The probiotics found in yogurt, kefir, kimchi and other foods have been marketed for years as healthful, though it wasn't clear why or how.
Researchers at the University of Maryland School of Medicine think they may have an answer. In research published in the April edition of the journal mBio, scientists including Claire M. Fraser, professor of medicine and director of the Institute for Genome Sciences, found that one type of probiotic may modify the way other organisms work in the gut and may reduce inflammation, which is linked to cancer and many chronic diseases such as diabetes.
"We have to remember that when we eat we're not just feeding ourselves but feeding all the millions of microbes that live in our gut," Fraser said. "And we know which types of food feed the beneficial microbes in our gut. Not surprisingly, they tend to be fresh fruits and vegetables, complex carbohydrates that you find in grains. It's not sugar, it's not high fat."
Probiotics are live microorganisms found in fermented foods. (They are also available in capsule form.) Previous research has suggested they may help treat gastrointestinal illnesses, boost immunity, and prevent or slow the development of certain types of cancer, according to the National Institutes of Health.
The researchers tested 12 healthy people between the ages of 65 and 80, feeding them the bacteria Lactobacillus rhamnosus GG, often known as LGG, twice a day for 28 days. They found that in some of the subjects, ingesting LGG appeared to make other microorganisms in the gut better able to reduce inflammation.
Fraser said the LGG appeared to turn on what she described as "outboard motors" on organisms in the gut that produce butyrate, an anti-inflammatory fatty acid. Those motors allow the butyrate-producers to bury themselves more deeply into the mucus lining the gastrointestinal tract.
The finding that LGG changes the way other microbes in the gut works was novel, according to the researchers at University of Maryland School of Medicine, though other research suggests LGG also may have a direct impact on the body.
"We're really delighted that we got an interesting relevant result, when a lot of people said you're going to see a slight change in the organisms and it'll go away and nobody cares," said Patricia L. Hibberd, chief of Massachusetts General Hospital's Division of Global Health, who collaborated with Fraser. "We didn't see a change in the bugs, we saw a change in what they're doing."
Mary Ellen Sanders, executive science officer for the International Scientific Association for Probiotics and Prebiotics, said the study "breaks new ground and moves the field forward."
"It's a nice study and shows that we need to think about probiotics as microbes that are transient components of a larger microbial ecosystem, and that they can have their effects in more subtle ways than just changing the populations of resident microbes," Sanders said. "It's important to understand mechanisms because it will help fine tune appropriate usage characteristics as well as give us something to assess when something changes."
Still, much is yet to be understood about how probiotics, which consist of many types of bacteria, work in the body.
Dannon, the maker of Activia yogurt, which includes a probiotic under the trade name Bifidus Regularis, agreed to settle Federal Trade Commission claims in 2010 that its marketing was deceptive and exaggerated the health benefits.
Fraser said consumers should be cautious about the marketing of probiotics with "vague" claims.
"I think it's important for consumers to understand, if they were able to say something more specific, they would be," she said.
Fraser also said that LGG is one of many types of probiotics widely available and that more research was needed to determine how other probiotics worked.
"If you go into the dairy aisle in the grocery store, or the digestive section in the pharmacy, you can find a lot of probiotics," she said. "We don't know any more about the rest of these than we knew about LGG then we did at the time we got involved in this study."
Added Hibberd: "It's really hard to generalize from one probiotic to another."
Scientists also need to study more the difference in how probiotics work when eaten in foods like yogurt and when taken in capsule form, Hibberd said.
Fraser said consumers may see some benefit to ingesting probiotics, and said people should eat healthy to maintain a healthy gut. Antibiotics are believed to have an adverse effect on gut health, and Fraser said doctors and patients should be aware of the harm of overprescribing them. Gut health also has been linked to brain function and mood, she said.
Fraser is working on a larger, randomized trial to test probiotics on participants of various ages, and Hibberd said she hopes that will bear fruit in a few months.
"I think this is probably promising data that really needs this next analysis," Hibberd said. But, "I've got a reason perhaps to believe that there's some benefit to reducing inflammation."