Clinical meaning
Malabsorption syndromes encompass a diverse group of disorders characterized by impaired absorption of nutrients, vitamins, minerals, and fluids from the gastrointestinal tract. To understand malabsorption, it is essential to understand normal absorption. The small intestine is the primary site of nutrient absorption, with a total surface area of approximately 200 square meters due to the presence of circular folds (plicae circulares), villi, and microvilli that form the brush border. The duodenum absorbs iron, calcium, and folate. The jejunum is the primary site for absorption of carbohydrates, proteins, fats, fat-soluble vitamins (A, D, E, K), and water-soluble vitamins. The ileum absorbs vitamin B12 (bound to intrinsic factor), bile salts, and remaining nutrients. Malabsorption occurs when any component of the digestive and absorptive process is disrupted. Celiac disease (celiac sprue, gluten-sensitive enteropathy) is an autoimmune disorder triggered by ingestion of gluten, a protein found in wheat, barley, and rye. In genetically susceptible individuals (carrying HLA-DQ2 or HLA-DQ8 genes), gluten triggers an immune response that damages the intestinal mucosa, causing villous atrophy, crypt hyperplasia, and intraepithelial lymphocyte infiltration. The resulting loss of absorptive surface area leads to malabsorption of virtually all nutrients, with iron and folate deficiency being among the earliest manifestations because these nutrients are absorbed in the proximal small intestine where damage is most severe. Tropical sprue is a malabsorption disorder of unknown etiology that occurs in residents of or travelers to tropical regions. It is thought to involve chronic bacterial infection of the small bowel, causing inflammation and villous atrophy similar to celiac disease but affecting the entire length of the small intestine, with particularly prominent ileal involvement (unlike celiac disease). This ileal involvement explains why vitamin B12 deficiency and megaloblastic anemia are more prominent in tropical sprue than in celiac disease. Whipple disease is a rare systemic infection caused by the bacterium Tropheryma whipplei that infiltrates the small intestinal mucosa (and other organs) with periodic acid-Schiff (PAS)-positive macrophages, distending the villi and blocking lymphatic drainage. This leads to fat malabsorption (steatorrhea) and malabsorption of other nutrients. Whipple disease can also affect joints (migratory polyarthralgia), the central nervous system (dementia, ophthalmoplegia), and the cardiovascular system (endocarditis). Common to all malabsorption syndromes is steatorrhea -- the passage of bulky, pale, foul-smelling, greasy stools that float in the toilet bowl and are difficult to flush. Steatorrhea results from fat malabsorption and is often the presenting symptom. The clinical consequences of malabsorption depend on which nutrients are poorly absorbed: iron deficiency causes microcytic anemia, folate and B12 deficiency cause megaloblastic anemia, calcium and vitamin D malabsorption cause osteoporosis and tetany, protein malabsorption causes edema and muscle wasting, and fat-soluble vitamin deficiency causes a range of specific syndromes (vitamin A deficiency causes night blindness, vitamin D deficiency causes rickets and osteomalacia, vitamin E deficiency causes neuropathy, and vitamin K deficiency causes bleeding disorders).