Dear Doctor: What's the difference between probiotics and prebiotics? I'm really interested in how gut bacteria impact health, but I'm not sure I always understand the terminology.

Dear Reader: We agree with you that the information emerging about gut bacteria is fascinating. And as with any area of research, the language used to discuss it can be confusing.

Probiotics are live microorganisms that are meant to have health benefits for the person who ingests them. They work by joining the many trillions of bacteria already living within our bodies, primarily in the gut. Collectively known as the human microbiome, these beneficial strains of bacteria harbor a diversity of genes that dwarfs that of the human genome.

Thanks to the rapid evolution of genetic sequence technology, researchers are learning new information about the human microbiome every day. Thus far, probiotics show promise in the areas of immune function and various digestive and bowel disorders. Probiotics have also been used to help preterm infants acquire a beneficial range of intestinal flora, which can prevent colonization by adverse bacteria.

You can ingest probiotics in food or in supplement form. When it comes to food-based probiotics, the common denominator is fermentation. Foods like yogurt, kefir, apple cider vinegar, pickles, kombucha, sauerkraut, kimchee, miso and certain soft cheeses are all sources of different types and varying amounts of probiotics.

The idea is that by ingesting these microorganisms, the portion of them that survive the acids of the stomach will take up residence in the gut. Once there, though, they need their own food source to grow and thrive. And that's where prebiotics come in.

Prebiotics are indigestible carbohydrates that act as food for probiotics. While some foods offer a greater percentage of prebiotic material than others, when you eat a diet high in a variety of vegetables, fruit and leafy greens, you are inevitably giving your gut bacteria plenty to eat.

You'll find a motherlode of prebiotics in foods like leeks, asparagus, Jerusalem artichokes, bananas, onions, garlic, artichokes, jicama and honey. Also beneficial is something known as "resistant starch," which is the indigestible portion of starch. It can't be broken down by the small intestine, so it moves to the colon, where it is fermented by the microbiota.

Whole cereal grains, many seeds and green bananas are good sources of resistant starch. So are cooked potatoes and rice, but only when they have cooled. Just as with probiotics, you can go the supplement route for prebiotics if you choose.

If you decide you'd like to add probiotic supplements to your diet, we suggest you check with your family doctor for guidance. Although regulated by the FDA, probiotic supplements are treated as a food and not a medication. That means that manufacturers don't have the burden of proof that their products will live up to the claims printed on the label. Your doctor can help you figure out which supplements to choose, and the best dose for you to start with.

Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. 

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