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The small intestine absorbs carbohydrates, proteins, fats, and micronutrients through coordinated brush-border enzyme activity and selective transporter expression. Glucose enters via SGLT1 (Na⁺-dependent), fructose via GLUT5 (passive), and amino acids via multiple amino acid transporters. Fats are emulsified by bile, hydrolyzed to monoglycerides and fatty acids, and reconstituted into chylomicrons for lymphatic transport. Different nutrients are absorbed preferentially in different intestinal regions based on transporter distribution.
You already know from epithelial transport that epithelial cells are architecturally polarized — apical membranes face the lumen and are distinct in composition from basolateral membranes facing the bloodstream. The intestinal enterocyte applies this principle with extraordinary specificity. The apical surface is densely packed with microvilli forming the brush border, amplifying absorptive surface area roughly 600-fold. Embedded in the brush border membrane are digestive enzymes (lactase, sucrase-isomaltase, peptidases) and an array of nutrient transporters, each tuned to a different molecule class. The strategy: break nutrients into absorbable units at the apical surface, import them into the cell using specific transporters, and export them across the basolateral membrane into portal blood or lymph.
For sugars, the mechanism depends on the sugar. Glucose and galactose enter through SGLT1 (Sodium-Glucose Linked Transporter 1) on the apical membrane — a secondary active transporter that co-transports one glucose molecule with two sodium ions, using the sodium gradient maintained by basolateral Na⁺/K⁺-ATPase as the energy source. This allows glucose uptake even against its concentration gradient, essential after a carbohydrate-rich meal when the lumen glucose concentration may still be lower than the cytoplasm. Fructose, however, uses GLUT5, a facilitated diffusion transporter that requires no energy — it simply flows down its concentration gradient. Both sugars then exit the cell through GLUT2 on the basolateral membrane into the portal circulation. This distinction explains why fructose absorption can be overwhelmed (GLUT5 has limited capacity), causing osmotic diarrhea with very high fructose loads.
Protein absorption follows the same vectorial logic. Pancreatic proteases cleave luminal proteins into dipeptides, tripeptides, and amino acids. Short peptides enter via PepT1, a proton-coupled oligopeptide transporter on the apical membrane — one of the most clinically relevant intestinal transporters because many oral drugs mimic di/tripeptides and exploit it. Amino acids enter through a family of sodium-dependent and independent transporters, each selective for a different chemical class (neutral, cationic, anionic). Intracellular peptidases cleave peptides to free amino acids before export.
Fat absorption is the most structurally complex pathway because fats are hydrophobic and cannot be simply dissolved and transported. Bile salts from the liver emulsify dietary triglycerides into small droplets, increasing the surface area for pancreatic lipase to act. Lipase cleaves triglycerides into 2-monoglycerides and free fatty acids, which are then incorporated into micelles — small bile-lipid assemblies that ferry the hydrophobic products to the brush border. Monoglycerides and fatty acids diffuse passively across the apical membrane into the enterocyte, where they are immediately re-esterified into triglycerides in the smooth ER. These are packaged with phospholipids, cholesterol, and apolipoprotein B-48 into large lipoprotein particles called chylomicrons, which are too large to enter the portal capillaries. Instead, they are secreted by exocytosis into the lacteals — lymphatic capillaries running through each villus — and travel through the thoracic duct to enter systemic circulation, bypassing the liver on first pass. This is why a fatty meal produces a characteristic milky appearance in lymph (chyle) and why fat-soluble vitamins (A, D, E, K) travel the same lymphatic route.