 If whatever gut flora enterotype we are could play an important role in our risk of developing chronic diet-associated diseases, the next question is, can we alter our gut microbiome by altering our diet? The answer is, diet can rapidly and reproducibly alter the bacteria in our gut. There's been growing concern that recent lifestyle innovations, most notably the high-fat, high-sugar Western diet, have altered the composition and activity of our resident gut flora. Such diet-induced changes to gut-associated microbial communities are now suspected of contributing to growing epidemics of chronic disease in the developed world. Yet it remained unclear how quickly our gut bacteria could respond to dietary change. So they prepared two diets, a plant-based diet, rich in grains, beans, fruits, and vegetables, and an animal-based diet, which is composed of meats, eggs, and cheeses. No refined sugars in either. They just wanted to test plant versus animal. And within just one day of the animal-based diet, hitting the gut, there was a significant shift. For example, the life-long vegetarian. What happens when you put him on an animal-based diet? Well, he started out privatella, like the one vegan in the typing study, but unlike everyone else, because they're reading a more standard American diet. Remarkably, the animal-based diet inverted the vegetarian's prevetilla-to-bacteroides ratio, causing the bacteroides to outnumber the prevetilla within just four days on an animal-based diet. His entire gut flora got turned on his head. The fact that our gut can so rapidly switch between herbivorous and carnivorous functional profiles is probably a good thing, evolution-wise. I mean, if you bring down a mammoth and you're eating meat for a couple of days before falling back to plants, you want your gut to be able to deal. And this flexibility is manifest in the diversity of human diets to this day, but what's the healthier state to be in most of the time? They looked at a number of different factors. First, the amount of short-chain fatty acids produced. Short-chain fatty acids like acetate butyrate function to suppress inflammation, suppress cancer, and our gut flora on plant-based diets produced more than on animal-based diets. Other microbial metabolites such as secondary biolacids promote the development of cancer, and with a significant increase in bacterial enzyme activity to create these secondary biolacids on an animal-based diet, no surprise, significant increase in carcinogens like DCA, a secondary biolacid known to promote DNA damage and liver cancer. Microbial enzyme activity to produce the rotten egg gas hydrogen sulfide also shoots up on an animal-based diet, which stinks because it stings, because it damages DNA, and has been implicated in development of inflammatory bowel diseases like ulcerative colitis. Hydrogen sulfide is made by pathogens like bilaphyla wadsworthia, which is increased on the animal-based diet, again within just days, supporting the link between diet and the outgrowth of microorganisms capable of triggering inflammatory bowel disease. Whereas the only pathogen you see more of on a plant-based diet is just a virus that infects spinach.