Just switching from your normal diet to eating only animal products can change gut bacteria within one day, a new study suggests.

For the study, participants who were on the animal-based diet were found to show an increase Bilophila wadsworthia, a type of bacteria that can tolerate bile (a fluid produced by the liver that helps break down fat). They also had a decrease in bacteria called Firmicutes, which break down plant carbohydrates. In contrast, the gut bacteria of another group of participants who stuck with a plant-based diet expressed genes that would allow them to ferment more carbohydrates. The differences between the gut bacteria of the people on the plant-only and animal-only diets were said to mirror "the differences between herbivorous and carnivorous mammals."
 
The finding supports a link between dietary fat (from animal fat), bile acids and an increase in growth of microbes that may affect the risk of inflammatory bowel disease. However, it still wasn't clear on why this change happens so quickly.

[Livescience]