Kefir has the potential to affect the gastrointestinal and oral microbial ecosystem, but there is insufficient evidence to support the concept strongly, as was found in the most recently published literature review in the journal Nutrients by Black E. G. and colleagues. The results of the study indicate kefir's potential to improve gastrointestinal and immunological disturbances related to the microbial modulation of the body. However, the composition of kefir varies significantly across studies, leading to differing levels of benefits due to its application to human physiological processes. Kefir has the potential to prove beneficial, but requires more accurate and high-quality studies to conclude its long-term benefits.
Fermented kefir milk has been a tradition for over 3,000 years in the Caucasus Mountains. It has been produced with the help of "kefir grains," which are a polysaccharide-based combination of lactic acid bacteria (LAB), acetic acid bacteria (AAB), and yeasts, forming a complex symbiotic microbial population. Contemporaries have become interested in "kefir" because it has been attributed properties related to being a probiotic agent and a potential modulator of the human microbiota.
Commercial kefir is typically produced by fermenting milk with kefir grains at a ratio of 1:30 to 1:50 for up to 24 hours at room temperature. Kefir can be made from cow, goat, or sheep milk, as well as plant-based substrates such as soymilk. A major challenge in kefir research is its inherent compositional variability, which is influenced by fermentation time, temperature, substrate type, and the microbial profile of starter grains. These factors determine the final concentrations of microbial species and bioactive metabolites, making it difficult to generalize findings across studies.
Some of the most frequently isolated lactic acid bacteria from kefir are Lentilactobacillus kefiri, Leuconostoc mesenteroides, and Lactococcus lactis. These bacteria ferment lactose into lactic acid and form bioactive compounds like bacteriocins, hydrogen peroxide, and acetaldehyde that might inhibit enteric pathogens. L. kefiri and L. mesenteroides have been identified as surviving gastrointestinal transit and adhering to intestinal epithelium, thus supporting their probiotic potential. Besides, L. kefiri is capable of toxic metals and mycotoxins, whereas L. mesenteroides produces linoleic acid with anti-inflammatory and antiatherogenic properties described in the literature. Recently, much attention has been focused on L. lactis as a vehicle for the delivery of therapeutic molecules or vaccines.
The presence of acetic acid bacteria like Acetobacter fabarum, Acetobacter lovaniensis, and Gluconobacter oxydans results in the production of acetic acid, a process that increases intestinal motility and colonic blood flow and helps maintain epithelial homeostasis. The production of ethanol and carbon dioxide by yeasts like Saccharomyces cerevisiae, Kluyveromyces marxianus, and Kluyveromyces lactis results in the development of the specific kefir flavor and effervescence. Importantly, the same species, Saccharomyces cere
Available evidence indicates kefir may have the ability to change the gut microbiota profile, including the increased abundance of LAB. In healthy subjects, the results were not profound and, in most cases, not statistically significant. In patients with metabolic syndrome, inflammatory bowel disease, and polycystic ovary syndrome (PCOS), the changes were more pronounced, however. For instance, the subjects with metabolic syndrome had an increased abundance of Lactobacillales, alongside within-group changes of improvements in fasting blood insulin, tumor necrosis factor-α, interferon-α, and blood pressure. In women with PCOS, there was an increased abundance of Bacilli, with improvements in the scores of physical function and mental health.
Reference:
Black, E. G. et al. (2025). The Effects of Kefir on the Human Oral and Gut Microbiome. Nutrients, 17(24). https://doi.org/10.3390/nu17243861. https://www.mdpi.com/2072-6643/17/24/3861
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