Chapter 26: Digestion & Absorption of Nutrients

Digestion & Absorption of Nutrients begins by outlining fundamental nutritional requirements, including caloric balance and the distinction between high-quality animal proteins and plant-based sources that may lack certain essential amino acids. The text details the systematic breakdown of complex carbohydrates, starting with salivary and pancreatic amylases and concluding with specialized enzymes at the intestinal brush border—such as sucrase and lactase—that yield simple sugars for absorption. A key focus is placed on the transport mechanisms for these sugars, specifically the sodium-dependent secondary active transport for glucose and galactose versus the facilitated diffusion used by fructose. Protein digestion is described as a coordinated effort involving gastric pepsin and a suite of pancreatic enzymes, such as trypsin and chymotrypsin, which are strategically activated in the small intestine by enterokinase to prevent self-digestion. The chapter further examines the unique pathway of lipid assimilation, which relies on the detergent action of bile acids to form micelles, allowing hydrophobic fats to reach the epithelial surface for re-esterification and the eventual formation of chylomicrons for lymphatic transport. Beyond macronutrients, the text clarifies the absorption of vital minerals like iron and calcium—noting the regulatory roles of ferritin, transferrin, and vitamin D—and the specialized uptake of vitamins, including the necessity of intrinsic factor for vitamin B12. Additionally, it addresses the role of the colon in fermenting dietary fiber into beneficial short-chain fatty acids that contribute to caloric intake and colonic health. Finally, the discussion moves to the systemic regulation of hunger and satiety, highlighting how peripheral hormones like the hunger-triggering ghrelin and the fat-signaling leptin act upon the hypothalamus to manage energy intake, while also touching upon the clinical challenges of malabsorption and the global health impact of obesity.