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Kefir

Kefir is a probiotic fermented milk drink made with a symbiotic culture of bacteria and yeasts. It may benefit oral health, may be a useful adjunct to stomach ulcer treatment, and may offer slight benefits to blood sugar. On the other hand, drinking kefir makes chemo side effects worse for some cancer patients. Since it contains live, active bacteria and yeasts, kefir should be used with caution by anyone with a compromised or weakened immune system.

Our evidence-based analysis on kefir features 40 unique references to scientific papers.

Research analysis led by and reviewed by the Examine team.
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Research Breakdown on Kefir


1Sources and Composition

1.1Sources

Traditionally, kefir is made by fermenting mammalian milk (usually cow, sheep, goat, or camel[2]) 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".[14] Some commercial kefirs are fermented only with a handful of bacterial cultures, omitting the yeasts, and aren’t carbonated.[1]

There is no generally accepted standard culture for kefir; the term refers to a wide range of active-culture lactic-acid-bacteria fermented milks.

1.2Composition

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.[1] Finally, the finished kefir contains complex polysaccharides, notably #kefiran, which give kefir its buttermilk-like consistency.

1.3Alcohol content

Kefir has a low alcohol content of 0.08% - 2%.[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%.[16]

1.4Microbial composition

Kefir grains are a complex mix of lactic-acid bacteria, acetic-acid-producing bacteria, lactose-fermenting yeasts, and non-lactose-fermenting yeasts.[1] 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.[1]

There are estimated to be more than 300 different microbial species that may be present in any given kefir culture.[1] That said, the most commonly found bacteria are Lactobacillus kefiranofaciens, Lacticaseibacillus paracasei, Lactiplantibacillus plantarum, Lactobacillus acidophilus, and Lactobacillus delbrueckii subsp. bulgaricus.[2] Lactococcus and acetic acid bacteria are also present in smaller numbers.[1] The most commonly found yeasts are Saccharomyces cerevisiae, Saccharomyces unisporus, and the lactose-fermenting yeast Candida kefyr/Kluyveromyces marxianus.[2]

1.5Kefiran

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.[17] 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.[6] However, clinical trials are needed to confirm any benefits in humans.

1.6Other fermentation metabolites

Finished kefir contains many fermentation metabolites in addition to kefiran, including amino acids, fatty acids (CLA), bacteriocins, organic acids, other polysaccharides, and bioactive peptides.[18]

1.7Fermentation substrate

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.

1.8Vegan kefirs

Milk kefir grains will ferment other sugary liquids, such as milk substitutes, fruit juice, or even sugared water[2]. 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[19]. However, this article is focused on milk kefir, made with mammalian milk. For a narrative review of nondairy kefir products, see this paper.

2Cancer

2.1Kefir and cancer: what does the non-human literature say?

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.[20] These results are promising, but keep in mind that none of these studies are human clinical trials.

2.2Kefir during cancer treatment

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, with more of the kefir group experiencing headache, nausea, vomiting, bloating, constipation, dry mouth, drowsiness, hair loss, weight loss, sweating, poor appetite, and oral mucositis. The exception was difficulty sleeping, which was less common among 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.[9][21]

2.3Kefir after cancer

One small study looked at kefir as a post-exercise beverage for cancer survivors who had undergone chemotherapy and/or radiation at most 2 years previously. The study put 24 cancer survivors through a 12-week exercise regimen of thrice-weekly hour-long sessions; 12 participants got kefir after each session, and 12 got nothing. Both groups benefited from the exercise, but the kefir group appeared to get more benefit, with greater improvements in lean body mass, depression, fatigue, and gastric distress. The study also saw a decrease in the kefir group's serum lipopolysaccharides (an endotoxin) and different changes in the white blood cell population from the control group. However, there was no milk control in this study, so we don't know whether these effects could have been seen with plain low-fat milk. This study didn't report on adverse effects, other than to note that "most" participants didn't report any.[22]

3Gut health

There's mixed evidence for kefir's effects on gut health. Some trials saw improvements in constipation, diarrhea, bloating, pain, and overall gut health; other trials found no effect. The only trial to find negative effects on gut health was in colorectal cancer patients, who were more likely to experience unpleasant GI side effects if they took kefir.

3.1Fecal microbiome

Kefir increases fecal microbial load[23][24], 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 bacteria than drinking unfermented milk.[25][24] One small study found a significant difference between two different kefirs.[14]

3.2Constipation

Three small trials have examined the effects of drinking kefir on constipation. Colorectal cancer patients saw an increase in the chemotherapy side effect of constipation[9], and children taking antibiotics saw no change[26]; 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[27].

3.3Diarrhea

Kefir didn't reduce the incidence of diarrhea or loose stools in children taking antibiotics for uper respiratory infections,[26] and didn't change stool consistency or frequency in a small study of people with inflammatory bowel disease (IBD),[24] but it did decrease the incidence of diarrhea in people taking medicine for stomach ulcers (a proton pump inhibitor (PPI) and two antibiotics).[28]

3.4Inflammatory bowel disease

One small study evaluated kefir in patients with inflammatory bowel disease (IBD). The Crohn's disease patients who took kefir (just 10 participants) self-reported an improvement in abdominal pain, bloating and feeling good after two weeks of kefir, but the study didn't report values and it's not clear whether these improvements were greater than control. Ulcerative colitis (UC) patients who took kefir didn't self-report any improvement. Neither Crohn's nor UC patients reported any change in stool frequency or stool consistency, nor any side effects.[24]

3.5Bloating

In patients receiving chemotherapy for colorectal cancer, kefir increased the incidence of bloating. A small study of patients with IBS saw less self-reported bloating in the Crohn's subgroup after two weeks of kefir, but that study didn't report the values, and it's not clear whether it was greater than control.[9][24]

3.6Overall

One small, non-randomised trial of kefir in cancer survivors in an exercise program recorded a pooled "gastric distress" score which incorporated urge to defecate, nausea, gastric reflux, stomach fullness, cramps, and flatulence. The study didn't report on individual symptoms, but did note a 64.7% reduction in this pooled GI distress score in the kefir group after 12 weeks of exercise, compared to a 23.6% reduction for the control group.[22]

4Oral health

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.[29][30][31]

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).[9][21]

5Cardiovascular health

5.1Blood pressure

There's limited evidence that kefir may help decrease blood pressure. Two small studies of people with metabolic syndrome noted a modest decrease in systolic and diastolic blood pressure, although it wasn't significantly different than milk control in one trial.[25][32]

5.2Framingham score

One study of 48 people with metabolic syndrome measured the Framingham Score, which is an estimate of an individual's 10-year cardiovascular risk. The Framingham Risk Score was lowered in the kefir group, compared to yogurt control, but the level of high-sensitivity C-reactive protein (hs-CRP, a measure of inflammation that may indicate increased risk of cardiovascular disease) was unchanged.[32]

5.3C-reactive protein

C-reactive protein is a measure of inflammation that may indicate increased risk of cardiovascular disease. Six trials measured either high-sensitivity C-reactive protein (hs-CRP) or C-reactive protein. Four trials measured C-reactive protein: one of patients with IBD, one of cancer survivors in an exercise program, one of endurance athletes, and one of healthy overweight subjects. Levels of CRP in the first three would be expected to reflect inflammation from IBD and exercise respectively. The participants' CRP levels in the trial with overweight participants could be relevant to cardiovascular health, but that trial didn't find any effect. The two trials measuring hs-CRP were conducted in patients with metabolic syndrome, so their results could be relevant to cardiovascular health, but neither trial found any effect from kefir consumption.[25][32] To sum up, there's no evidence that kefir consumption reduces C-reactive protein associated with increased risk of cardiovascular disease.

5.4Atherosclerosis

No clinical trials have studied kefir's effect on atherosclerosis. However, one small clinical trial of kefir in people with metabolic syndrome did measure levels of oxidised LDL, which is associated with atherosclerosis, and found a modest reduction in the kefir group compared to yogurt control.[32] See the lipid profile section below as well.

6Weight loss

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.[11][25][32]

Several studies whose main outcome wasn't weight loss did track related body 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.[11][32] None of the four studies that measured body fat found any effect.[33][34][25][32] Neither did the study that measured body density,[34] nor did the one study that measured fat-free mass,[33] nor the one that measured hip circumference,[25], nor the one that measured lean mass[32], nor the one that measured muscle mass.[9]

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.[33][9]

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.[11] There is one unfortunate exception: patients undergoing chemotherapy for colorectal cancer were more likely to experience negative side effects, including weight loss, if they took kefir.[9]

7Inflammation

Several clinical trials of kefir reported on biomarkers of inflammation, mostly as secondary or experimental endpoints. There is no to weak evidence for any effects.

7.1Cytokines

Cytokines are small proteins that are involved in intercellular communication and used by the immune system for cell signaling.

One small, non-controlled Turkish study investigated kefir's effect on cytokines specifically. It found a decrease in IL-8 levels and an increase in serum IL-5 levels during kefir consumption. It also found an increase in TNF-alpha levels, but the increase was insignificant in paired comparisons and the level was borderline between 0th and 6th weeks (p= 0.013). The study noted no significant change in hemoglobin, serum creatinine, ALT, interleukin 1 or serum-tgf levels during kefir consumption. Note: this study is available in Turkish only, so Examine.com was not able to review the full study; this information is taken from the study's English-language abstract.[35]

Several other studies measured cytokines as secondary or experimental endpoints. Three of these also measured TNF-α; in contrast to the above study, which saw an increase, one study found a decrease (but no greater than milk control) and the other two found no effect.[21][25][22]

One study found that interferon gamma was significantly lower after 12 weeks of kefir (p ≤ 0.05). However, there was no significant difference between the kefir and control (unfermented milk) groups (p > 0.05). [25]

No effect was found by any study for: IL-1 beta[21][35], IL-10[25], IL-6[25][21][22].

7.2C-reactive protein

C-reactive protein is a measure of inflammation. There are two tests for C-reactive protein (CRP). They both measure the same protein, but one test is more sensitive and is called high-sensitivity C-reactive protein (hs-CRP). There's little to no evidence that kefir has any effect on C-reactive protein.

Two clinical trials of kefir have measured hs-CRP, and neither found any effect from kefir consumption.[25][32] Four clinical trials of kefir measured C-reactive protein: one of patients with IBD, one of cancer survivors in an exercise program, one of endurance athletes, and one of healthy overweight subjects. Among the patients with IBD, the subgroup of Crohn's disease patients saw a decrease in CRP, but not significantly more than control. The studies of kefir in cancer survivors and healthy overweight participants didn't find any effect on CRP. The study in endurance athletes found no significant change in CRP in the kefir groups, in contrast to a significant increase in the training+milk control group; the study's authors noted it's possible that kefir mediated an exercise-related spike in CRP, but since the training+milk group started with a lower average CRP than the other groups, this is tentative at best.

8Glycemic control

A 2021 meta-analysis examined six RCTs, with a total of 323 participants, which used fasting glucose (FBS)(N=323), hemoglobin A1c (glycated hemoglobin, HbA1c)(N=271), and/or insulin (n=169) as indicators of glycemic control. The patient groups in the six studies were homogenous (diabetic in 3 studies). The meta-analysis found that the pooled kefir treatment group had significantly lower fasting glucose compared to control (mean reduction -10.3), with a high degree of concordance between the six studies; significantly lower insulin compared to control (mean reduction -2.9), with low concordance between three studies; but did not have significantly lower HbA1c compared to control, with low concordance between three studies.

At least 50% of the trials were assessed as having an unclear risk of bias in four of the seven domains of the risk of bias assessment. The considerable levels of heterogeneity, the relatively small number of trials, and unclear risk of bias in most of the trials reduce our confidence in the results.[36]

8.1Diabetes

There is limited evidence to suggest that kefir consumption may help with a modest reduction in fasting glucose and HbA1c in people with type 2 diabetes.

Two clinical trials have tested kefir in people with type 2 diabetes (excluding one poor-quality paper[13]); both found reductions in fasting glucose and HbA1c. One trial, a 42-person trial of adult men newly diagnosed with diabetes and treated with metformin, found a significant decrease in glycated hemoglobin (HbA1c) and fasting glucose in the participants who also received kefir.[37] In another trial, 60 men and women with type 2 diabetes took either kefir or doogh (an Iranian yogurt drink). After eight weeks, the kefir group's fasting blood glucose was significantly lower than the doogh group's. This trial didn't find any change in insulin levels, but did note that one calculated measure of insulin resistance, HOMA-IR, decreased by about 2 points (likely due to the reduction in fasting glucose). Although HbA1C wasn't one of the trial's preregistered endpoints, the researchers did measure it and noted that the kefir group's serum HbA1C reduced during the study.[38]

It's important to note that the reductions in fasting glucose and HbA1c weren't huge, certainly not enough to move them out of the diabetic range. There's no evidence to suggest that kefir alone could be an adequate treatment for diabetes. At best, regular kefir consumption could provide a slight reduction in blood sugar and HbA1c.

8.2Metabolic syndrome

Two clinical trials have tested kefir in people with metabolic syndrome, with mixed results In one study of 48 men and women with metabolic syndrome, kefir did not affect any anthropometric measures, but the kefir group saw reductions in blood pressure, fasting blood glucose levels, LDL-C, non-HDL cholesterol, triglycerides, and oxidized LDL. Women in the kefir group saw an increase in HDL-C levels. The Framingham Score of cardiovascular risk was lowered in the kefir group, but the level of hsCRP (a measure of inflammation) was unchanged.[37] Another study of 22 people with metabolic syndrome didn't find any significant differences from milk control, but had a 45% dropout rate.[25]

9Lipid profile

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 possible exception of non-HDL cholesterol (total cholesterol minus HDL cholesterol).

9.1LDL

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. [11][32] However, 6 studies found no effect.

9.2HDL

The evidence is mixed but leans toward no effect. Six studies found no effect on HDL.[23][11][25][38][39][37] One study saw an increase, but only in female particpants.[40] And one study saw a decrease, but saw the same decrease for the milk group.[33]

9.3Triglycerides

The evidence leans toward no effect. Only one study found a decrease in triglycerides.[32] Seven other studies found no effect.[25][33][38][23][11][39]

9.4Non-HDL cholesterol

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.[11][32]

9.5Total cholesterol

Kefir doesn't appear to have an effect on total cholesterol. No effect was seen in any of the 8 clinical trials of kefir that measured total cholesterol.[25][33][38][23][11][39][37][32]

10 Other body systems

10.1 Kidneys

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.[35][32][39][37]

10.2Liver

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.[25][35][39][32]

10.3Bone health

One clinical trial examined kefir's effect on bone mineral density. The average bone mineral density increased in both control and kefir groups, but the difference between control and kefir wasn't significant. The study took blood samples before, during and after the intervention, and found weak evidence for some differences between the kefir group and the control group. However, a lot of irregularities in how this study was registered, analyzed, and reported, coupled with a large loss to follow-up, make this study very weak evidence for any beneficial effects of kefir. Future research is needed to explore this issue.[41]

11Kefir and exercise

Three studies have looked at kefir as a post-exercise beverage (excluding one which used a heat-killed, non-probiotic product made from kefir.[12])

One trial examined the effects of kefir on healthy young adults undergoing endurance training. Kefir didn't affect body density, body composition, heart rate, blood pressure, injury rate, self-reported sickness, or the time it took the athletes to run 1.5 miles. The study did find that C-reactive protein (CRP) increased during endurance training in the control group but not in the kefir group, which could indicate that kefir mediated exercise-induced inflammation; on the other hand, the pre-intervention C-reactive protein was much lower in the endurance control group than in any other group, and the post-intervention levels of CRP matched all the other groups, so it's hard to know how to interpret this finding.[34]

Another trial compared kefir, boza (a traditional Turkish yeast-fermented grain beverage) and control as postexercise beverages in healthy, sedentary young men. They found that the serum levels of alanine aminotransferase (ALT) and total oxidant status (TOS) were significantly reduced in the exercise + kefir group. However, while the authors used a statistical test to compare the exercise+(kefir/boza) groups to the exercise-only group, they didn't include the results of that test in the paper. Between that omission, the small sample size, and the lack of statistical correction for multiple comparisons, these findings should be treated as preliminary. [39]

A third trial put cancer survivors, who had undergone chemotherapy and/or radiation at most 2 years previously, in a 12-week structured exercise program, with either kefir or nothing as a post-exercise beverage. The participants who drank kefir after each exercise session saw greater improvements in lean body mass, depression, fatigue, and gastric distress than the control participants, who weren't given any post-exercise beverage. The kefir group also saw a significant reduction in circulating lipopolysaccharides (an endotoxin), and changes in their leucocyte population. However, there was no milk control in this study, so we don't know whether these effects could have been seen with low-fat milk. The kefir group didn't have any greater improvement than control in oxygen uptake or in measurements of strength.[22]

To sum up, there's no evidence that kefir as a post-exercise beverage increases gains in strength, speed or endurance, but one small study suggests it may beneficially effect lean body mass as well as reducing depression, fatigue, and gastric distress.

12Kefir as complementary therapy

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[28], but is not recommended for patients undergoing chemotherapy.[9]

12.1For stomach ulcers

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.[28]

12.2For chemotherapy

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.[9] 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.[21]

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