Fermented dairy products, such as yogurt and kefir, are considered synbiotic because they contain live bacteria and the fuel they need to thrive. Probiotics are found in foods such as yogurt, while prebiotics are found in whole grains, bananas, onions, garlic, honey and artichokes. In addition, probiotics and prebiotics are added to some foods and available as dietary supplements. Although more research is needed, there's encouraging evidence that probiotics may help: 1  Treat diarrhea, especially following treatment with certain antibiotics.