Alglucanase deficiency belongs to one of the clinical classifications of disaccharidase deficiency. This disease, also known as disaccharide intolerance, refers to a variety of congenital or acquired diseases that make the small intestinal mucosa brush border disaccharidase deficiency, so that the digestion and absorption of disaccharides are impaired, and a series of symptoms and signs occur when eating food containing disaccharides. 1. Giardia lamblia: Giardia lamblia for short is mainly parasitic in the small intestine of human and some mammals, named in honor of the researchers of this worm, A. Jahr and W. D. Lambre, causing diarrhea as the main symptom of Giardia lamblia. Initially, Giardia was regarded as a commensal intestinal parasite, but its pathogenicity was recognized only after epidemics of the disease occurred worldwide since 1976. It is also known as “traveler’s diarrhea” because it is most commonly seen in children and travelers. Nowadays, Giardia has been listed as one of the ten major parasitic diseases endangering human health worldwide. Lymphosarcoma of small intestine: Lymphosarcoma mostly originates from the lymphatic tissue in the submucosa of the intestinal wall. When the lesion develops along the intestinal wall and to the depth, it can invade the plasma membrane layer, mesentery and its lymph nodes outwardly, and infiltrate the mucosa inwardly, making the mucosal folds flat and stiff. The intestinal canal can be narrowed or slightly wider than normal, and the demarcation with normal intestinal canal is not as obvious as that of cancer, and generally there is no limited nodular mass or obvious ulcer formation. Malignant lymphoma of small intestine: adenoma, carcinoid tumor, smooth muscle sarcoma, lymphoma and other malignant tumors originating from small intestine. The common manifestations are obstruction, bleeding and perforation. Diagnosis is difficult and requires wire-guided small intestine air-barium contrast imaging and small intestine endoscopy and mesenteric arteriography for diagnosis. The main thing is to restrict the diet and abstain from milk and foods containing lactose. In mild cases, milk is limited, and in severe cases, complete abstinence is imposed. Infants can be given unsweetened milk or lactase, and sucrose-isomaltase deficient individuals should limit sucrose intake and starch intake if necessary. Secondary disaccharidase deficiency is often a multiple enzyme deficiency. Small intestinal brush border enzyme impairment secondary to other diseases involving the intestine such as adult celiac disease, lymphoma, small intestinal bacterial overgrowth, and extensive Crohn’s disease may be normalized by treatment of the primary disease.