Think of Kefir as yogurt's tangy cousin, a fermented milk product native to the Caucasus Mountains of southeastern Europe that boosts gut health a few different ways. First, it's a probiotic, thanks to its live, active bacteria cultures. But it also does double duty as a prebiotic. "Kefir is made with oligosaccharides, a complex carbohydrate that feeds beneficial bacteria in the gut," says Rumsey. And since it's meant to be sipped rather than eaten with a spoon, kefir makes a good on-the-go snack. Look for a low-fat brand or variety with no added sugar, she says, and as with yogurt, make sure the label clearly states that it has live cultures.