A change in diet quickly alters the types of bacteria living in the human gut, a finding that suggests this rapid adaptability to different foods can be used to control illnesses tied to stomach microbes, researchers said.
Switching to an animal-based diet increased the number of microorganisms that process protein, while a plant-based diet increased the number of bacteria that help process starch and cellulose, according to a study led by Harvard University researchers published Wednesday in the journal Nature. The change in the bacteria populations occurred within a day.
Trillions of microorganisms live in the human gut, helping to digest food, fight disease-causing germs and process nutrients. Research has suggested that diets high in fat and sugar may change the human gut's bugs, perhaps contributing to chronic illness, the study authors wrote. Previous work in mice suggested that the microbiome could change within a day, though until now, the effect hadn't been replicated in humans.
"It's exciting and gratifying to find out this holds up in people," said Lawrence David, who was one of the Harvard researchers and is now an assistant professor at Duke University's Molecular Genetics & Microbiology and Institute for Genome Sciences & Policy. "We're getting an increasing appreciation of how flexible and responsive the microbiome is, even on a very short time scale."
Humans are home to more than 10,000 species of microbes, mostly bacteria that live in healthy symbiosis, according to the Human Microbiome Project. The trillions of microorganisms that live in and on the body outnumber human cells by 10 to 1, according to research published in 2012 in Nature and the Public Library of Science journals.
Scientists are just beginning to explore the composition of these ecosystems, said David. Knowing how these organisms interact with their host can reveal more about illnesses such as inflammatory bowel disease and obesity.
The 11 people studied were allowed to eat as they normally did for four days, writing down what they ate and submitting fecal samples to the researchers. Then they consumed what was provided to them by researchers for four days, and were watched for six days afterward. That meant that each person essentially served as their own control group, David said.
The plant-based diet, which boosted fiber intake significantly and dropped fat and protein intakes, led to very few changes in the existing microbes. The animal-based diet had almost no fiber intake had a "really big shift," David said.
The animal-based diet caused changes in the population, including an increase in the anaerobic bacteria Bilophila wadsworthia, which is known to cause colitis in mice. The bacteria seem to thrive with the increased intake of fat, David said. Some bacteria also changed their gene expression with the diet, the study found.
"Perhaps in prehistoric groups, when there was a lot more volatility in terms of what you can forage or hunt for, this could have been very useful," David said. "It creates a way of buffering nutritional changes and may have enabled ancient humans to be a little more flexible with their diet."
Further research may focus on how food preparation affects the bacterial colonization, David said.