Human physiology textbooks tend to limit their discussion of colonic functions to those of absorbing water and electrolytes and storing waste material. However, the colon is a highly active metabolic organ, containing an exceedingly complex society of microbes. By means of fermentation, gastrointestinal microbes break down nutrients that cannot be hydrolyzed by mammalian host enzymes and thus play an important role in digestive physiology, not only in plant-eating animals (herbivores) but also in humans (omnivores). Colonic fermentation yields both gases (hydrogen, methane, and carbon dioxide) and short-chain fatty acids (SCFAs; acetic, propionic, and butyric acids), which exert several effects of major physiological and pathophysiological importance. This article discusses the lack of emphasis on the study of human colon as a digestive organ and encourages the need for colonic fermentation to be included in every textbook of human physiology.