The gut microbiota is a changing ecosystem, containing trillions of bacteria, continuously shaped by many factors, such as dietary habits, seasonality, lifestyle, stress, antibiotics use, or diseases. Diet is the key determinant of the microbiota configuration, through modulation of the abundance of specific species and their individual or collective functions. Most complex plant polysaccharides from diet are not digested by humans and enter the colon as a potential nutrient source for the microbiota. Gut microbiota can convert these undigestible polysaccharides into short-chain fatty acids (SCFAs). Besides, SCFAs are likely to have broad impacts on various aspects of host physiology. Dietary fibers interact directly with gut microbes and lead to the production of key metabolites such as short-chain fatty acids. Dietary fibers can be considered key ancestral compounds that preserve gut ecology, especially regulating macronutrients and host physiology. Here we summarize the composition of intestinal symbiotic bacteria, the utilization of polysaccharides entering the intestinal tract, and the potential impact of metabolites on human health.