How Are Dietary Carbohydrates Digested

Normally, just about all non-fiber dietary carbohydrate will be absorbed across the wall of our small intestine. Monosaccharides are the absorbed form of carbohydrate, therefore disaccharides and starch must be digested into monosaccharides. Carbohydrate digestion begins in the mouth as chewing breaks up food and mixes it with saliva. Saliva contains salivary amylase, which is an enzyme that begins to break down starch. The activity of salivary amylase is short lived due to the rather brief period of time that food stays in the mouth. As the swallowed food/saliva mixture reaches the stomach, the acidic juice reduces the activity of salivary amylase, which halts carbohydrate digestion.

Chemical digestion of carbohydrates picks up again in the small intestine as the pancreas delivers pancreatic amylase along with a battery of other digestive enzymes. Pancreatic amylase resumes the assault upon starch molecules, breaking them into smaller links of glucose. The cells that line the small intestine will play the final role in carbohydrate digestion as they produce enzymes that digest the smaller carbohydrates, such as disaccharides and the remaining branch points on what was once starch. The enzymes that split sucrose, maltose, and lactose into monosaccharides are called sucrase, maltase, and lactase, respectively.

Carbohydrates are primarily absorbed as monosaccharides, thus disaccharides and starch must be digested.

Once monosaccharides are liberated they can move into the cells lining the wall of the small intestine. They can then move out the back end of these cells and then into tiny blood vessels (capillaries) in the wall of the small intestine. These capillaries drain into a larger blood vessel that leaves the intestines and travels to the liver (Figure 4.2). It should be mentioned that the absorption of glucose and galactose requires energy (ATP) but fructose does not.

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