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The assimilation of ingested nutrients involves a complex integrated process of digestion and absorption with subsequent transport of the breakdown products across the intestinal mucosa into the systemic circulation. Normal digestion and absorption is discussed in Chapter 381. The term malabsorption is broadly used to characterize abnormalities of both digestion and absorption. The schema in Table 408-1 lists the major pathogenic mechanisms of various specific malabsorptive disorders that are generally due to disorders in luminal digestion, mucosal function, or lymphatic transport. Malabsorption may involve multiple nutrients (as occurs with celiac disease or pancreatic insufficiency) or only a single molecule (as found in isolated glucose-galactose malabsorption or vitamin B12 malabsorption). Defects may be congenital, with onset of symptoms shortly after birth, or acquired, when the age of onset of symptoms is variable.

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Table 408-1. Mechanisms of Maldigestion and Malabsorption 
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The nutritional consequences of malabsorption vary from mild or none to severe, with malnutrition and even death. Abnormalities that impair absorption tend to have greater nutritional consequences than those impairing digestion. In general, growth is affected, manifest first by poor weight gain or weight loss and subsequently by linear growth retardation. Additional consequences may involve specific nutrients causing conditions such as rickets, osteoporosis, or tetany (vitamin D and calcium deficiency), coagulation disturbances (vitamin K deficiency), or anemia (iron, folate, or vitamin B12 deficiency).

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Clinical Features

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The evaluation of children with chronic diarrhea is discussed in Chapter 385. The most prominent clinical manifestation of children with disorders of digestion or absorption is diarrhea, except in those conditions involving malabsorption of a single molecule such as vitamin B12. Diarrhea is often accompanied by symptoms of abdominal distension, excessive flatulence, and growth failure in the older infant, toddler, or child. In the newborn or in early infancy, growth failure may not be evident and diarrhea may lead to dehydration. Very watery stools may soak into the diaper and be mistaken for urine, especially in infants with disorders such as congenital chloride-losing diarrhea....

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