Insights into digestion and absorption of major nutrients in humans

Nutrient digestion and absorption is necessary for the survival of living organisms and has evolved into the complex and specific task of the gastrointestinal (GI) system. While most people simply assume that their GI tract will work properly to use nutrients, provide energy, and release wastes, few nonscientists know the details about how various nutrients are digested and how the breakdown products traverse the cells lining the small intestine to reach the blood stream and to be used by the other cells of the body. There have been several recent discoveries of new transporters that likely contribute to the absorption of oligopeptides and fatty acids. In addition, details are being clarified about how transporters work and in what forms nutrients can be absorbed. The enzymes that digest basic carbohydrates, proteins, and fats have been identified in various segments of the GI tract, and details are becoming clearer about what types of bonds they hydrolyze. Usually, detailed information about the digestion of basic nutrients is presented and learned in biochemistry courses and detailed information about absorption via transepithelial transport of the breakdown products of digestion is studied in physiology courses. The goal of this Staying Current article is to combine the details of the biochemistry of digestion with the updated information about the physiology of nutrient absorption into one source for teachers of physiology. Insights are included about some of the diseases and conditions that can bring about malabsorption of food in the GI tract and their consequences.