Which of the following statements regarding digestion and absorption of carbohydrates is TRUE? Lactose intolerance results from an insufficiency of the enzyme amylase. Digestible polysaccharides are broken down into the monosaccharides glucose, galactose, and fructose, which can be absorbed. Sucrose is the main form of carbohydrate that can be absorbed by active transport across the intestinal epithelium. Carbohydrate digestion begins in the stomach. Cellulose from plants is a polymer of glucose that can be easily digested and absorbed by the human GI tract.

For dietary carbohydrates to be utilized by the body, they must be converted during digestion to monosaccharides. In addition to starch, the other major dietary carbohydrates are the disaccharides lactose and sucrose, and the monosaccharide fructose. The monosaccharides produced by complete digestion of these dietary carbohydrates are glucose, galactose, and fructose.

The digestive process begins with salivary amylase, which randomly cleaves the α-1,4 linkages of starch. Although amylase digestion begins in the saliva, pancreatic α-amylase is more important to the complete digestion of starch (Fig. 19-1). Starch is degraded first to dextrins and then to a mixture of glucose, maltose, and isomaltose (containing the α-1,6 linkages that are not digested by amylase).

The major disaccharidases, located in the brush border of the intestinal lumen, are

•

Maltase—hydrolyzes maltose

•

Sucrase-isomaltase—hydrolyzes sucrose and isomaltose

•

Lactase—hydrolyzes lactose

Whenever lactose goes undigested, it is not absorbed and passes into the large intestine. Here lactose is acted on by the intestinal flora that ferment it, producing large quantities of CO2, hydrogen gas, methane, and organic acids; the last irritate the intestines, increasing intestinal motility. All these products have only one way out. Thus, the symptoms that characterize lactose intolerance are bloating and flatulence and, in extreme cases, a frothy diarrhea. Lactose intolerance is least common in Northern Europeans and their descendants and most common in descendants of Asian, African, and South American origin.

HISTOLOGY

Brush Border Dynamics

The unstirred brush border of the intestinal lumen consists of finger-like processes, known as microvilli, of the surface absorptive cells. Many enzymes associated with the process of digestion and absorption are located on the surface of these microvilli. This allows the products of digestion such as free fatty acids, amino acids, and monoglycerides to be absorbed by the cells rather than be swept into the lumen itself.