The study found people who spent one day eating only animal products such as meat, poultry, and dairy instead of their regular diet had a drastic change in stomach bacteria, LiveScience reported.

The bacteria that became more populous during the 24-hour all-animal product diet were more adapted to tolerate bile (a fluid produced by the liver that helps break down fat) than other microbes. Meanwhile the bacteria Firmicutes, which is known to break down plant proteins, took a vacation.

The bacteria appeared to turn on certain genes during the meaty diet that helped the body process protein. A second group of study participants that ate a plant-based diet expressed genes that promoted the fermentation of carbohydrates.

This phenomenon had been observed before in rodents, but this study marked the first time it had been seen in humans, Bloomberg BusinessWeek reported.

"It's exciting and gratifying to find out this holds up in people," Lawrence David, a Harvard researchers who is now an assistant professor at Duke University's Molecular Genetics & Microbiology and Institute for Genome Sciences & Policy, said. "We're getting an increasing appreciation of how flexible and responsive the microbiome is, even on a very short time scale."

The microbial makeup of the participants' guts was consistent with what would be seen in herbivorous and carnivorous animals, LiveScience reported.

The study suggests diet and microbial populations in the digestive tract can be linked to an increased or decreased risk of certain diseases.

Participants on the animal-based diet had higher levels of the bacteria Bilophila wadsworthia, which has been linked to inflammatory bowel disease.

The diet phenomenon may have ancient roots.

"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 told Bloomberg. "It creates a way of buffering nutritional changes and may have enabled ancient humans to be a little more flexible with their diet."