You don't necessarily need probiotics — a type of "good" bacteria — to be healthy. However, these microorganisms may help with digestion and offer protection from harmful bacteria, just as the existing "good" bacteria in your body already do.

Prebiotics are nondigestible carbohydrates that act as food for probiotics. When probiotics and prebiotics are combined, they form a synbiotic. 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:

• Treat diarrhea, especially following treatment with certain antibiotics
• Prevent and treat vaginal yeast infections and urinary tract infections
• Treat irritable bowel syndrome
• Speed treatment of certain intestinal infections
• Prevent or reduce the severity of colds and flu

Side effects are rare, and most healthy adults can safely add foods that contain prebiotics and probiotics to their diets. If you're considering taking supplements, check with your doctor to be sure that they're right for you.

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Oct. 15, 2014 See more Expert Answers