Disaccharide intolerance is a characteristic symptom complex resulting from the ingestion of ordinary dietary quantities of disaccharides, which produces a symptomatic diarrhea. The cause is a deficiency of one or more disaccharidases, but not all people with such a deficiency will experience symptoms.

Dietary carbohydrates are presented to the surface of the jejunal mucosa in the form of isomaltose, maltotriose and three major disaccharides - maltose, sucrose and lactose. Trehalose, a disaccharide contained in young mushrooms and in certain insects, is a minor component of modern Western diets. Deficiencies of disaccharidases may be primary (hereditary) or secondary (acquired) deficiencies. Characteristically in primary deficiencies, which are rare, only one enzyme is involved; the deficiency is present at birth (with the exception of the adult-onset form of lactase deficiency), not associated with intestinal disease, and irreversible. Secondary deficiencies usually involve all the disaccharidases, may occur at any age, are associated with a disorder of the small intestinal mucosa, and may be reversed if the intestinal disorder (e.g., celiac disease, stasis syndromes or acute enteritis) heals. Because primary lactase deficiency is uncommon in Canadians with northern European ancestors, the appropriate tests need to be performed to exclude secondary causes such as celiac disease.

The clinical manifestations of enzyme deficiency result from the osmotic diarrhea following ingestion of the disaccharide. The affected individual develops crampy, abdominal distress and distention, relieved by the expulsion of liquid stool and flatus. The severity of the diarrhea varies with the disaccharide load, the degree of deficiency of enzyme activity and any associated/ causal intestinal disease. The clinical diagnosis can be confirmed by direct enzyme assay of jejunal mucosal biopsies or by indirect methods for detecting disaccharide malabsorption (e.g., the breath hydrogen test). Treatment of hereditary deficiencies is usually by elimination diets. For children and adolescents (who have high nutritional requirements) and for adults who enjoy milk, low-lactose milk is available in some localities. It can also be prepared by adding yeast lactase (available in commercial form) to milk and refrigerating it for 24 hours; 80% lactose hydrolysis can be obtained with this method.

Delayed-onset (adult-onset) hereditary lactase deficiency is extremely common and probably "normal" for humans. Beginning as early as age 2 years in some racial groups, and as late as adolescence in others, the activities of lactase in the majority of the world's populations drop sharply. This is the result of the genetically controlled "switching off" of lactase synthesis by intestinal cells. Individuals of northern European ancestry and certain groups in Africa and India, however, maintain lactase activity throughout adulthood.