Fermented milk products have long been associated with the ability to provide health benefits to those who consume them on a regular basis, and have been analyzed both clinically and in the laboratory for at least a century, primarily by Ellie Metchnikoff’s original research [15].

Russian scientist and immunologist Ellie Metchnikoff, who worked at the Pasteur Institute in Paris, was awarded the Nobel Prize in Medicine for his work on immunology (1907), and developed the first theories about the effects of the fermentation process on dairy products, and their contribution to health and longevity.

Kefir: The King of Probiotics

The word kefir comes from the Turkish word keyif, which means “good feeling after eating”, and is a natural, complex, fermented, dairy product created through the symbiotic fermentation of milk by lactic acid bacteria and yeasts, contained within an exopolysaccharide complex (kefiran) and protein, called kefir grain [1].

Kefir has been consumed by the indigenous people of Caucasus and the Middle East for many centuries now, where it was traditionally prepared in goatskin bags, which were hung next to the door of the house, so that when tenants would knock, they would mix the kefir grains.

Kefir is a tart, acidic, viscous, lacto-fermented product, which means that during its production process, lactic acid is generated.

Lactic acid is created through the action of the Lactobacillus bacteria, and is a compound very important for the absorption of iron.

The Kefir fermentation process essentially pre-digests the milk and it’s macronutrients (proteins, carbs, and fats), which explains why this beverage is so tolerable by even lactose intolerant individuals.

Kefir grains are off-white, irregularly shaped clumps that look like mini-cauliflower and have a spongy texture [10].

Milk kefir, in addition to the extremely beneficial bacteria it contains, is a rich source of many different vitamins, minerals, and essential amino acids that promote healing, repair, and general well-being.

Kefir contains high levels of thiamine (B1), B12, calcium, folate (B9), vitamin A, vitamin K2, Vitamin C, and vitamin D.

It is a good source of biotin, a B vitamin that helps the body assimilate other B vitamins.

Its proteins are high in nutritional value and are already significantly digested, and thus used directly by the body.

Kefir is also a great source of minerals, such as calcium and magnesium, as well as phosphorus, which helps the body use carbohydrates, fats, and proteins for cell growth, maintenance, and energy.

Finally, kefir contains more than 50 species of probiotic bacteria and yeast, which are beneficial to health, with the most common being the LactobacillusLactococcus, Streptococcus, and Leuconostoc [16].

That’s the main difference between kefir and other fermented dairy products, as kefir is not the result of the metabolic activity of just a single or a few microbial species, but 50+.

Milk Protein Intolerance

It is been known from previous observations that cow’s milk can have some unwanted side effects on health, mainly due to some indigestible proteins and sugars, such as lactose and casein, which are found in cow’s milk in greater quantities than in human breast milk.

So at older ages, the enzymes that break down these milk proteins may not be functional and thus create various problems.

However, it has become clear from many studies that the microorganisms of kefir affect milk proteins by breaking them down, producing 609 peptides that are unique to kefir.

The fact that kefir breaks down milk proteins into functional peptides, which have positive effects on health, makes kefir a useful food choice for people suffering from protein indigestion and milk protein intolerance.

Lactose Intolerance

Lactose is a naturally occurring sugar found in dairy products.

Many people, especially adults, are not able to digest lactose properly.

This condition is called lactose intolerance.

Kefir is a safe food to be consumed by people with lactose intolerance, due to the action of the enzyme β-galactosidase produced by lactic acid bacteria.

Lactic acid bacteria, which are present in fermented dairy products (such as kefir and yogurt), convert lactose (which is their food) into lactic acid, glucose, and galactose so that the final drink contains a lot of less lactose than regular milk.

They also produce enzymes that can help the body break down lactose even more, making kefir suitable for people who are lactose intolerant.

The fact that kefir is one of the few lactose-free foods, combined with its high content of probiotics and other valuable and nutritious ingredients, including the 50 “good” bacteria and yeasts, has made it one of the top foods on the healthy foods list of the 21st century.


Casein is the main protein found in milk, that has been blamed for various health problems, from autoimmune conditions to autism.

However, casein plays an important role in the conversion of milk to kefir, almost 80 percent of the protein in milk comes from the four members of the casein family.

These substances include alpha-casein, capa-casein, and two types of β-casein.

A study published in the January issue of the Journal of Dairy Science showed that the enzymes in kefir break down milk casein, and that the resulting substances/peptides – have a wide range of health-promoting effects, such as strengthening the immune system and exerting anti-cancer effects [11].

From these data, it is understood that kefir, not only does not have the harmful effects of milk, but breaks down and converts the indigestible and harmful components of milk, into compounds that are beneficial to human health.

Therapeutic Properties

Kefir has been shown to have multiple properties that provide proven health benefits, including treating obesity, osteoporosis, arthritis, irritable bowel syndrome (IBS), hepatic steatosis, and ischemic heart disease [5].

Some other benefits of kefir also include improved glucose control, tumor suppression [9], increased wound healing rate [12], asthma relief, antioxidant activity, antimicrobial activity [17], anti-hypertensive effect, anti-allergic action [18], anti-inflammatory action [8], and upregulation of the immune system [2].

Gastrointestinal Benefits

The effects of kefir consumption on the intestinal composition of the microflora may be due to a combination of factors, such as the direct inhibition of pathogens by acids, the production of bacteriocin (anti-microbial peptides produced by bacteria), and the exclusion of pathogens in the intestinal mucosa (Rattray and ‘Connel, 2011).

According to Marquina (2002) [19], the consumption of kefir significantly increases the epithelial integrity of the intestinal mucosa and reduces enterobacteriaceae and clostridial populations.

In addition, kefir consumption prevents the colonization of the pathogen C. jejuni [20], and has been effective in the postoperative treatment of patients with gastrointestinal disorders.

In Russia, kefir has been long used by researchers as a treatment for peptic ulcers in the stomach and duodenum (Farnworth and Mainville, 2008).

IBS and Ulcerative Colitis

Probiotics have a beneficial effect on irritable bowel syndrome, through several proposed mechanisms including suppressing the growth and binding of pathogenic bacteria, improving epithelial barrier function, and altering the host’s immune response [21].

Probiotics secrete short-chain fatty acids (SCFAs), which results in a decrease in lumen pH and the production of bactericidal proteins.

Butyric acid, a byproduct of the bacterial fermentation of kefir, has been shown to nourish colonic enterocytes, enhancing mucosal integrity.

The DNA of probiotic organisms has also been shown to inhibit epithelial cell apoptosis and to improve intestinal motility.

Thus, probiotics reduce overt symptoms of the disease, improve the patient’s quality of life, as well as increasing the chance of successful treatment.

Probiotics are also effective against ulcerative colitis and their effectiveness in maintaining remission and preventing relapse can be compared to the drug mesalazine.

Data from 23 randomized trials of ulcerative colitis patients showed that probiotics reduced the incidence of the disease by 12%.

Probiotics should be given immediately when the symptoms of relapse appear until they subside.

In particular, a probiotic product called VSL # 3, which is a mixture of 8 strains of lactic acid-producing bacteria, has been shown to be particularly effective for this purpose [22].

Candida and Candidiasis

Under normal circumstances, members of the genus Candida are non-pathogenic, symbiotic microorganisms that inhabit the gastrointestinal tract, oral cavity, and female reproductive system [23].

Some species, such as Candida albicans, and Candida glabrata are known to turn to opportunistic pathogens and cause mucosal infections in these areas, which in turn can lead to invasive candidiasis.

The pathogenesis of Candida begins with fungal overgrowth, followed by adhesion, tissue invasion, and mucosal infection [24].

The lactobacilli contained in kefir can act against the overgrowth of candida, through mechanisms, such as immunomodulation of the epithelial cell barrier, reduction of the adhesion of Candida strains by co-aggregation, as well as by competition for binding sites [24].

These beneficial effects of lactobacilli, protect the intestine from the overgrowth of pathogens.

A great example is Lactobacillus rhamnosus GG ATCC 53103 (L. rhamnosus GG or LGG).

This strain, which has been isolated from healthy human intestinal microflora, and is found in Kefir, is one of the probiotic strains with the highest number of documented health benefits, especially in the prevention and recovery of intestinal tract infections [2526].

Liver Disease

According to the results of 14 studies, probiotics help in the clinical treatment of hepatic encephalopathy.

They significantly reduce the symptoms during the early stages of the disease and reduce the chance of transition to later stages [3].

They also reduce hospitalization rates.

Effects on Mental Health

The latest data show that probiotics have uses in systems other than the gastrointestinal tract.

Probiotics can be helpful in treating various mental disorders, as research has shown a clear connection between bowel and brain health [28].

According to new data, the microbial flora of the intestine communicates with the brain through hormones, through the immune system, or even directly (e.g. neurotransmitters produced in the intestine communicate with the brain via the vagus nerve).

This pathway has given birth to psychobiotics- drugs that improve brain function by interacting with the microbial flora of the gut [29].


The gut microbiome also impacts neurotransmission.

It is capable of directly producing various neurotransmitters, such as serotonin, noradrenaline, dopamine, and γ-aminobutyric acid [31].

Gut bacteria have been shown to modulate tryptophan metabolism and serotonin production [32].

One of the most effective applications of psychobiotics is the treatment of depression.

In early 2017, studies in mice were published that showed that the probiotic Lactobacillus restored to normal levels a number of metabolites that are blamed for the onset of depression [30].

As a result, mice showed significant improvements in their mental health and behavior.

The results of the study were confirmed in humans a little later, in a meta-analysis of more than 2,100 adults who participated in two health studies.

The researchers found that the levels of two specific groups of gut bacteria – Coprococcus and Dialister – were “steadily depleted” in people with depression, while people with higher levels of Coprococcus and another group called Faecalibacterium achieved the highest scores in terms of quality of their psychological state [33].

Both of these types of bacteria break down fiber to produce an anti-inflammatory compound called butyric acid.

A placebo-controlled randomized study showed that the probiotic Bifidobacterium longum NCC3001 significantly reduced depression in patients with irritable bowel syndrome (IBS) [34].

Finally, in patients with mild or moderate depression who had not been treated, the symptoms of the disease subsided significantly after 4 weeks of probiotic treatment. 


Probiotics have been used in the past to treat infections.

One study found that patients who received prebiotics and probiotics the day before a liver transplant had a significantly reduced chance of post-operative infections [35].

Other research has shown that probiotics reduce the risk of respiratory infections and pneumonia in ICU patients [36].

Two recent studies have linked the use of probiotics to a reduced risk of sepsis in newborns and premature infants [37].

Antibacterial and Anti-Fungal Properties

Kefir, and its strains, have shown several antibacterial and antifungal effects [13].

Milk kefir has been used in disk diffusion tests against a wide range of bacterial pathogens and fungal species, and has been found to exert antimicrobial activity equal to ampicillin, azithromycin, ceftriaxone, amoxicillin, and ketoconazole against many of them (Cevikbas et al., Rodrigues et al, 2005.) [38].

Strengthening the Immune System

The formation of kefir bioactive peptides during fermentation or digestion has shown a variety of physiological effects, including the stimulation of the immune system in animal models and in humans (Farnworth, 2005) [39].

Thoreux and Schmucker in 2001, after feeding mice with kefir, observed an increase in the specific mucosal immune response (IgA) against the cholera toxin [40].

Stimulation of the immune system can also occur due to the action of exopolysaccharides found in kefir grains.

Researchers in 2011 observed that kefir was able to modify the balance of intestinal mucosal immune cells, and demonstrated the immunoregulatory effects of kefir, first in mice, and later in humans. (Vinderola et al., 2011).

Kefir administration also elicits a response in the intestinal mucosa, suggesting that kefir components may stimulate cells of the innate immune system by suppressing the Th2 immune response, or by promoting the activity of immune cells against tumors, infections, and intracellular pathogens.

Anti-Diabetic Action

The antidiabetic activity of Lactobacillus and Bifidobacteria has been investigated in several animal and human studies [4142].

Many of them have stated that probiotic treatment can lower blood glucose levels in diabetic patients.

The mechanisms used by probiotics to exert their antidiabetic activity are based on stimulating the production of insulinotropic polypeptides and glucagon-like peptides, thereby causing glucose uptake by muscle and the liver, as well as stimulating the absorption of more glucose from the bloodstream in the form of glycogen [6].

Oral administration of Lactobacillus johnsonii strain La1 for two weeks has been shown to reduce the increase in blood glucose and glucagon levels in diabetic patients [43].

In addition, probiotics have been shown to be effective in reducing glucose absorption from the intestinal tract and can regulate glucose metabolism [44].

Anti-Oncogenic Effects

Kefir has significant anti-cancer activity against multiple types of cancer cells.

The L. kefiri bacterial species has been shown to increase apoptosis of human myeloid leukemia cells, which are resistant to in vitro chemotherapy by activating caspase-3 in a dose-dependent manner [45].

In other studies, kefir extract reduced the number of human breast cancer cells by 56%.

The free kefir cell fraction has also shown anti-cancer activity in vitro against gastric cancer [46].

This study also showed that kefir was able to induce apoptosis in SGC7901 cells through the regulation and apoptosis of the bax and bcl-2 gene, which are apoptosis inhibitors and known oncogenes, as well as by promoting and increasing the action of anti-oncogenes (Sorenson, 2004).

In another study, oral administration of milk and soy kefir to sarcoma-vaccinated mice (180 tumor cells) resulted in 64.8% and 70.9% inhibition of tumor growth, respectively [4].

These results suggest that milk and soy kefir can be considered among the most promising food ingredients in terms of preventing and enhancing mucosal resistance in gastrointestinal and other types of cancer.

Kefir, in addition to promoting cell death in cancer cells, also carries anti-mutagenic effects as shown in a study (2006) by the Ames test, where kefir was tested against known carcinogens, such as methyl methanosulphate, methyl lazoxymethanol, sodium azide, aflatoxin B1, and 2-amino-anthracene [47].

Many studies have shown that kefir acts on different types of cancers, such as colorectal cancer [48], malignant T-lymphocytes, breast cancer [7], and lung cancer.

How to Make Milk Kefir

Put 50 grams of kefir grains in a glass jar.

Add 1/2 liter of milk of your choice and close the lid tightly.

Leave it at room temperature for 24 hours.

If you prefer your kefir to be on the thinner, less viscous side, leave it for about 12 hours.

The more you leave it, its consistency gets thicker and its taste more acidic.

Generally, you should not leave it for more than three days and less than 12 hours.

Also, make sure it is away from direct sunlight.

When the 24 hours have passed, pass the mixture through a plastic or stainless steel strainer.

What passes through the strainer is the actual, drinkable kefir, and what is left in the strainer are the live kefir grains, which you will reuse for your next badge.

Rinse the jar and the grains as they are in the strainer with cold water.

Make sure that they are drained well and store them for future use.

Keep in mind that with continuous cultivation the number of kefir grains is multiplied.

Water Kefir

Water kefir is a probiotic, sour, carbonated beverage with a fruity flavor and small amounts of alcohol [14].

It is a symbiotic colony of about 40 species of friendly bacteria and yeast that can be fermented in water with sugar, coconut water, fruit juices, maple syrup, or other sweet liquids.

Water kefir is a beverage made with other types of grains, not the ones you use to make milk kefir.

The process is essentially the same with the difference that the microorganisms that feed on the milk lactose will now feed on sugars derived from the plant-based drink you will use.

Fungi break down simple sugars, such as glucose and fructose, and convert them to ethanol and acetic acid.

Bacteria, such as Lactobacilli, convert sugars, such as sucrose and complex carbohydrates (starch) into monosaccharides / simple sugars, lactic acid, acetic acid, carbon dioxide, and ethanol.

For any type of plant-based kefir you may also use vegan milks, such as rice, almond, hazelnut, coconut, cashews, or any other vegan milk alternative you like.

Water kefir will not come out as thick as milk kefir, has no lactose, calcium or protein, and its nutritional value is poorer compared to products based on regular milk.

However, water kefir is the right choice for those who are vegan, have a milk allergy or do not like to include dairy products in their diet, but also want to take advantage of the gut-healing benefits of kefir.

How to Make Water Kefir

For water kefir, you will only need 1 tablespoon of water kefir grains, 1 glass of water, 1 tablespoon of cane sugar, half a lemon (optional), or any other dried or fresh fruit you like.

Most people usually prefer dried apricots or fresh figs, as these fruits also give water kefir a richer nutrient profile and taste.

Some form of sugar must always be used as it serves as food for the fungi and bacteria you will cultivate.

If you don’t have a high-quality water filter and use plain tap water, you should boil it first, so that it does not contain any chlorine, which is anti-bacterial and extremely bad for your thyroid.

In that case and after the water cools completely, put in the grains and sugar, and cover with a lid, keeping the concoction at room temperature.

In 2-3 days, your water kefir will be ready for consumption.

Use a strainer to separate the grains from the actual drink, rinse the grains gently with a little water and store them for the next use.

As with milk kefir grains, water kefir grains may be used to repeat this process indefinitely.

If you accidentally make a large badge of kefir, you may keep it in the refrigerator in a glass bottle or tightly-closed jar.

Keep in mind that in water kefir, you may also add ginger, cinnamon sticks or cloves, or lemon peels to further enhance the taste and positive health benefits.


Kefir is a dynamic fermented dairy product with multiple health benefits associated with its consumption.

These benefits originate from the variable yeast and bacterial species present, as well as metabolites such as kefiran and other exopolysaccharides.

Kefir as well as other fermented foods have been used by humans for centuries due to their therapeutic effects, which have been proven both empirically over the years and academically with hundreds of studies shedding light on their physiologic and biochemical effects.

Whole kefir, as well as specific probiotic bacteria isolated from kefir, provide a multitude of positive effects when consumed.

These range from improved gastrointestinal function and wound healing, to the modulation of the immune system and microbiome, and even the potential alleviation of allergies and cancers.

Further research into the mechanisms behind these effects will allow scientists to better understand exactly how kefir and other fermented dairy products confer these benefits, as well as how to harness these traits outside of kefir itself.

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About George Kelly




This content is for informational and educational purposes only. It is not intended to provide medical advice or to take the place of such advice or treatment from a personal physician. All readers/viewers of this content are advised to consult their doctors or qualified health professionals regarding specific health questions. Neither Metabolic Body nor the publisher of this content takes responsibility for possible health consequences of any person or persons reading or following the information in this educational content. All viewers of this content, especially those taking prescription or over-the-counter medications, should consult their physicians before beginning any nutrition, supplement or lifestyle program.