[ARTICLE] Kefir and Kombucha: are health promises justified?

Fermented beverages, such as kefir and kombucha, are gaining increasing place on the shelves of supermarkets, parallel to increasing research on their supposed health benefits. Are they substantiated by scientific knowledge? 

Gut microbiota and metabolic health 

Kefir is a fermented drink made from milk or fruit juice, traditionally consumed in Russia, Eastern Europe, Serbia and Turkey, and is renowned for its positive effects on gut health. Fermentation is obtained through the action of many microorganisms, including lactic bacteria and yeast. At least transiently, kefir consumption enhances microbial diversity in the gut, considered as a marker of overall health (1). Kefir’s metabolic benefits have also been suggested. A randomized controlled study reveals that a 12-week consumption period significantly increased high density lipoproteins levels, as compared to non-fermented milk, reflecting improved cardiovascular health (2). However, a recent review of literature focused on kefir’s therapeutic potential in managing metabolic syndrome and type 2 diabetes, could not draw conclusions due to insufficient or non-robust data (3). 

Kombucha, said to have originated in Mongolia, is a beverage fermented from sweetened tea by a symbiotic culture of bacteria and yeasts. It exhibits remarkable anti-inflammatory and antioxidant properties. It helps reducing inflammatory markers and positively modulating gut microbiota (4). A recent randomized study suggests that kombucha consumption favorably impacts glucose and lipid homeostasis in healthy subjects fed a Western diet, via changes on gut microbiota (5). 

Oral and gastrointestinal health 

Benefits of kefir and kombucha for oral health have also been highlighted, as they contribute to the balance of oral microbiota (6). Probiotics, organic acids and polyphenols from these beverages help control the proliferation of pathogens (such as Streptococcus mutans) and the formation of biofilm, involved in tooth decays and periodontal diseases. Antioxidants also participate in decreasing gingival inflammation. However, the acidity of these beverages can contribute to erosion of tooth enamel if consumed excessively (7). Kefir also reduces gastrointestinal symptoms in healthy subjects (8). As a complementary intervention, it may help eradicate Helicobacter pylori, responsible for stomach ulcer (1). 

Neurological and cognitive function 

Some research suggests a link between fermented beverages and improved cognitive function. In a randomized cross-over study comparing kefir to an isocaloric control, the fermented dairy beverage was shown to improve relational memory in healthy adults while strongly increasing the relative abundance of Lactobacillus, suggesting the involvement of the gut-brain axis (9). Furthermore, kefir may prevent neurodegenerative diseases by reducing risk factors such as oxidative stress, hypertension and systemic inflammation, as shown in animal models. However, the effects of fermented beverages on cognitive functions in patients with Alzheimer’s disease remain to be demonstrated (10). 

Conclusion 

Current data on kefir and kombucha suggest potential benefits linked to their probiotics and bioactive metabolites. Their ability to improve metabolic, gut, oral and cognitive health could make them valuable allies in a balanced diet. However, their effectiveness remains based on preliminary evidence, often limited by small sample sizes and heterogeneous methodologies. Formulations of kefir and kombucha vary widely (ingredients, fermentation duration, microbial strains), complicating the uniform evaluation of their effects. While these fermented beverages offer intriguing prospects for health, robust and well-controlled clinical studies are essential to validate their benefits, better understand their mechanisms of action and explore their long-term effects. 

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