Does Probiotic Yogurt Really Affect Digestion?

Reporting in Science Translational Medicine, researchers write that the bacteria in yogurt affect people's digestion—but not by repopulating gut flora. Microbiologist Jeffrey Gordon talks about these findings and the future of using bacteria as therapy for digestive disorders such as diarrhea.

JOHN DANKOSKY, HOST:

This is SCIENCE FRIDAY. I'm John Dankosky.

Next up, our microbial hour continues, but this time with some friendly bacteria, like the ones in yogurt. Now, I've always thought that eating yogurt was good for my gut because it lets those good bacteria set up shop, warding off the bad guys by out-populating them. At least that's what I thought. Well, maybe not, because my next guest has found that yogurt bacteria don't actually re-colonize your gut, but they do alter what's going on down there as you digest your food. That research appears in the journal Science Translational Medicine.

How important are these bacteria to our digestion? Might certain strains help us get more nutrients from what we eat? If you have questions, 1-800-989-8255. 1-800-989-TALK.

Joining me now is Jeffrey Gordon, a professor of pathology and immunology at the Washington University School of Medicine in St. Louis. He's also director of the school's Center for Genome Sciences. Welcome to SCIENCE FRIDAY, Dr. Gordon.

JEFFREY GORDON: Thank you for having me.

DANKOSKY: So you first studied yogurt bacteria in mice. How did you do that?

GORDON: Well, let's have a few definitions to begin with. Yogurt refers to a product obtained from fermentation of milk by cultures of bacteria. Common bacteria in yogurts are streptococcus thermophilus and, another one with a bigger name, lactobacillus. We wanted to understand the effects of yogurt strains, as you said, on both the structure and the operations of our gut microbes, and we set up an experiment with two arms.

On the one hand, we had mice that were raised under sterile condition. They're called germ-free mice. In a particular stage of their life, we introduced a collection of 15 normal members of our human gut community. By the way, their genomes were completely sequenced, so we knew all the genes in this model community of our human gut bacteria. And we studied these mice for a time, looking at the operations of this community, and then assessed the impact of introducing the bacterial strains that are present in a popular, commercially available fermented milk product.

At the same time, we were studying a group of adult, healthy identical twins. And we followed them for a period of time, sampling their gut communities, and then administered the same commercially available product and looked at the impact of consuming that product over time, and then had them all stop consuming the product and looked to see whether the live bacterial strains associated with the product disappeared in a predictable way.

DANKOSKY: Oh, so...

GORDON: So that was the setup.

DANKOSKY: That's the setup. What else did you find?

GORDON: Well, we found in both the mice and in the humans that consumption of these live bacteria didn't really disrupt the representation of preexisting bacteria in the guts of humans or mice. It didn't affect the representation of genes. Rather, it affected the way these communities operated, specifically the pattern of expression of microbial genes involved in certain aspects of metabolism, most notably the metabolism of complex sugars, polysaccharides.

And what we saw in mice we also saw in humans, which gave us encouragement because we felt that, in this field, where lots of claims are made about the health benefits of probiotics, it's very important to be able to establish a rigorous testing scheme, where we could, under very controlled conditions, look at the impact of these strains on gut communities, take the lessons learned from these models and then apply it in an informed and directed way to humans.

DANKOSKY: Just so we understand, when I eat a little bit of yogurt, the bugs that are in the yogurt, the microbes that are in there, are just a tiny, tiny fraction of how many microbes are in my gut. Maybe you can explain the scale here.

GORDON: Absolutely. So a typical helping of yogurt will have a few billion bacteria. Now, of course that's a large number. But consider for a moment the fact that our human guts are home to tens and tens of trillions of microbes, largely bacteria. So, really, it's a small fraction of the population.

And what we noted in mice and what we noted in humans shouldn't be surprising. We dosed the mice at a level that's equivalent to the dosing in humans, and there wasn't a change in the overall architecture, the structure, of these communities. Rather, through processes that we don't fully understand, there was a form of communication between the yogurt strains and the normal resident bacteria in the gut. And that communication resulted in a change in the properties of the gut communities; notably, it affected the way these communities processed polysaccharides. Why is that important? Well, polysaccharides, which are common components of our diet, are molecules, large molecules that have complex chemical linkages. We don't have the dining utensils necessary to break down these polysaccharides on our own. Rather, we depend upon our gut bacteria to mobilize these utensils - they're actually enzymes - to break down polysaccharides in ways that are useable to us and to them. So we see...

DANKOSKY: And what are the polysaccharides we're breaking down again?

GORDON: Well, in this particular case, the change involved a series of polysaccharides that are common ingredients of our foods. I'll give you their names. Xylans, these are present in fruits and vegetables, milk, honey, wheat. Pectins, they're prominent in apples, plums, oranges, carrots. They are the jelling agent used in jams and jellies. And the other type of polysaccharide, again, common in our diet can be classified as fructans. They're present in wheat, barley, garlic, onion, asparagus. And again, these are normal components of our diets. These are components that we depend upon our gut microbes to break down. And the introduction of these yogurt strains appeared to improve the capacity to break down these components of our diet.

They not only appeared at the level of gene expression, but we actually measured the products of digestion in these mouse models, and they were very informative. They confirmed what we saw at the level of gene expression and told us that, yes, in fact, a small number of bacteria ingested can affect the properties of a gut community.

DANKOSKY: But - yeah.

GORDON: Of course, once the twins stopped consuming yogurt, the strains in the yogurt rapidly cleared from the system.

DANKOSKY: Yeah. So how long do the effects last? And how quickly do they get cleared?

GORDON: Well, in all cases, within about two weeks, the bacterial strains that were contained in this fermented milk product fell to below the limits of detection in each of the seven sets of twins that we studied.

DANKOSKY: So to get any gut benefit here, does it matter how much yogurt I eat? Is there a minimum effective dose to actually do what you're getting at here?

GORDON: There probably is some variations in the amount of bacteria from the ingested yogurt that are present in your gut at the time you're consuming it. We really don't know the question of dosing. We really don't know the effect of the diets of the people who are consuming the yogurt on the effects of the yogurt strains in digestion of other types of food in our diets. We hope that these mouse models will allow us to answer the questions of whether a particular type of yogurt or fermented milk product has similar effects on people having different dietary habits, different ages - the types of questions that are going to be important to get answers to if we rigorously assess some of the health claims that are made by manufacturers of probiotics.

DANKOSKY: We're talking with Jeffrey Gordon from Washington University School of Medicine in St. Louis. You can join us at 1-800-989-8255 or 1-800-989-TALK. Let's go to Chris in Springdale, Arkansas. Hi, Chris.

CHRIS: Hi. How are you guys doing today?

DANKOSKY: Good. What's up?

CHRIS: Well, I had a question about the different types of yogurt. I know when you walk into a yogurt aisle at any Wal-Mart or any kind of big-box store, there's so much selection. I was wondering if there's one type of yogurt that's better for you than another, and really what it is in the yogurt that we should looking for in the label to find out what it is really that gives us these probiotic benefits.

GORDON: Well, that's a wonderful question and one of the reasons that we embarked on this set of experiments. We really wanted to create an analysis pipeline, where we could use models - in this case, an animal model where we reconstructed a human gut community - to answer some of the questions that you ask. We do know that individuals vary in terms of their collection of gut microbes. Even genetically identical twins have somewhat different collections of gut microbes. We know that diet plays an important role in shaping the structure and operations of these communities. There are different types of yogurt. There are different types of fermented dairy products. As I said in the beginning of this episode, there are - a minimum of two types of bacterial strains that are required to be present in a fermented milk product in order for it to be labeled yogurt.

There are some types of products that have more than these two strains. You can also, as you know, go to supermarkets and various stores and pick up probiotics that have a variety of different components. We really don't have sufficient information to answer the question you have asked. We do, we think, have a set of tools in a new toolbox to address this. How do foods and how do gut bacteria interact with one another? Is the nutritional value of food influenced, in part, by the microbes we normally harbor? Can it be further modified by these live microbes we ingest deliberately? And in the future, if we open up a medicine cabinet in the 21st century, can we find - should we discover a series of new probiotics that can enhance the nutritional value of the particular diets that we consume?

DANKOSKY: Well, you mentioned the off-the-shelf probiotics. I know a lot of people are interested in that. If you take probiotics in capsule form, is it different somehow than eating yogurt in the morning?

GORDON: That also is a great question. And just relating to the episode that preceded this one, it's going to be very important for the formulation of these products to be carefully validated. Is there a set number or an indicated number of live microbes in that formulation as advertised? Do we know the genome sequences of the bugs that are contained in these products? Is manufacturing such that from lot to lot we have consistency? I know that issue of consistency is taken very seriously by the manufacturers of a number of yogurts. But as you indicate, probiotics are sold widely, they're advertised having very - a variety of different health effect. And for those claims to be validated, we'd need the types of tools that we described in this study and others.

DANKOSKY: I'm John Dankosky. And this is SCIENCE FRIDAY from NPR. The yogurt company Dannon, which partially funded this research that we're talking about today, recently settled a suit for claiming on its packaging that yogurt can improve digestion and immune system. What do you think of labels like this? Does this study back up these claims?

GORDON: Well, actually, Dannon funded part of our research in order to construct this type of analysis pipeline, to test the types of claims that are being made, not only by themselves but by others. I do think that this particular study indicated that there's an effect on the digestion of a component of our diet, polysaccharides. There have been other studies in mice, for instance, that shows that certain consortium of bacteria that are found in fermented milk products, including products made by Dannon, may modulate immune function in a way that would be beneficial, at least in the setting of colitis.

So I think that there's much to learn. I think we have to be very rigorous in terms of testing the claims in order for the public to gain additional trust that we should be equipped to address the complexity of our gut microbial communities in the form of representative animal models, learn from those models, and then design, execute and carefully interpret clinical studies. A lot of public will know.

DANKOSKY: Yeah. Lyle is in Eagle, Michigan. Lyle, a quick question for the doctor?

LYLE: Yeah. After reading the China study, I quit eating dairy and meat. And I was real curious and overjoyed to hear about the billions of bacteria I have in my belly. Good to hear that because I was kind of concerned that because I was not going to eat any more yogurt that maybe there was a challenge. I never really had any issues. I was wondering if your research has gone that way with people that didn't eat any meat or dairy, and how their bacteria as well as digestions have been affected.

DANKOSKY: Yeah. How is Lyle's gut, Doctor?

GORDON: Well, I don't know, Lyle, but thank you for sharing your personal story with me. Lyle, I'll tell you something. Your thought is a very important one because there is an emerging set of observations, in part, made by our group and others, that diet has a huge effect in shaping the structure and operations of your gut communities. We've studied many different mammalian species, including humans, to look at the impact of different diets on how our gut communities are configured. And not billions, Lyle, but trillions of microbes live in our gut. And people on different diets have different gut structures. And when they switch diets, the representation of members of your gut community will change.

It's part of an important adaptation, part of the fitness. We have to learn how to digest the foods that we eat. As humans, we change what we eat over time. What is the code that relates the nutritional value of what we consume in the structure and operations of our gut communities? That's going to be a very important issue to address because looking forward, we heard this week that the population of our planet has reached seven billion humans, by 2050, 9 billion humans. What types of crops we plant, what kind of recommendations we make about what to eat in the future will be informed by deeper knowledge of the operations of this vast collection of microbes that live inside of us.

DANKOSKY: Jeffrey Gordon, you eat yogurt every day?

GORDON: I don't eat yogurt every day, but I do eat yogurt intermittently.

DANKOSKY: OK. And it has any health benefits for you?

GORDON: Not noticeable, but I enjoy the experience.

DANKOSKY: Excellent. Jeffrey Gordon is professor of pathology and immunology at the Washington University School of Medicine in St. Louis. He's also director of the school's Center for Genome Sciences. Thank you so much for joining us. I appreciate it.

GORDON: My pleasure.

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