Impact of Fermented Dairy on Gastrointestinal Health and Associated Biomarkers.

In this narrative review, we examined observational and randomized controlled trials investigating the effects of fermented dairy foods, including yogurt, fermented milk, kefir, and cheese, on gastrointestinal (GI) symptoms and/or GI biomarkers of health. Studies that recorded GI symptoms such as intestinal discomfort, flatulence, gastroenteritis, diarrhea, and constipation were included. GI health biomarkers encompassed measurements of intestinal integrity or permeability (eg, endotoxemia, zonulin, 2-arachidonoylglycerol), immune responses (eg, TNFα, IL-6, high-sensitivity CRP [hs-CRP], IFNγ, IL-1β, CCL5, TGFβ, IL-10, secretory IgA, α- and β-defensin, and LL-37), fecal microbiota, and fecal short-chain fatty acids (SCFAs). Studies on probiotic-containing fermented dairy foods were included if the primary focus was the fermented dairy food, not specific probiotic strains. Thirty-seven reports met the inclusion criteria and encompassed studies on healthy children, healthy adults, or individuals with underlying conditions. Twenty-one of these studies included fermented dairy products with probiotic strains. No study reported a harmful impact of fermented dairy on gut health. Ten studies reported no benefit of fermented dairy on GI symptoms or immune biomarkers compared with milk or when no dairy was consumed. The remaining studies described significant changes in one or more gut symptoms or biomarkers with fermented dairy intake. Improvements in GI symptoms, such as abdominal pain or discomfort, flatulence, constipation, and IBS severity, were found in most studies for which such symptoms were assessed. Reductions in intestinal inflammatory markers, specifically serum TNFα levels, were found to be associated with fermented dairy intake. In several trials, significant alterations to the gut microbiota or increased levels of fecal SCFAs following fermented dairy intake were measured, but not all of those studies incorporated clinically relevant outcomes. New investigations evaluating the impact of fermented dairy on gut health should build upon the findings of these prior studies, considering target populations, underlying health conditions, and relevant gut health end points.