Chapter 24: Nutrition, Metabolism and Energy Balance

Nutrition, Metabolism and Energy Balance begins by classifying essential nutrients—carbohydrates, lipids, proteins, vitamins, and minerals—and describing their specific roles in supporting life processes. Carbohydrates serve as the body's immediate energy currency, lipids function as long-term energy reserves and structural components, and proteins enable tissue synthesis, enzymatic catalysis, and metabolic regulation. Water-soluble and fat-soluble vitamins act as coenzymes and cofactors, while minerals contribute to enzyme function and structural integrity. The chapter then traces nutrient processing through three sequential stages: initial breakdown and absorption in the digestive system, cellular-level processing through metabolic pathways, and energy capture via the electron transport chain and oxidative phosphorylation. Carbohydrate metabolism receives detailed attention through the glycolytic pathway, the citric acid cycle, and coupled ATP synthesis in mitochondrial membranes. Lipid catabolism is examined through beta-oxidation and lipolysis, contrasted with lipogenesis during nutrient storage. Protein metabolism is addressed through transamination and deamination, showing how amino acids contribute to energy production. The liver emerges as a metabolic hub, orchestrating nutrient distribution, regulating blood glucose, storing glycogen and fat-soluble vitamins, and detoxifying harmful substances. The chapter distinguishes between absorptive and postabsorptive metabolic states, highlighting how insulin and glucagon coordinate the transition between nutrient uptake and mobilization. Energy balance is presented as the fundamental principle governing body weight, with basal metabolic rate and total metabolic rate influenced by age, sex, body composition, and hormonal status. The chapter concludes by exploring thermoregulation as an essential metabolic function, describing mechanisms of heat generation, dissipation, and hypothalamic control through physiological responses including sweating, vasodilation, and behavioral adaptation.