This study investigates the rapid and reproducible effects of short-term dietary changes on the human gut microbiome. The researchers compared the gut microbial communities of 11 healthy volunteers who consumed either a plant-based or an animal-based diet for five consecutive days. The animal-based diet, rich in animal products, significantly altered the microbial community structure, increasing the abundance of bile-tolerant microorganisms and decreasing the levels of Firmicutes that metabolize dietary plant polysaccharides. The microbial activity mirrored the differences between herbivorous and carnivorous mammals, reflecting trade-offs in carbohydrate and protein fermentation. Foodborne microbes from both diets transiently colonized the gut, including bacteria, fungi, and viruses. Notably, the animal-based diet increased the abundance and activity of *Bilophila wadsworthia*, which is linked to inflammatory bowel disease. The study demonstrates that the gut microbiome can rapidly respond to dietary changes, potentially facilitating the diversity of human dietary lifestyles.This study investigates the rapid and reproducible effects of short-term dietary changes on the human gut microbiome. The researchers compared the gut microbial communities of 11 healthy volunteers who consumed either a plant-based or an animal-based diet for five consecutive days. The animal-based diet, rich in animal products, significantly altered the microbial community structure, increasing the abundance of bile-tolerant microorganisms and decreasing the levels of Firmicutes that metabolize dietary plant polysaccharides. The microbial activity mirrored the differences between herbivorous and carnivorous mammals, reflecting trade-offs in carbohydrate and protein fermentation. Foodborne microbes from both diets transiently colonized the gut, including bacteria, fungi, and viruses. Notably, the animal-based diet increased the abundance and activity of *Bilophila wadsworthia*, which is linked to inflammatory bowel disease. The study demonstrates that the gut microbiome can rapidly respond to dietary changes, potentially facilitating the diversity of human dietary lifestyles.