Carbohydrate Foods of the Diet. Only three major sources of carbohydrates exist in the normal human diet. They are sucrose, which is the disaccharide known popularly as cane sugar; lactose, which is a disaccharide found in milk; and starches, which are large polysaccharides present in almost all non-animal foods, particularly in potatoes and different types of grains. Other carbohydrates ingested to a slight extent are amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, and minor quantities of carbohydrate derivatives in meats.
The diet also contains a large amount of cellulose, which is a carbohydrate. However, enzymes capable of hydrolyzing cellulose are not secreted in the human digestive tract. Consequently, cellulose cannot be considered a food for humans.
Digestion of Carbohydrates Begins in the Mouth and Stomach. When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin (an α-amylase) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecules, as shown in Figure 1. However, the food remains in the mouth only a short time, so probably not more than 5 percent of all the starches will have become hydrolyzed by the time the food is swallowed.
Fig1. Digestion of carbohydrates.
Starch digestion sometimes continues in the body and fundus of the stomach for as long as 1 hour before the food becomes mixed with the stomach secretions. Activity of the salivary amylase is then blocked by acid of the gastric secretions because the amylase is essentially inactive as an enzyme once the pH of the medium falls below about 4.0. Nevertheless, on average, before food and its accompanying saliva become completely mixed with the gastric secretions, as much as 30 to 40 percent of the starches will have been hydrolyzed, mainly to form maltose.
