In recent years, kefir has gone from being a virtually unknown fermented food to becoming a staple in Spanish refrigerators. It is credited with improving digestion, the immune system, and even mood, leading many to consider it the "healthiest yogurt."
Beyond trends, some of these effects are scientifically supported , but there are also important nuances: not all supermarket kefirs are the same, not everyone tolerates them well, and their consumption can cause discomfort if introduced without supervision. Understanding what it really is, how it works, and who should be cautious helps in making more informed decisions.
What is kefir and why is it not just any yogurt?
Kefir is a fermented dairy product made from cauliflower-shaped nodules known as kefir grains , which contain a complex community of bacteria and yeasts. This symbiotic mixture transforms milk into a tangy, slightly thickened, and often somewhat carbonated beverage.
During fermentation, lactic acid bacteria convert lactose into lactic acid , which gives it its characteristic sour taste, while yeasts produce carbon dioxide and small amounts of ethanol. This process also yields organic acids, bioactive peptides, and kefiran, a polysaccharide with potential prebiotic effects.
Unlike conventional yogurt, which typically contains only a few well-defined bacterial strains, kefir can contain dozens of different species . This microbial diversity is associated with a greater impact on the gut microbiota and the immune system, provided the product contains live cultures.
In Spain, one of the problems is the lack of specific regulations on what can be labeled as kefir. This means that some products are actually fermented dairy products with a limited variety of microorganisms or no yeast at all, despite being marketed under that name.
Science-backed benefits of kefir
Several specialists in cardiology, nutrition, and gut microbiota agree that authentic kefir is not just a fad: a growing body of research supports its role as a natural probiotic . Among its best-documented effects are those related to digestion, immunity, and certain cardiometabolic markers.
First, its combination of bacteria and yeasts helps to balance the gut microbiota . Regular consumption is associated with an increase in beneficial microorganisms such as Lactobacillus and Bifidobacterium and with greater production of short-chain fatty acids, including butyrate, which is key to the health of the intestinal mucosa.
This adjustment of the gut ecosystem often results in easier digestion, less bloating, and better tolerance to certain foods. Furthermore, during fermentation, the amount of lactose is reduced and enzymes are produced that facilitate its breakdown, which is why many people with mild lactose intolerance tolerate kefir better than milk.
On the other hand, effects on the immune system have been observed. Studies in humans and animal models show decreases in inflammatory cytokines (such as IL-6 or TNF-α) and an increase in intestinal IgA, an immunoglobulin that acts as the first line of defense in the gut.
This anti-inflammatory action, combined with the production of bioactive compounds, is associated with a potential benefit in conditions where chronic low-grade inflammation plays a significant role, such as some digestive and metabolic disorders. It does not replace medical treatment, but it can serve as a dietary adjunct within a comprehensive plan.
How it helps the gut microbiota, digestion, and the immune system
When we consume kefir regularly, some of its microorganisms manage to survive the acidic environment of the stomach and reach the intestine. Although a fraction is lost along the way, studies indicate that a significant amount of bacteria and yeasts withstand gastric passage , especially if the kefir is ingested with other foods that buffer acidity.
Once in the gut, these microbes interact with the resident microbiota, compete with potentially harmful bacteria, and can temporarily adhere to the mucosa. They don't usually colonize permanently, but they are capable of modifying the microbial composition and activity while they are being consumed.
Furthermore, the structure of the kefir ecosystem acts as a kind of "barrier" against external contamination. Research on traditional fermented foods shows that the combination of bacteria and yeasts can limit the invasion of opportunistic microorganisms , similar to what happens in a healthy gut when the microbiota is well balanced.
This ability to compete with undesirable bacteria and modulate the immune response contributes to kefir being considered a valuable food for indirectly strengthening the immune system. However, the effect depends on regular consumption and whether the product contains a sufficient quantity of live cultures.
Due to its connection with the so-called gut-brain axis, modest effects on mood have also been described. Some preliminary studies suggest a slight reduction in anxiety and an improvement in subjective well-being, perhaps due to changes in the gut microbiota or the production of compounds such as GABA and serotonin during fermentation.
Impact on the heart, metabolism, and body weight
Beyond its effects on the gut, kefir can influence cardiometabolic health. As a fermented dairy product with high-quality protein , it provides essential amino acids, calcium, magnesium, phosphorus, and vitamins such as D and K—nutrients that support muscle function, bone health, and cardiovascular health.
Some research has shown that continued consumption of kefir can modestly reduce total cholesterol and LDL cholesterol . This is partly explained by the fact that certain kefir bacteria bind to bile salts in the intestine, decreasing cholesterol absorption, while the short-chain fatty acids produced during fermentation help limit cholesterol synthesis in the liver.
Effects on insulin sensitivity and blood glucose control have also been described . Its organic acid profile and lower lactose content compared to milk generally result in a milder impact on blood sugar. This makes it an attractive option within a balanced diet for individuals who need to monitor their glucose levels, always under medical supervision.
Regarding body weight, kefir isn't a "miracle" product, but it can help indirectly. Its protein content and its effect on the gut microbiota and inflammation often promote a feeling of fullness and regular bowel movements. As part of a healthy lifestyle, it can be another ally in maintaining a healthy weight.
Some experts also point out that improved gut health and reduced low-grade inflammation are linked to a more efficient metabolism. Again, this is just one factor among many: a varied diet, physical activity, rest, and stress management.
Kefir side effects and most common discomforts
Despite its potential benefits, kefir is not without its drawbacks. Several experts agree that it is not a food for everyone and that introducing it suddenly in large quantities can cause problems, especially for people with sensitive digestion.
One of the most frequent complaints is the appearance of gas, bloating, or diarrhea at the beginning of consumption. These discomforts are usually related to the fermentation of sugars and abrupt changes in the gut microbiota when going from not taking probiotics to incorporating them intensively.
In people with lactose intolerance, although kefir contains less lactose than milk, it can still cause symptoms if consumed in large quantities or if the intolerance is severe. In these cases, some professionals recommend considering alternatives such as water kefir or fermented plant-based versions , always carefully checking the ingredients.
Another possible reaction is linked to the presence of histamine . This fermented food can contain significant levels of this molecule, so those with histamine intolerance may experience headaches, skin flushing, palpitations, or digestive discomfort after consuming it.
It is also common for changes in bowel habits or stool consistency to occur during the first few weeks. If these effects are mild and temporary, they are usually interpreted as part of the gut microbiota's adaptation process, but if they are intense or prolonged, it is advisable to consult a healthcare professional.
Who should avoid it or take extra precautions
Certain population groups should be especially cautious with kefir. On the one hand, people with severe lactose intolerance may not tolerate even small amounts of the traditional milk version, even though this fermentation process reduces the lactose content compared to fresh milk.
For those with milk protein allergies , dairy kefir is completely contraindicated. There is no margin of safety here: since it is an allergy, the reaction can be severe even with minimal amounts, so only water kefir or other specific plant-based fermented products should be considered under expert guidance.
Another sensitive situation is that of people with severe immunosuppression , such as patients undergoing intensive chemotherapy, transplants, severe neutropenia, or other conditions that severely compromise the immune system. In these cases, although the risk is very low, there is a possibility that certain bacteria in kefir could cross the intestinal barrier and cause an opportunistic infection, so it is recommended to avoid it unless specifically advised.
People with acute gastrointestinal outbreaks (gastroenteritis, active colitis, severe diarrhea) are also not good candidates to consume kefir during the active phase of the condition, as the intestine is especially sensitive and the introduction of a fermented food so rich in microorganisms could aggravate the symptoms.
Caution is also advised for those suffering from SIBO (small intestinal bacterial overgrowth) or irritable bowel syndrome, as fermented foods can worsen bloating and pain if not handled carefully and with the guidance of a specialist.
How to start taking kefir and what quantity is recommended
For those who can tolerate it, the general recommendation is to introduce kefir gradually . Starting with 50-100 milliliters per day, or 2-3 tablespoons for very sensitive individuals, is usually a prudent strategy to observe how the body responds.
If no significant discomfort arises, the amount can be gradually increased to about 150-250 milliliters per day for adults, which is a typical consumption range. For children, the amounts are adjusted to their size: around 100 grams per day is usually sufficient, while athletes can consume somewhat larger portions due to their higher energy and protein requirements.
In babies under one year old, due to digestive immaturity and the general recommendation against introducing fermented dairy products of this type so early, many experts advise against offering kefir . It is also not ideal for people suffering from acute infections or recovering from delicate procedures without professional supervision.
Besides the quantity, the rate at which the dose is increased also matters. Suddenly going from a few spoonfuls to a whole glass can trigger gas, bloating, and diarrhea due to excessive fermentation, even in healthy people. The key is to increase the amount gradually and pay attention to how you feel.
At the same time, it is always advisable to review the individual response: if persistent or very bothersome symptoms appear, it may be necessary to reduce the dose, space out the doses or stop consumption until consulting with a health professional or a dietitian-nutritionist.
When to drink kefir and what foods to combine it with
Many experts suggest that one of the best times to drink kefir is in the morning, accompanied by fresh fruit rich in vitamin C , such as kiwi, strawberries, or citrus fruits. This combination provides prebiotics (fermentable fibers) and antioxidants that can enhance the activity of probiotics.
Taking it on a relatively empty stomach, but alongside foods that neutralize acidity, allows a greater proportion of microorganisms to survive passage through the stomach. Some people prefer to include it in their breakfast in a bowl with oats, seeds, or berries , which increases fiber intake and makes the meal more filling.
It can also be taken mid-morning or in the afternoon, as long as it's taken some time before or after main meals. After eating, the stomach tends to become more acidic, which could interfere with the survival of some of the probiotics, so it's advisable to leave at least a couple of hours between meals.
In general, it is not advisable to ingest large quantities of kefir just before going to sleep , especially in people prone to nighttime abdominal distension or reflux, as the residual fermentation can generate gas and discomfort.
A practical way to incorporate it into your daily routine is to use it as a base for sauces and dressings, mixed with lemon and fresh herbs, to replace mayonnaise and dressings loaded with fats and additives. It can also be combined with cinnamon, cooked quince, or mild spices for more digestible and palatable preparations.
Milk kefir, water kefir, and commercial versions: key differences
Although they are all grouped under the same name, not all types of kefir have the same composition or effects. Traditional milk kefir is currently the most studied: it combines the nutritional benefits of dairy products (proteins, calcium, vitamins D and K2, magnesium, and phosphorus) with a wide variety of bacteria and yeasts.
Water kefir and plant-based kefir also contain beneficial microorganisms, but their microbial community is more variable and, in general, less documented in the scientific literature. They can be an acceptable alternative for those who do not consume dairy, provided they are prepared or purchased according to safety and quality standards.
Regarding the difference between homemade and commercially produced kefir , many nutritionists point out that homemade kefir made with traditional grains tends to have a higher probiotic potency due to the abundance and diversity of microorganisms. The drawback is that it is less standardized: the result depends on the fermentation time, temperature, type of milk, and the hygiene of the process.
Industrial products offer greater batch-to-batch consistency and stricter sanitary controls, but often contain a more limited number of species for safety and stability reasons. Furthermore, some include added sugars, thickeners, flavorings, or preservatives that can alter their nutritional profile.
In Spain, nutritionists like Boticaria GarcÃa insist on the importance of reading the label: a truly interesting kefir should include only milk and live cultures , specify the type of milk used and, if possible, indicate the number of CFU (colony forming units) per serving, aiming for high figures without resorting to exaggerated claims.
How to recognize a good kefir in the supermarket
In the absence of very detailed regulations, consumers should pay attention to several basic points when choosing a commercial kefir. In the ingredient list, it's advisable to check that live and active cultures are mentioned and that it refers to specific kefir yeasts, not just generic lactic ferments.
It is also preferable to opt for products without added sugars , high fructose corn syrup, maltodextrin or artificial sweeteners, as these ingredients can negatively impact both the nutritional value and the glycemic response and the microbiota.
Another aspect to watch out for is the absence of preservatives and subsequent heat treatments , such as pasteurization after fermentation, which inactivate some of the beneficial microorganisms. Sometimes the packaging indicates that the product has not been subjected to heat after fermentation or that it contains live cultures.
Texture and flavor also offer clues. Traditional kefir is usually somewhat more acidic, with a certain effervescence and an uneven creaminess, a direct result of yeast activity and active fermentation. An excessively homogeneous, very sweet product with no acidic note may indicate a limited fermentation process or significant industrial intervention.
Finally, it's advisable to pay attention to the expiration date and storage instructions. To keep the microorganisms healthy, kefir should be kept refrigerated and consumed within the timeframe indicated by the manufacturer , avoiding leaving it open in the refrigerator for too many days.
Comparison with other fermented foods such as kombucha
Within the family of fermented foods, kombucha has become another very popular product. Unlike kefir, it is made from sweetened tea fermented using a culture of bacteria and yeast (SCOBY), so it is dairy-free and appealing to those who follow vegan diets or don't consume milk.
Kombucha typically has fewer calories than dairy kefir, but it offers a different profile of microorganisms and, most importantly, contains caffeine and small amounts of alcohol produced during fermentation. This makes it unsuitable for everyone, especially children, pregnant women, or people with caffeine sensitivity.
Regarding the gut, both kombucha and kefir can cause bloating or gas in people unaccustomed to fermented foods or with marked gut dysbiosis. In many cases, kefir provides a more studied and better-characterized combination of probiotics and dairy nutrients.
The choice between one or the other will depend on personal preferences, individual tolerance, and the clinical situation. In any case, it's important to remember that these are dietary supplements , not sole solutions or substitutes for medical treatment.
When incorporating them into your daily diet, it is helpful to observe your body's response, introduce them gradually, and always prioritize versions with simple ingredients and live cultures over excessively sugary or highly processed drinks.
Recommendations for safely preparing kefir at home
Those who choose to make homemade kefir are usually looking for a more economical, fresh product with a potentially higher probiotic content. However, as a live food , it requires certain precautions to avoid contamination and maintain the quality of the fermented product.
The first step is to use thoroughly washed and disinfected glass jars , as this material is inert and easy to sanitize. It is recommended to avoid deteriorated plastic containers or those with lingering odors, as these can harbor undesirable microorganisms.
As for utensils, it is generally advised not to use metal in direct and prolonged contact with the kefir grains, opting for spoons and strainers made of plastic, silicone or good quality stainless steel, as recommended by reference sources and the instructions of the supplier of the nodules.
To wash the grains, it's best to use filtered or boiled and cooled water , avoiding heavily chlorinated tap water, which can harm some of the microorganisms. Fermentation should take place in a cool place, away from direct sunlight and excessive heat sources.
The fermentation time will depend on the ambient temperature and personal preference, but generally it's best not to prolong it excessively, as the acidity increases and the flavor can become too harsh. If the kefir changes color, gives off a strange odor, or develops abnormal bubbles , it's best to discard it.
Once strained, kefir should be stored in the refrigerator and consumed within a short period, usually around three days, to ensure it retains its properties and prevents spoilage. The grains can be kept active by going through regular fermentation cycles or, if necessary, by following specific instructions for temporary storage.
Integrating homemade kefir into your routine makes sense as long as you observe these basic hygiene measures and listen to your body's response, adjusting quantities and fermentation times to individual tolerance.
Overall, kefir is emerging as an interesting fermented food within a varied diet, capable of providing probiotics, proteins, and micronutrients with scientific backing in areas such as the microbiota, inflammation, and certain cardiometabolic parameters; however, it requires common sense: choosing quality products, introducing it gradually, respecting contraindications, and understanding that its real effect will always depend on the overall context of lifestyle.