Lactobacillus fermentum is a probiotic bacterium that has been studied for its effects on gut health, including conditions associated with increased intestinal permeability (commonly referred to as "Leaky Gut Syndrome"). While the concept of "Leaky Gut Syndrome" is somewhat controversial in mainstream medicine, scientific research has explored the effects of probiotics, including L. fermentum, on intestinal barrier function.

Several animal and in vitro studies have shown that L. fermentum strains can enhance the expression of tight junction proteins (such as occludin and zonula occludens-1), which are critical for maintaining gut barrier integrity. For example, research published in the journal Gut Microbes (2016) demonstrated that L. fermentum administration reduced gut permeability and inflammation in rodent models. Additionally, a 2015 study in Beneficial Microbes found that certain strains of L. fermentum could attenuate intestinal damage induced by antibiotics or inflammatory agents.

However, direct evidence in humans with "Leaky Gut Syndrome" is limited. Most clinical studies focus on related conditions, such as irritable bowel syndrome (IBS) or inflammatory bowel disease (IBD), where some improvements in symptoms and markers of gut permeability have been observed with multi-strain probiotics containing L. fermentum. Therefore, while there is a scientific rationale and some supportive preclinical data, robust clinical evidence in humans with leaky gut is lacking, justifying a modest evidence rating.