Congenital sucrase-isomaltase deficiency (CSID) is a rare inherited metabolic disorder characterized by the deficiency or absence of the enzymes sucrase and isomaltase. This enzyme complex (sucrase-isomaltase) assists in the breakdown of sucrose/saccharose and certain products of starch digestion (dextrins). The sucrase-isomaltase enzyme complex is normally found within the tiny, finger-like projections (microvilli or brush border) lining the small intestine. When this enzyme complex is deficient, nutrients based on ingested sucrose and starch cannot be absorbed properly from the gut.