Carbohydrate digestion initiates in the mouth with the release of salivary amylase from the salivary glands. This enzyme kickstarts the breakdown process by cleaving large-chain polysaccharides into smaller segments. Chewing food and mixing it with saliva aid in digestion by increasing its surface area.
In the stomach, food combines with gastric juices, and the highly acidic environment continues the digestive process.
Upon entering the duodenum of the small intestine, chyme is neutralized by sodium bicarbonate from the pancreas, allowing pancreatic amylase to further break down carbohydrates in the jejunum. Here, digestible carbohydrates are further broken down into disaccharide and monosaccharide forms. These then progress to the ileum, where most carbohydrate absorption occurs. While monosaccharides are readily absorbed, disaccharides require additional digestion into single sugar units.
Once digestion is complete, monosaccharides are absorbed by absorptive cells in the intestinal wall and transported via the hepatic portal vein to the liver.
Carbohydrates primarily serve as immediate fuel, with glucose or fructose utilized by the liver or circulated to target cells. Glucose enters energy pathways directly, while only a portion of absorbed fructose undergoes oxidation in these pathways. After meeting immediate fuel demands, surplus carbohydrates are allocated to replenishing liver and muscle glycogen stores. Glucose molecules combine to form larger glycogen chains, with a portion of fructose converted into glucose for glycogen synthesis. While absorbed fructose is either oxidized or converted into glucose, the remainder is converted into lactate or triglycerides.
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