Scientists have identified several bioactive compounds in kefir, including peptides, polysaccharides, and organic acids.
Some laboratory experiments suggest that these substances may influence processes involved in cancer cell growth.
For example, kefiran and other kefir-derived compounds have demonstrated biological activities that researchers are investigating for their possible effects on cell signaling, oxidative stress, and inflammatory responses.
Studies involving cultured cells have reported that certain kefir extracts can inhibit the growth of selected cancer cell lines.
Experimental research has investigated these effects in models involving:
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Breast cancer cells.
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Colon cancer cells.
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Gastric cancer cells.
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Skin cancer cells.
Some animal studies have also explored whether kefir-derived compounds could influence tumor development.
One review published in Medical Oncology in 2017 discussed the potential anticancer properties of fermented milk products, including kefir.
The researchers described promising laboratory and animal findings while emphasizing the need for further investigation.
It is essential to understand that laboratory results do not establish that a food can prevent or treat cancer in humans.
Cancer is a complex group of diseases with different causes, biological mechanisms, and treatment requirements.
There is currently no reliable clinical evidence that drinking kefir prevents cancer or eliminates cancerous cells in the human body.
Nevertheless, unsweetened kefir can be included in a nutritious diet that supports general health during and outside cancer treatment, provided the individual can safely consume it.
People receiving chemotherapy, undergoing transplantation, or experiencing severe immune suppression should ask their medical team before consuming unpasteurized or homemade fermented products.
15. Kefir May Support the Gut Microbiome During Dietary Changes
The final potential benefit of kefir is closely connected to its effect on the intestinal microbial environment.
The gut microbiome is not a fixed collection of microorganisms. It changes in response to diet, lifestyle, medications, illness, and environmental influences.
Eating a varied diet rich in plant foods provides different types of dietary fiber that beneficial intestinal bacteria can use.
Some fibers are fermented by gut microorganisms, producing short-chain fatty acids such as acetate, propionate, and butyrate.
These compounds are involved in several aspects of intestinal health and metabolic regulation.
Kefir introduces another source of microbial diversity through its live cultures.
However, it is important to distinguish between probiotics and prebiotics.
Probiotics are live microorganisms that provide a health benefit when consumed in adequate amounts.
Prebiotics are substances, usually specific types of dietary fiber, that are selectively used by microorganisms in ways that provide a health benefit.
Traditional kefir is primarily valued as a fermented food containing microorganisms and their fermentation products. Its exact prebiotic activity depends on its composition and preparation.
Harvard's Nutrition Source emphasizes that supporting a healthy gut microbiome involves more than consuming a single probiotic food.
A long-term dietary pattern rich in different types of fiber from vegetables, fruits, legumes, whole grains, nuts, and seeds is also important.
Therefore, rather than relying exclusively on kefir, consider combining fermented foods with a wide variety of plant-based foods.
This approach helps provide both beneficial microorganisms and the nutrients that support a healthy intestinal environment.
Possible Side Effects of Drinking Kefir
Kefir is generally well tolerated by healthy adults when consumed in moderate amounts. However, like other fermented foods, it may cause digestive discomfort in some people, particularly when they first introduce it into their diet.
Understanding the possible side effects can help you decide whether kefir is suitable for you.
1. Bloating and Excessive Gas
One of the most common initial complaints associated with probiotic foods is increased gas production.
Kefir contains live microorganisms that interact with the digestive environment. When someone who rarely consumes fermented foods suddenly introduces a large quantity, they may experience temporary changes in digestion.
Possible symptoms include:
These symptoms are often temporary, but persistent or severe discomfort should not be ignored.
A simple way to reduce the likelihood of digestive discomfort is to begin with a small serving and gradually increase the amount according to your tolerance.
2. Diarrhea or Changes in Bowel Movements
Some people experience looser stools when they begin consuming kefir.
This may occur because of changes in digestive activity, sensitivity to remaining lactose, or the introduction of unfamiliar microorganisms.
People with lactose intolerance may still experience symptoms if the kefir contains more lactose than they can tolerate.
If diarrhea develops after drinking kefir, reduce the serving size or stop consuming it temporarily.
Persistent diarrhea, dehydration, fever, or severe abdominal pain requires medical evaluation.
3. Allergic Reactions to Milk Proteins
Traditional milk kefir contains milk proteins, including casein and whey.
People with a milk allergy can experience an immune reaction when consuming these proteins.
Symptoms may include:
Severe allergic reactions can be life-threatening and require emergency medical attention.
Fermentation does not reliably remove the milk proteins responsible for allergic reactions.
People with milk allergies should avoid conventional milk kefir unless an allergy specialist specifically advises otherwise.
4. Small Amounts of Alcohol
Because kefir fermentation involves yeasts as well as bacteria, small quantities of alcohol may naturally develop during fermentation.
The amount varies depending on the ingredients, microorganisms, fermentation duration, and storage conditions.
Most conventional milk kefir contains only a small amount, but homemade preparations can vary considerably.
People who need to avoid alcohol completely should check product information and seek professional advice if necessary.
5. Potential Risks for People With Weakened Immune Systems
Although probiotics are generally considered safe for healthy people, they are not appropriate for every individual.
According to the U.S. National Center for Complementary and Integrative Health (NCCIH), serious infections associated with probiotic microorganisms have occasionally been reported, particularly in people who are severely ill or have compromised immune systems.
People who have undergone organ transplantation, are receiving intensive chemotherapy, or have certain serious medical conditions should consult their healthcare providers before regularly consuming probiotic products.
This precaution is particularly important for homemade kefir because its microbial composition is not standardized.
6. Food Safety Concerns With Homemade Kefir
Homemade kefir requires appropriate hygiene and temperature control.
If the equipment is contaminated or the fermentation process is poorly managed, unwanted microorganisms may grow alongside the intended cultures.
Using pasteurized milk is generally the safer choice.
The U.S. Centers for Disease Control and Prevention (CDC) explains that raw milk can contain harmful microorganisms responsible for foodborne illnesses. Pasteurization significantly reduces these risks while preserving the nutritional benefits of milk.
For additional safety:
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Wash your hands before preparing kefir.
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Use clean glass jars and utensils.
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Avoid contaminated or damaged equipment.
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Use fresh pasteurized milk.
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Keep the finished drink refrigerated.
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Discard kefir that develops unusual discoloration, visible mold, or an unpleasant rotten smell.
Pregnant women, older adults, young children, and people with weakened immune systems should be particularly careful about homemade fermented products.
How Much Kefir Should You Drink Per Day?
