![]() The descriptions and diagrams are aimed at an audience of teachers of physiology who want to understand the details of the biochemistry of digestion and the physiology of epithelial transport of nutrient components. This review article seeks to highlight insights learned in studying the digestion, absorption, and transport of dietary carbohydrates, proteins, and lipids. New transporters have been discovered (such as H +-oligopeptide transporters and fatty acid transporters). In addition, since the late 1970s, many of the details about digestion and transport have been elucidated. Teachers in medical biochemistry for first-year medical students may give lectures on “Digestion and absorption of carbohydrates/proteins/fats.” Whereas undergraduate physiology textbooks tend to gloss over the details of the digestion of the various nutrients (what the enzymes are and how they work), medical biochemistry textbooks tend to gloss over the details of the transporters needed for the uptake of the breakdown products of the nutrients and the fate of the nutrients in the body. The diagrams found in most undergraduate physiology textbooks seek to clearly explain the details of these steps to the students. The information presented in class generally has ∼10 basic steps for the digestion and complete absorption of each major nutrient group. ![]() Teachers of undergraduate physiology courses may routinely assign students the following question: “Be able to describe in detail the steps in the entire mammalian gastrointestinal (GI) tract for digestion and absorption of ONE of the three nutrient groups.” In other words, tell how carbohydrates, proteins, OR fats are broken down (in which organs and by which enzymes) and then describe how the final breakdown products are absorbed (how they enter intestinal epithelial cells, cross the cell, and how they leave the cell, including whether they go into the bloodstream or the lymph system). Insights are included about some of the diseases and conditions that can bring about malabsorption of food in the GI tract and their consequences. 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. 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 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. ![]() In addition, details are being clarified about how transporters work and in what forms nutrients can be absorbed. There have been several recent discoveries of new transporters that likely contribute to the absorption of oligopeptides and fatty acids. 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. 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. ![]()
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