Chapter 16: Regulation of Organic Metabolism and Energy Balance

The fundamental principle underlying these regulatory systems is the maintenance of stable blood glucose concentration, which serves as the critical fuel source for the brain and central nervous system. The chapter traces how ingested carbohydrates, lipids, and amino acids are processed through the liver, skeletal muscle, and adipose tissue following absorption. During the absorptive or fed state, elevated insulin secretion from pancreatic beta cells orchestrates the storage and utilization of newly acquired nutrients by promoting glucose entry into cells, activation of glycogen synthesis, triglyceride deposition in adipose tissue, and protein synthesis in muscle. Conversely, during the postabsorptive or fasting state, glucagon released from pancreatic alpha cells initiates the mobilization of stored energy reserves through glycogenolysis, gluconeogenesis from noncarbohydrate substrates, and lipolysis to preserve blood glucose availability. The chapter further explores how counter-regulatory hormones including epinephrine, cortisol, and growth hormone amplify these responses during extended fasting or physiological stress. Energy homeostasis is explained through the balance between total daily energy intake and total metabolic expenditure, which comprises basal metabolic rate, the thermogenic cost of food processing, and activity-related energy consumption. The hypothalamic regulation of appetite is detailed through neural circuits in the arcuate nucleus and melanocortin signaling pathways, with emphasis on how appetite-suppressing leptin and appetite-stimulating ghrelin provide feedback on energy stores and nutritional status. The chapter concludes with clinical applications including the metabolic consequences of insulin deficiency in type 1 diabetes, insulin resistance in type 2 diabetes, metabolic syndrome, obesity from dysregulation of appetite centers, and the endocrine adaptations during starvation and stress.