A comparative review of microbial composition and health benefits of “milk-based” and “water-based” kefir grains

As a traditional fermented food, kefir is produced by symbiotic microbial communities in kefir grains, and its composition and functionality are strongly shaped by the culture medium. The main types are milk-based kefir grains and water-based kefir grains, the latter commonly cultivated in sugar water with various additions and widely consumed in some South American regions. Given the substantial nutritional differences between milk and sugar water, these two kefir types likely differ in microbial community structure, metabolic activity, and health-related properties. However, current research remains heavily biased toward milk-based kefir, with systematic investigation of water-based kefir still limited. In addition, claims that milk-based kefir is superior in health benefits are often based more on assumptions about its nutrient-rich medium than on direct comparative evidence. To address this gap, this review systematically summarizes and compares the available literature on the microbial composition and documented health benefits of milk-based and water-based kefir grains. The evidence suggests that milk-based kefir generally exhibits greater microbial diversity and a richer metabolite profile. This comparison clarifies the distinct characteristics of the two kefir types and provides a reference for optimizing traditional fermentation processes and guiding future research and applications.