Chapter 14: Metabolism Overview & Metabolic Fuels

Metabolism is categorized into anabolic pathways that construct complex structures, catabolic pathways that liberate chemical energy, and amphibolic pathways like the citric acid cycle that serve as vital links between both functions. Central to these processes is the production of acetyl-CoA, a universal intermediate where the breakdown of carbohydrates, lipids, and proteins converges for eventual oxidation. Carbohydrate metabolism revolves around the provision of glucose, which provides essential energy for the brain and red blood cells through glycolysis and oxidative phosphorylation, while excess is stored as glycogen in the liver and muscle. Lipid metabolism involves the efficient storage of triacylglycerols in adipose tissue and the mobilization of fatty acids for beta-oxidation or the synthesis of ketone bodies during periods of nutrient scarcity. The liver serves as the primary metabolic hub, regulating blood glucose levels via glycogenesis, glycogenolysis, and gluconeogenesis, while also managing nitrogen waste through the urea cycle. Metabolic flux and regulation are achieved through nonequilibrium reactions, where specific flux-generating enzymes are influenced by allosteric modifiers and systemic hormonal signals. Insulin dominates the fed state to promote energy storage and glucose uptake, whereas glucagon facilitates the mobilization of reserves during fasting to maintain homeostasis. Clinical correlations highlighted in the text include metabolic disorders such as diabetes mellitus, where impaired insulin signaling leads to chronic hyperglycemia and potentially fatal ketoacidosis, and the impact of conditions like cachexia on protein wasting. Understanding these interconnected pathways provides a comprehensive basis for grasping how the human body adapts to various physiological demands like exercise, starvation, and pregnancy.