THURSDAY, Sept. 1 (HealthDay News) -- You are what you eat -- especially when it comes to the microbes that live in your gut.

New research shows that people who eat a diet that's high in fats and animal proteins have a certain group of bacteria that flourish in their digestive tract, while the guts of people who eat a more plant-based, higher carbohydrate fare favor other microbes.

What that means for human health is still unknown. But there's increasing evidence that the "microbiota" that live in the human gut may play an important role in health, including possibly contributing to obesity and other ailments, researchers said.

The findings are published in the Sept. 1 issue of Science.

In the study, researchers asked 98 healthy, non-obese America adults to report on their usual diet and the diet they ate in the week prior to giving a stool sample. From each sample, researchers then isolated the DNA of the bacteria present.

The analysis showed that participants could be generally grouped into one of two categories, or "enterotypes", based on the prevalence of certain species of bacteria in the gut. People in the first group had high levels of the bacteria Bacteroides. In type 2, Prevotella was more prevalent.

"You could see the people who consumed more animal protein and fat tended to fall into an enterotype characters by Bacteroides, whereas those who tended to have a diet high in carbohydrates [more plant-based] fell into an enterotype characterized by Prevotella," said study co-senior author Dr. James Lewis, a professor of medicine and epidemiology at University of Pennsylvania Perelman School of Medicine.

In a second experiment, researchers had 10 participants, all of whom fell into the Bacteroides group, stay in a research lab for 10 days. Both groups were fed an identical diet and an identical amount of calories, with one exception: one group was put on a high fat/low fiber diet, while the other group was put on a low fat/high fiber diet.

The dietary change did impact bacteria levels in the gut, the study found, but not enough to move the Bacteroides group into the Prevotella group.

That suggests that long-term dietary habits, rather than any short term changes, have a bigger impact on gut microbiota, Lewis said.

The next step for researchers is getting a better handle on how the bacteria that resides in our gut may influence the development of disease, said Justin Sonnenburg, an assistant professor of microbiology and immunology at Stanford University School of Medicine. He praised the researchers for being able to correlate specific enterotypes with actual human diets.

Though no one has yet proven a cause-and-effect relationship, researchers have linked altered microbiota with many diseases and conditions, including obesity, inflammatory bowel disease, irritable bowel syndrome and potentially colorectal cancer.

What's almost certain is that gut microbes play a significant -- and underestimated -- role in human health, he added.

One theory is that our immune systems may react to certain bacteria in the gut, triggering an inflammatory response that could contribute to several diseases, Lewis said.

"There's also a whole another line of research that's looking into to what extent the bacteria living in our intestines is related to the host's risk of becoming obese, perhaps by influencing the efficiency of absorbing nutrients," he said.

It's known that the bacteria living in the gut help humans harvest energy from the food we eat. If the bacteria there are really good at that, some people may be getting more calories from a given food that others, he theorized.

Prior studies in mice have shown that if you transplant the bacteria in the intestines from an obese animal to an ordinary mouse, that second one will become obese.

"The major question that springs from this work is, will long-term dietary change be able to move somebody out of their dietary enterotype?" Sonnenburg said. "This study suggests that dietary change will not do it in the short term, but may require a long term change in diet and lifestyle."

More information

The U.S. National Institutes of Health has more about how gut microbes may impact metabolic syndrome.