DIGESTION AND ABSORPTION
Monosaccharides—glucose, fructose, and galactose—are simple sugars that may be absorbed from the intestine directly into the bloodstream. They are subsequently carried to the liver, where fructose and galactose are changed to glucose. The blood then carries glucose to the cells.
Disaccharides—sucrose, maltose, and lactose—require an additional step of digestion. They must be converted to the simple sugar glucose before they can be absorbed into the bloodstream. This conversion is accomplished by the enzymes sucrase, maltase, and lactase.
Polysaccharides are more complex, and their digestibility varies. After the cellulose wall is broken down, starch is changed to the intermediate prod-uct dextrin; it is then changed to maltose and finally to glucose. Cooking can change starch to dextrin. For example, when bread is toasted, it turns golden brown and tastes sweeter because the starch has been changed to dextrin.
The digestion of starch begins in the mouth, where the enzyme salivary amy-lase begins to change starch to dextrin. The second step occurs in the stomach, where the food is mixed with gastric juices. The final step occurs in the small intestine, where the digestible carbohydrates are changed to simple sugars by the enzyme action of pancreatic amylase and are subsequently absorbed into the blood.