Knowing the difference between a prebiotic and a probiotic will help your health, says columnist Carrie Dennett.

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Do you know the difference between a prebiotic and a probiotic? You should, because that knowledge could benefit your health — from the inside out.

Your gut — or intestine — is home to a population of beneficial bacteria species (your microbiota). Think of it as an ecosystem living inside your body’s “inner tube.” Your microbiota affects more than just digestive health: an imbalance of “good” and “bad” bacteria has been linked to chronic inflammation, obesity, some cancers and a weakened immune system.

About 80 percent of the body’s immune cells live in the gut. From birth, the immune system and microbiota help each other develop and mature. Your microbiota is as individual as you are, like a bacterial fingerprint — and it’s influenced by the food you eat.

Prebiotics are food for the health-promoting bacteria in your gut. You need good food to thrive — so does your microbiota. Probiotics are foods that contain live friendly bacteria. They can help maintain the balance between your intestinal environment and your immune system.

The most common probiotic species are Bifidobacteria and Lactobacilli. They’re added to milk to make yogurt and other fermented dairy foods like kefir (you’ll see them on the ingredient list). You can sometimes find them in fermented nondairy foods like tempeh, miso, kimchi, kombucha tea and sauerkraut.

Trouble is, many fermented foods don’t contain living beneficial bacteria. When you buy yogurt, make sure it has a “Live and Active Cultures” seal. If the label just says “made with active cultures,” the bacteria may have been killed by heat processing.

Prebiotics are nondigestible carbohydrates, often called “functional fiber.” They make it through your stomach and small intestine undigested, but once in the large intestine, your friendly bacteria feed on them. Resistant starch is found in beans, underripe bananas, whole grains and cold cooked potatoes.

Fructans (inulin and oligosaccharides) — found in vegetables and fruits, such as onions, garlic, leeks, asparagus, bananas, raisins, rye, barley and wheat — are good for you, but too many can cause intestinal discomfort, so eater beware.

Your microbiota also has enemies. “Broad-spectrum” antibiotics aren’t particular about which bacteria they kill. A high-fat diet can harm your population of Bifidobacteria. Plus, stress, disease and injury affects intestinal functions.

Should you take probiotics in pill form? If you’re healthy, probably not. The health claims often exceed the evidence. Nurture the good bacteria you have with prebiotic-containing foods and give the population a boost with probiotic foods. If you have irritable bowel syndrome (IBS) or a history of antibiotic use, talk to your doctor. If you could benefit from probiotic supplements, it’s important to get one with bacterial strains you need.

Next time: Five unhealthful foods that aren’t.

Carrie Dennett:

Dennett is a graduate student in the Nutritional Sciences Program at UW; her blog is