Short-chain fatty acids are products of nondigestible oligo- and poly-saccharides fermented by vast numbers of colonic microorganisms called microbial flora. Produced in the colon they are readily absorbed and are metabolized in the liver and muscle tissues providing energy to the human body. The colonic mucosal cells consume butyric acid, a kind of short-chain fatty acid, as a main and indispensable energy source. A deficiency in butyric acid leads to a malfunction of the colon. This means that nondigestible sugars, including prebiotics, are essential for human health. Short-chain fatty acids are not merely sources of energy; they provide beneficial physiological actions. These include improving the absorption of water and several minerals, especially calcium, magnesium, and iron, and suppressing the synthesis of liver cholesterol. Butyric acid is marked as a suppressor of colon cancer, a lifestyle-related disease. This fermentation product reduces cell proliferation and induces apoptosis in the mutated cells. Butyric acid is very likely to be effective for the prevention of this fatal disease. How do short-chain fatty acids work in various cells? This has not been revealed.The research into short-chain fatty acids may clarify the physiological implications of the colonic fermentation and prebiotics.