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LINDA WERTHEIMER, HOST:

This is MORNING EDITION, from NPR News. I'm Linda Wertheimer.

RENEE MONTAGNE, HOST:

And I'm Renee Montagne.

Today in Your Health, hospitals take on junk food. First, though, let's take on yogurt. As the yogurt section has expanded, so too have the claims about probiotics. The message that good bacteria is healthy for you has gained traction. Still, as NPR's Allison Aubrey reports, the science behind that message is tricky.

ALLISON AUBREY, BYLINE: If you want to understand the surge of interest in probiotics, Dr. Athos Bousvaros of Harvard Medical School says first, you have to picture the human gut. Our GI tracts are home to lots of bacterial cells - trillions of them, from all kinds of bacteria.

DR. ATHOS BOUSVAROS: And it's incredibly clear that these bacteria in our gut are not just innocent bystanders hanging out, you know. They, first of all, help us digest foods. They help make vitamins, such as vitamin K.

AUBREY: And good bacteria probably also help protect us against infections from harmful pathogens. The idea is that when good microbes colonize our guts, they help displace the bad ones.

BOUSVAROS: So I think that's all totally real.

AUBREY: Now, as scientists have learned more about the importance of beneficial microorganisms, or probiotics, inevitably, the question has become: How do we get more of these good bugs to set up shop in our guts? Some people have turned to yogurt, with the belief that the bacteria used to make yogurt is helpful. Others are trying specialty yogurts or supplements made with specific strains of probiotics. But Bousvaros says this is where it gets tricky. It's not necessarily clear how much of which kinds are helpful.

BOUSVAROS: We don't know, because the studies haven't been done.

AUBREY: And Kirsten Tillisch, a gastroenterologist at UCLA, has a similar take. She says there are hundreds of probiotic products on the market.

KIRSTEN TILLISCH: Most of them have never been studied, and they contain all different kinds of strains of bacteria. So what we don't know right now is: Do you have to take a specific strain, or does each person have a strain that they need? We just don't know.

AUBREY: Tillisch says when her patients are eager to try probiotics, she steers them to ones with some clinical research behind them. For instance, if the issue is irritable bowel syndrome, there's a probiotic called Align, made with a bifidobacterium.

TILLISCH: So there's been a couple studies of that that have shown in irritable bowel syndrome, specifically, that people do better.

AUBREY: And when it comes to probiotic yogurts, particularly brands such as Dannon's Activia, she helps set realistic expectations. The yogurt is made with a trademarked strain of probiotic called bifidus regularis, which supposedly helps regulate the digestive system. There is some research behind it.

TILLISCH: In those studies, people are having yogurt twice a day or three times a day, every day, and most people are not aware of that. They think, well, I'll have a yogurt once in a while and that'll make a difference. I don't know if having yogurt twice a week will make any difference at all.

AUBREY: And it's important to note that most of the research has been sponsored by yogurt makers. So the bottom line is that there are still more questions about probiotics than concrete answers. Lots of small studies point to possible benefits. Take, for instance, a recent analysis that found people who ate fermented foods like yogurt or took probiotic supplements were less vulnerable to upper respiratory illnesses.

Kirsten Tillisch says unraveling these connections will take years. And this includes her own research, investigating a possible mind-body connection. When she told me about it, it sounded a little out there. She's studying whether probiotics may be able to subtly influence mood or behavior. She says it's been shown in animals.

TILLISCH: Pretty dramatic effects can happen in animals when you change their gut flora.

AUBREY: She points to studies with rodents that document a connection between irritated or inflamed guts and anxious behavior.

TILLISCH: And so if you take an animal that has an inflamed gut and you give them a probiotic, they don't act anxious anymore.

AUBREY: It's too soon to say if this same gut-brain interaction can be documented in humans. But curious to test this, Tillisch recently completed a very small, preliminary study of healthy women. After taking probiotics, brain scans found that the women were less reactive when shown angry or sad faces.

TILLISCH: At the most basic level, we show that by changing the bacteria in the gut, we change the way the brain responds to environmental cues.

AUBREY: It's intriguing stuff, but it's early days. And Tillisch says she hopes to keep testing this theory with future studies.

Allison Aubrey, NPR News.

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