Lactobacillus kefiri is a probiotic bacterium commonly found in kefir, a fermented dairy product. Its use for supporting "Leaky Gut Syndrome" (also known as increased intestinal permeability) is based primarily on emerging scientific interest rather than longstanding traditional use or robust clinical evidence. Several in vitro and animal studies suggest that various probiotics, including some strains of L. kefiri, may help improve gut barrier function by modulating tight junction proteins, reducing inflammation, and influencing the gut microbiota composition.

Specifically, L. kefiri has been shown in a few laboratory and animal studies to exert anti-inflammatory effects and to inhibit the growth of pathogenic bacteria (such as Salmonella and Shigella). Some studies indicate that kefir and its bacterial components can reduce gut permeability in animal models. However, direct clinical studies in humans with "leaky gut" or increased intestinal permeability are very limited, and most available research involves either mixed strains from kefir or animal models rather than isolated L. kefiri in human subjects.

In summary, while there is a plausible scientific rationale and some early preclinical evidence supporting the potential benefit of L. kefiri for gut barrier integrity, human clinical data are sparse. Therefore, the evidence level is rated as 2 out of 5, reflecting early but not yet definitive scientific support.