Chapter 33: Animal Nutrition

Animals require dietary sources of essential amino acids, polyunsaturated fatty acids, vitamins, and minerals; their absence produces specific deficiency diseases ranging from protein-energy malnutrition to scurvy and goiter. Undernutrition occurs when energy intake falls below metabolic demands, forcing sequential depletion of glycogen reserves, adipose tissue, and ultimately structural proteins, with severe physiological consequences. Food processing involves four integrated stages—ingestion, digestion, absorption, and elimination—executed through diverse feeding mechanisms adapted to ecological niches. While primitive organisms conduct digestion intracellularly within vacuoles, most animals employ extracellular digestion in compartmentalized systems ranging from simple gastrovascular cavities to complex mammalian alimentary canals with specialized regions and accessory organs. The human digestive system demonstrates this complexity: mechanical and chemical breakdown begins in the oral cavity through mastication and salivary amylase activity; the stomach functions as a temporal storage vessel and acid-enzyme chamber where pepsin initiates protein hydrolysis; the small intestine serves as the primary site of enzymatic digestion and nutrient absorption, facilitated by pancreatic enzymes, hepatic bile, and absorptive epithelial architecture including villi and microvilli; the large intestine recovers water and hosts a mutualistic bacterial community synthesizing vitamins and modulating immune and metabolic function. Structural and physiological digestive adaptations reflect dietary specialization: carnivores possess sharp incisors and canines for flesh processing; herbivores display grinding molars and elongated alimentary canals; many herbivorous species harbor fermentation chambers with resident microorganisms that enzymatically break down plant cellulose. Ruminant digestion exemplifies this adaptation through a four-chambered stomach system enabling regurgitation and microbial fermentation. Homeostatic regulation of digestion and energy metabolism involves coordinated nervous and endocrine signaling: hormones including gastrin, secretin, and cholecystokinin orchestrate secretion and motility; basal metabolic rate establishes baseline energy requirements; appetite-regulating hormones such as ghrelin promote feeding while leptin and peptide YY signal satiety; insulin and glucagon maintain blood glucose stability. Dysregulation of these systems produces pathologies including diabetes mellitus, characterized by impaired glucose homeostasis, and obesity, reflecting chronic imbalance between energy intake, storage capacity, and expenditure.