There is a range of clinical disorders in which sugar digestion or absorption is disturbed and gives rise to sugar intolerance, creating symptoms by the undigested or unabsorbed sugar and causing water to enter the intestine, which activates peristalsis and induces passage of frequent fluid stools. The undigested carbohydrate can also enter the colon and become fermented into diarrheic agents. The disorders are usually classified as (a) congenital or (b) secondary to some other disease, to impaired digestion of disaccharides, or to impaired absorption of the monosaccharides. The congenital deficiencies, although relatively rare, are life threatening; examples are sucrase-maltase deficiency (watery diarrhea after ingesting sucrose-containing foods), alactasia (absence of lactase, diarrhea from ingestion of milk), glucose-galactose malabsorption (diarrhea from ingestion of glucose, galactose, or lactose), and the very rare trehalase deficiency (intolerance to trehalose in mushrooms). Sugar intolerance secondary to underlying gastrointestinal disease is the commonest type, especially in pediatrics. Infections of the gastrointestinal tract, for example, often induce a temporary intolerance to lactose.
Lactose Intolerance. Adult mammals and most human groups after weaning keep only a fraction of the intestinal lactase activity of neonates (who need it to digest the lactose of breast milk). The persistence of lactase activity in Europeans has been regarded as the exception to the rule, since most human groups are hypolactasic and lactose malabsorbers (56). However, small amounts of dietary lactose, up to 250 mL of milk, can be tolerated by most adult lactose maldigesters. The decrease in lactase in adults is a programed event, and feeding high-lactose diets does not prevent the decrease. The mechanisms of the decline in activity have been studied in rats. As the animal matures, more and more mRNA message for lactase is needed to maintain the decreasing lactase activity in the enterocytes, suggesting that translational events may be of great importance in lactase gene expression ( 57).
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