Chapter 43: Nutrition, Digestion, & Absorption

Nutrition, Digestion, & Absorption details the mechanical and chemical breakdown of carbohydrates, lipids, and proteins into their constituent monomers—monosaccharides, fatty acids, and amino acids—to facilitate absorption across the intestinal epithelium. Carbohydrate digestion is highlighted through the action of salivary and pancreatic amylases, followed by specialized brush border enzymes like lactase and sucrase, with specific transport mechanisms like the sodium-dependent SGLT 1 protein ensuring nutrient uptake. The text introduces the glycemic index as a measure of how different carbohydrates impact blood sugar and insulin responses. Lipid metabolism is described as a complex process of emulsification by bile salts and micelle formation, allowing hydrophobic molecules to navigate the aqueous environment of the gut before being re-esterified into chylomicrons for lymphatic transport. Protein digestion involves a cascade of proteolytic enzymes, starting with gastric pepsin and continuing with pancreatic zymogens like trypsinogen, which must be precisely activated to prevent self-digestion. Beyond macronutrients, the chapter emphasizes the regulated absorption of vitamins and minerals, noting the critical roles of Vitamin D in calcium uptake and the sophisticated hormonal control of iron via hepcidin and ferroportin to prevent toxicity. Energy balance is a central theme, defined by the relationship between Basal Metabolic Rate (BMR), physical activity levels, and dietary intake. The discussion extends to the clinical consequences of nutritional imbalances, contrasting the metabolic wasting of marasmus with the inflammatory edema found in kwashiorkor, and explaining the hypermetabolic state of cachexia often seen in chronic illnesses like cancer. Finally, the concept of nitrogen balance is used to determine protein requirements, distinguishing between essential amino acids that must be ingested and nonessential ones the body can synthesize. This comprehensive overview links molecular biochemistry to global health issues, ranging from the rising tide of obesity and metabolic syndrome to the persistent challenges of undernutrition and specific vitamin deficiencies.