Chapter 22: Metabolism and Energy Balance

Energy balance is explored through the laws of thermodynamics, distinguishing between energy input from diet and energy output via transport, mechanical, and chemical work, alongside heat production. The text explains how metabolic rate is quantified using direct calorimetry or indirect calorimetry via oxygen consumption and carbon dioxide production, which determines the respiratory quotient (RQ). Metabolism is divided into two primary phases: the anabolic fed (absorptive) state and the catabolic fasted (postabsorptive) state, which are regulated by the push-pull control of enzymatic pathways. In the fed state, insulin dominates, facilitating the cellular uptake of glucose for ATP production via glycolysis and the citric acid cycle, while excess energy is stored through glycogenesis and lipogenesis. The chapter details lipid transport, explaining how chylomicrons, low-density lipoproteins (LDL), and high-density lipoproteins (HDL) manage cholesterol and triglyceride levels, which are critical markers for cardiovascular health. Conversely, the fasted state is governed by glucagon, which triggers glycogenolysis in the liver and gluconeogenesis to maintain plasma glucose levels for the brain. Simultaneously, adipose tissue undergoes lipolysis to release fatty acids for beta-oxidation, potentially leading to the formation of ketone bodies and a state of ketosis during prolonged fasting or low-carbohydrate intake. A significant portion of the chapter is dedicated to the pathophysiology of diabetes mellitus, contrasting the autoimmune destruction of beta cells in Type 1 diabetes with the insulin resistance characteristic of Type 2 diabetes, and outlining complications such as metabolic acidosis, osmotic diuresis, and metabolic syndrome. Finally, the chapter addresses thermoregulation, describing how the hypothalamus acts as a thermostat to balance heat gain and loss through mechanisms like radiation, conduction, convection, evaporation, and thermogenesis (both shivering and nonshivering via brown fat) to maintain homeothermy.