CARBOHYDRATES provide a substantial proportion of the total caloric content of our diet. The dietary carbohydrates of quantitative importance are the polysaccharide starch and the disaccharides sucrose and lactose. Hydrolysis of each of these to monosaccharide form is necessary before significant absorption can occur.

The principal product of starch digestion by amylases of the salivary glands and pancreas is maltose, a disaccharide in which two glucose units are joined by 1-4 α linkages. The other products are oligosaccharides including the disaccharide isomaltose, in which 1-6 α linkages are present.

Further hydrolysis of maltose and oligo 1-6 glucosides, produced from starch, and the hydrolysis of sucrose and lactose ingested as such, is dependent on enzymes of the small intestinal mucosa, the disaccharidases.

Figure 1 summarizes the sequence of digestion of the common dietary carbohydrates.

For many years, clinicians have recognized that diarrhea in some patients is related to the ingestion of certain forms of carbohydrate. Recent years have seen an increasing interest in the detection of such patients, and the application of new techniques to their investigation.

The development of instruments for intraluminal biopsy of the small intestinal mucosa was a notable advance, since it allowed specimens of this tissue to be obtained and some of its digestive capacities, including those concerned with disaccharide digestion, to be studied directly.

By these means, it has been established that deficiency of one or more of the intestinal disaccharidases is often the basis of disaccharide intolerance.

The terms "disaccharide intolerance" and "disaccharidase deficiency" are frequently used in this article, but have rather different meanings, so that a preliminary discussion of the sense in which each is used seems warranted.

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