophan, valine, lysine, methionine, phenylalanine and isoleucine, than the substrate. Its vitamin profile depends both on the fermentation substrate and on the specific cultures used; one study found that kefir fermentation increased the levels of pyridoxine, vitamin B12, folic acid, biotin, thiamin and riboflavin relative to the substrate. Kefir’s lipid profile also depends on the fermentation substrate. Finally, the finished kefir contains complex polysaccharides, notably #kefiran, which give kefir its buttermilk-like consistency.
Kefir has a low alcohol content of 0.08% - 2%. The exact value depends on the bacteria and yeasts used and the fermentation process. Commercially made kefir, made with modern manufacturing processes, has an extremely low ethanol content of 0.01% - 0.1%.
Kefir grains are a complex mix of lactic-acid bacteria, acetic-acid-producing bacteria, lactose-fermenting yeasts, and non-lactose-fermenting yeasts. Kefir can also be prepared with a starter culture of bacteria and yeasts isolated from previous batches of kefir; this method is used for most commercially prepared kefir. Kefir fermented with such a starter culture contains a lower number and variety of bacteria and yeasts than kefir fermented by kefir grains.
There are estimated to be more than 300 different microbial species that may be present in any given kefir culture. That said, the most commonly found bacteria are Lactobacillus kefiranofaciens, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, Lactobacillus acidophilus, and Lactobacillus delbrueckii subsp. bulgaricus. Lactococcus and acetic acid bacteria are also present in smaller numbers. The most commonly found yeasts are Saccharomyces cerevisiae, Saccharomyces unisporus, and the lactose-fermenting yeast Candida kefyr/Kluyveromyces marxianus.
Kefiran is the bacterially produced polysaccharide that coats the outside of kefir granules and gives kefir its creamy mouthfeel. It’s produced by the bacterium L. kefiranofaciens. Kefiran is available as a standalone product (not to be confused with KEFIRAN™, a commercial probiotic encapsulated along with 20mg of kefiran).
In animal and in vitro studies, kefiran has been found to have antitumor, antimicrobial, immunomodulatory, healing, anti-inflammatory, and antioxidant properties. However, clinical trials are needed to confirm any benefits in humans.
Finished kefir contains many fermentation metabolites in addition to kefiran, including amino acids, fatty acids (CLA), bacteriocins, organic acids, other polysaccharides, and bioactive peptides.
Kefir has been made from many mammal milks (cow, goat, sheep, yak, bison, camel…), and several milk substitutes including soy milk. The studies referenced in this article all studied kefir made with mammalian milk.
Milk kefir grains will ferment other sugary liquids, such as milk substitutes, fruit juice, or even sugared water. A kefir-like drink, called “water kefir” or tibicos, can also be made using sugared water and transparent “water kefir grains,” which are a SCOBY related to the kefir SCOBY. However, this article is focused on milk kefir, made with mammalian milk. For a narrative review of nondairy kefir products, see this paper.
To date, there haven’t been any human clinical trials of kefir as a cancer treatment. A 2015 review of the literature found 7 in vitro studies of the effects of finished kefir on various human cancer cell lines. These consistently showed positive effects (antiproliferative, cancer-cell-killing, or reduction of DNA damage). The review found only 4 relevant in vivo studies, none of them recent, all conducted in mice. These four studies found reduced tumor growth or decrease in tumor size. These results are promising, but keep in mind that none of these studies are human clinical trials.
There have been two small human trials of kefir during 5-FU chemotherapy for colorectal cancer. One study found that kefir made almost all of the chemo’s side effects notably worse (headache, nausea, vomiting, bloating, constipation, dry mouth, drowsiness, hair loss, weight loss, sweating, poor appetite, oral mucositis), with the exception of difficulty sleeping, which was less for those taking kefir. The other study, which focused specifically on the side effect of oral mucositis, found that kefir didn't make a significant difference in the incidence of oral mucositis.
Kefir increases fecal microbial load, but the evidence is mixed on whether drinking kefir increases the proportion of probiotic bacteria in the stool; it may not have a greater effect on the proportion of probiotic baceria than drinking unfermented milk. One small study found a significant difference between two different kefirs.
Three small trials have examined the effects of drinking kefir on constipation. Colorectal cancer patients saw an increase in the side effect of constipation, and children taking antibiotics saw no change; but a small, uncontrolled study of constipated patients saw a decrease in many of the symptoms of constipation (stool frequency, "bowel satisfaction", improved stool consistency) but not in self-reported scores of straining.
Kefir didn't reduce the incidence of diarrhea or loose stools in children taking antibiotics for uper respiratory infections, but it did decrease the incidence of diarrhea in people taking medicine for stomach ulcers (a proton pump inhibitor (PPI) and two antibiotics).
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Streptococcus mutans lives in the mouth and starts cavity development. In three studies, regular kefir consumption was associated with a decrease in S. mutans. Once cavities have started, they're deepened by lactobacilli; even though there are lactobacilli in kefir, the two studies that measured salivary lactobacilli didn't find that kefir consumption increased salivary lactobacilli, and one even found that it decreased. Kefir didn't have any effect on salivary pH or on saliva buffering capacity.
Based on these three small studies, drinking kefir regularly may benefit your oral health. However, if you're undergoing colorectal cancer chemotherapy, there's mixed evidence that kefir may worsen one of its more unpleasant side effects, oral mucositis (painful mouth sores).
There's been one human clinical trial focused on kefir and weight loss. In that small study, overweight and obese women who drank kefir lost more weight than control (2.4 kg vs. 1 kg), but didn't lose significantly more than those who drank unfermented low-fat milk (2.1 kg). The study found similar results for waist circumference, which decreased by 2.1 cm for those who drank kefir, 2.0 cm for milk, and .9 cm for control. However, other small studies which happened to track waist circumference and weight didn't find any effect.
Several studies whose main outcome wasn't weight loss did track related anthropometric measurements. In the two studies that measured BMI, one found no change; the other found a decrease, but it wasn't significant compared to unfermented milk. None of the four studies that measured body fat found any effect. Neither did the study that measured body density, nor did the one study that measured fat-free mass, nor the one that measured hip circumference,, nor the one that measured lean mass, nor the one that measured muscle mass.
People who want to lose weight often look to reduce appetite. There's no evidence that kefir will have any effect for healthy people here. Chemotherapy patients who took kefir were more likely to have reduced appetite, but a study of healthy overweight people found no effect on appetite.
To sum all this up, drinking kefir isn't likely to help with weight loss or body composition. Increasing low-fat dairy, if you're able to keep total calories the same, may lead to a small (~1 kg) loss over several months, but the dairy doesn't have to be kefir.
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Five clinical trials of kefir measured C-Reactive protein, an inflammatory biomarker for heart disease and arterial plaque, or high-sensitivity C-reactive protein (hs-CRP), and didn't find a significant difference from control.
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Several human clinical trials have measured kefir's effect on serum lipids, whether as primary, secondary or experimental endpoints. While these are small trials, the preponderance of the evidence suggests that kefir has little effect on the lipid profile, with the exception of non-HDL cholesterol (total cholesterol minus HDL cholesterol).
The evidence is mixed but leans toward no effect. 2 human clinical trials found LDL decreased in kefir group, but the decrease in one of these trials was similar to that seen with milk.  However, 6 studies found no effect.
The evidence is mixed but leans toward no effect. Six studies found no effect on HDL. One study saw an increase, but only in female particpants. And one study saw a decrease, but saw the same decrease for the milk group.
Kefir may have a lowering effect on non-HDL cholesterol, based on two studies that measured non-HDL cholesterol as a secondary/experimental endpoint. Both studies found a decrease in the kefir group, but evidence is mixed on whether the effect is kefir-specific or just due to the dairy content: One study saw the same reduction in non-HDL cholesterol in its milk group as in its kefir group, but the other saw a reduction in the kefir group and not in its yogurt group.
This review didn't find any clinical trials designed to study kefir's effect on the kidneys. Creatinine, a metabolic byproduct of creatine, is sometimes used as a biomarker for kidney damage. Creatinine levels weren't a primary outcome for any study, but the 4 clinical trials of kefir which measured creatinine found no effect.
This review didn't find any clinical trials designed to study kefir's effect on the liver. However, several clinical trials of kefir measured liver enzymes and found no significant effect, with the exception of a single trial that found that ALT levels decreased by 13 U/L in kefir+exercise group.
A few human trials have looked at how kefir performs as an add-on to established clinical therapies. It performed well for H. pylori infection, reducing side effects and increasing cure rate, but is not recommended for patients undergoing chemotherapy.
In a study of 80 patients undergoing "triple therapy" drug treatment for H. pylori infection (i. e. a proton pump inhibitor (PPI) plus two antibiotics), the patients given kefir had a significantly higher H. pylori eradication rate (78% vs 50%) 45 days after starting treatment, and reported significantly less occurrence of the adverse effects diarrhea, headache, nausea, and abdominal pain at follow-up.
There have been two small human trials of kefir during 5-FU chemotherapy for colorectal cancer. One study found that kefir made the chemo’s side effects notably worse. The other study, which focused specifically on the side effect of oral mucositis, found that kefir didn't make a significant difference in the incidence of oral mucositis.