1.Intraluminal digestion, in which proteins, carbohydrates, and fats are broken down into assimilable forms. The process begins in the mouth with saliva, receives a major boost from gastric peptic digestion, and continues in the small intestine, assisted by the detergent action of bile salts.

2.Terminal digestion, which involves the hydrolysis of carbohydrates and peptides by disaccharidases and peptidases in the brush border of the small intestinal mucosa.

3.Transepithelial transport, in which nutrients, fluid and electrolytes are transported across the epithelium of the small intestine for delivery to the intestinal vasculature. Absorbed fatty acids are converted to triglycerides and with cholesterol, are assembled into chylomicrons for delivery to the intestinal lymphatic system.

This classification is most helpful for disease in which there is a single, clear cut abnormality. In many malabsorptive disorders, adefect in one pathophysiologic process predominates, but others may contribute. Although many causes of malabsorption can be established clinically, diagnosis may require small intestinal mucosal biopsy to exclude celiac sprue satisfactorily.

Clinically the malabsorption syndromes resemble each other more than they differ. The consequences of malabsorption affect many organ systems, as follows:

Alimentary tract: Diarrhea (both from nutrient malabsorption and from excessive intestinal secretions) flatus, abdominal pain, weight loss, and mucositis resulting from vitamin deficiences.

Hematopoietic system: Anemia from iron, pyridoxine, folate, or vitamin B12 deficiency and bleeding from vitamin K deficiency.

Musculoskeletal system: Osteoopenia and tetany from calcium, magnesium, vitamin, and protein malabsorption.

Endocrine system: Amenorrhea, impotence, and infertility from generalized malnutrition and hyperparathyroidism from protracted calcium and vitamin D deficiency.

Epidermis: Purpura and petechiae from vitamin K deficiency; edema from protein deficiency; and determines and hyperkeratosis from deficiencies of vitamin A, zinc essential fatty acids, and niacin

Nervous system: Peripheral neuropathy from vitamin A vitamin B122 deficiencies.

The passage of abnormally bulky, forthy, greasy, yellow or gray stools (steatorrhea) is a prominent feature of malabsorption, accompanied by weight loss, anorexia, abdominal distention, borborygmi, and muscle wasting. The malabsorptive disorders most commonly encountered in the United States are celiac sprue, pancreatic insufficiency, and Crohn disease

Pancreatic insufficiency, primarily from chronic pancreatitis or cystic fibrosis is a major cause of defective intraluminal digestion. Excessive growth of normal bacteria within the proximal small intestine (bacterial overgrowth) also impairs intraluminal digestion and can damage mucosal epithelial cells. Immunologic deficiencies, inadequate gastric acidity, and intestinal stasis as from surgical alteration of small intestinal anotomy, predispose to bacterial overgrowth. Typical features of defective intraluminal digeswtion are an osmotic diarrhea from undigested nutrients and steatorrhea, which is excess output of undigested fat in stool. The intestinal mucosa in bacterial overgrowth either is normal or is minimally damaged.