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If you eat yogurt, the phrase live and active cultures may ring a bell. It refers to good bacteria, or probiotics, thought to promote health. The buzz around probiotics has lead to hundreds of new foods and supplements, as NPR's Allison Aubrey reports.

ALLISON AUBREY reporting:

The idea that fermented mild might aid digestion and immune function goes back centuries. There are biblical references. And in the early 1900s, a Russian Nobel prize winner promoted yogurt rich in one strain of bacteria that he believed to be good for health.

Today, food companies and supplement makers are trying to do the same. physician Robynne Chutkan says with so many new products, she's had to go to the grocery store to do her homework.

Dr. ROBYNNE CHUTKAN (Gastroenterology and Internal Medicine, Georgetown University Hospital in Washington): Actually, here it is in the diary section.

AUBREY: A row of specialty yogurts containing probiotics. Chutkan is a digestion specialist with Georgetown University. She says her patients are asking about these products.

She picks one up made by Dannon, called Activia.

Dr. CHUTKAN: Well, a probiotic by definition is any substance that contains live organisms that, when ingested, has a beneficial effect on the host by altering the body's intestinal microflora.

AUBREY: Basically, helping the gut replace bad bacteria with good. At least this is the theory. It may sound simple, but the challenge is figuring out which combination of bacterial strains - at which doses - might be effective. Chutkan tells her patients not to assume all products will be helpful, since they don't need FDA approval. Instead, she urges them to be skeptical, ask questions.

Dr. CHUTKAN: Number one, has this been studied? Or did a manufacturer give it to ten of their best friends, and guess what, all ten said this really helps.

AUBREY: Anecdotal accounts aren't good evidence.

In the case of Dannon Activia, which contains a strain of bifido bacterium, she's learned the product has been studied. In one trial, healthy volunteers were broken down into two groups: one ate authentic Activia while another ate an inactive version of the product where the bacteria had been killed off by heat.

At the end of the study, the volunteers eating the authentic stuff had a 21 percent decrease in something called colon transit time. This means that food passed more quickly out of their bodies. Chutkan says this may reduce the discomfort of constipation and…

Dr. CHUTKAN: From a theoretical point of view, we think moving substances through the colon quickly is important for good health.

AUBREY: So when patients ask her about Activia now, she can't tell them its superior to other products with bifido bacterium, but she does tell them that there's some evidence the product contains enough good bacteria.

Dr. CHUTKAN: For most claims, we need five to ten billion live bacteria.

AUBREY: To see an effect. Many probiotics, she warns - particularly those sold as non-refrigerated pill form supplements - probably don't contain this much.

Dr. CHUTKAN: Because many of these bacteria are inactivated by heat, by oxygen, by moisture.

AUBREY: Making them ineffective. The extent to which products are studied varies greatly. Most good research - testing probiotics as a medical treatment for bowel or allergy immune disorders - have used either Lactobacillus GG, sold under the name Culturelle, or one called ViaCell III.

In one study completed a few years ago, researchers gave Lactobacillus GG to pregnant women with a history of allergies, and then to their infants. They found the babies developed a significantly lower rate of allergic eczema than the control group that did not take probiotics.

Harvard Gastroenterologist Athos Bousvaros says the findings are very intriguing, but they also raise many more unanswered questions.

Dr. ATHOS BOUSVAROS (Gastroenterologist, Harvard Medical School): For example, is that affect on allergy a long-lived effect? Or as soon as you stop taking the probiotic, you know, does the eczema come back, for example?

AUBREY: Bousvaros' own research testing probiotics on kids with well-managed cases of Crohn's disease found the probiotics did not reduce GI flare-ups. He hopes to do follow-up research on a different population of patients that may benefit more.

For now, he tells patients that probiotics probably can't hurt, and might - he stresses, might - help.

Allison Aubrey, NPR News, Washington.

YDSTIE: And a correction. We aired a report last week that said a doctor had found evidence of heart disease in the small arteries that feed blood to the major ones. Well, that's actually backwards. Blood flows from the major arteries of the heart into the smaller ones.

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