What Drinking Kefir Really Does to Your Gut and Oral Microbiome

What Drinking Kefir Really Does to Your Gut and Oral Microbiome

A comprehensive review of human studies shows that kefir can alter certain gut and oral bacteria. However, due to inconsistent results and small studies, the actual impact on health remains an open scientific question.

Study: The Effects of Kefir on the Human Oral and Gut Microbiome. Photo credit: xamnesiacx84/Shutterstock.com

Scientists recently reviewed the existing literature on the health effects of kefir consumption and published their findings inNutrients. Kefir consumption may be associated with changes in the balance of microbiota in specific niches, which could support digestive, immune and metabolic health.

Exploring Kefir: From Tradition to Science

Kefir is a probiotic milk drink that dates back more than 3,000 years to the Caucasus. It is made from kefir grains, which are complex, symbiotic clusters of lactic acid bacteria (LAB), acetic acid bacteria (AAB), and yeast held together in a polysaccharide matrix. When added to milk, these grains trigger a fermentation process that thickens the milk and gives it a slightly sour taste. Although kefir is primarily made from cow's milk, it can also be made from goat's or sheep's milk and soy milk.

Commercially, kefir is made by inoculating milk with kefir grains in a ratio of 1:30 to 1:50; Then let it ferment at room temperature for up to 24 hours. After filtering out the grains, the newly made kefir can be consumed immediately or stored at low temperatures for later use.

The health benefits of kefir largely depend on the substrate used for fermentation. A key challenge in kefir research is the inherent variability in composition, which can vary significantly due to numerous factors, including the microbiota of the starter grain, fermentation time and temperature. These variables shape the final mix of microbial species, their relative abundance, and the content of bioactive metabolites in the beverage.

In kefir research, findings from one study may not be directly applicable to another because different kefir preparations may affect the oral microbiome in different ways. Therefore, evidence of changes in the gut and oral microbiome after kefir consumption can vary widely.

Microbes in kefir and their health effects

LAB, such as Lentilactobacillus kefiri, Leuconostoc mesenteroides and Lactococcus lactis, ferment milk substrates by metabolizing lactose and producing lactic acid. In addition to lactic acid, these microbes produce bacteriocins, cathelicidin, carbon dioxide, acetaldehyde, and hydrogen peroxide, which have the potential to attenuate or eliminate common intestinal pathogens.

L. kefiri and L. mesenteroides can survive passage through the intestine and adhere to the epithelial lining, which is an essential feature of effective probiotics. These species have antibacterial and antifungal effects in the intestine. In particular, L. kefiri can bind toxic metals and mycotoxins, highlighting its potential for applications in emergency toxicology. L. mesenteroides produces linoleic acid, which has antiatherogenic, anti-inflammatory and anticarcinogenic effects. L. lactis is often referred to as a “cell factory” due to its proven potential to deliver therapeutics and vaccines.

The proportion and occurrence of individual bacterial species in kefir can vary significantly depending on the region, substrate and manufacturer.

AAB, including Acetobacter fabarum, Acetobacter lovaniensis, Acetobacter orientalis, Gluconobacter oxydans and Gluconobacter liquefaciens, have also been identified in kefir. Acetic acid and its metabolites produced by these bacteria can increase ileal motility, improve colonic blood flow, and help maintain epithelial homeostasis.

Yeasts such as Saccharomyces cerevisiae, Kluyveromyces marxianus and Kluyveromyces lactis are contained in kefir. These yeasts produce ethanol and carbon dioxide and give kefir its distinctive taste and mild tanginess. Numerous studies indicate that Saccharomyces cerevisiae var. boulardii possesses antimicrobial, antioxidant, anticarcinogenic and anti-inflammatory properties that may help with conditions such as irritable bowel syndrome and Crohn's disease.

Consuming kefir changes the gut and oral microbiome

Previous studies have shown varying effects after consuming kefir. For example, some healthy adults showed a slight, non-significant increase in Lactococcus raffinolactis, while those with metabolic syndrome or IBD showed an increase in Actinobacteria and Lactobacillus, respectively. In critically ill patients, kefir increased the gut microbiome wellness index despite reduced diversity. Women with PCOS experienced a significant increase in Bacilli frequency and a statistically significant improvement in physical function and mental health compared to pretreatment.

Kefir consumption alters the amount of LAB in the gut microbiome, which may contribute to gut changes and systemic health outcomes. For example, research found that participants with metabolic syndrome in the kefir group, even with a small increase in Lactobacillales, had positive changes in fasting insulin, TNF-α, IFN-γ and blood pressure within the groups.

Kefir consumption reduced salivary levels of Streptococcus mutans, an important cause of dental caries in adults and children. However, only four studies reported the effects of kefir on the oral microbiome, specifically a reduction in saliva production of S. mutans and Lactobacillus. However, a major limitation in all four studies was the use of culture-based methods, which only detect certain bacteria and do not provide a complete picture of the oral microbiome. None of the studies used DNA sequencing, so the broader effects of kefir on oral microbial diversity remain unknown.

Conclusions

Kefir consumption may impact both the gut and oral microbiome, but the magnitude, consistency, and clinical relevance of these effects remain uncertain.

Different sources of kefir, inconsistent study designs, and the lack of standardized products make it difficult to draw firm conclusions about its specific effects and long-term benefits. Well-controlled, larger and longer-term studies will clarify the true influence of kefir on the microbiome and the associated health consequences.

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