Digestion breaks down complex food substances like starches, proteins, and lipids into simpler forms such as glucose, amino acids, and fatty acids that cells can absorb. Absorption involves transporting these end products from the intestinal lumen into the blood or lymphatic systems. For instance, glucose and amino acids enter the portal circulation and then the liver. Fatty acids, however, use the lymphatic system to reach the heart.
Carbohydrate digestion primarily involves dietary sugars like starch, sucrose, and lactose. Sucrose and lactose pass through the mouth and stomach without breaking down, heading straight to the small intestine. Cooked starch, on the other hand, begins digestion in the mouth with salivary alpha-amylase. This enzyme transforms starch into maltose, isomaltose, maltotriose, and alpha-limit dextrins.
Starch digestion continues in the small intestine, where pancreatic alpha-amylase breaks down both cooked and uncooked starch into maltose and dextrins. Following this, intestinal enzymes like maltase, isomaltase, and dextrinase further reduce these substances into glucose. Sucrose and lactose are also digested here by sucrase and lactase, converting them into glucose, fructose, and galactose. These simple sugars are the final products of carbohydrate digestion.
Absorption of these sugars occurs in the small intestine, moving from the lumen into enterocytes and then into the blood. Fructose enters cells through facilitated diffusion, while glucose and galactose use secondary active transport. These sugars eventually reach the liver, where glucose is stored as glycogen or used for energy. Through processes like glycolysis and the TCA cycle, a single glucose molecule can produce 38 ATP molecules, carbon dioxide, and water.