Sucrase-isomaltase deficiency presents with a history of chronic diarrhoea and abdominal pain and discomfort when the diet contains sucrose or starch. If this diet is replaced by one containing lactose, the symptoms disappear. Milk is tolerated well, as is glucose; but the usual clear liquid diet of water containing table sugar, fruit juices or carbonated beverages may aggravate the diarrhoea.
After oral administration of a test dose (1-2 gm/kg) of lactose, glucose, galactose or maltose a normal rise of blood sugar concentration; this is not seen after the ingestion of sucrose, which may be followed by explosive diarrhoea. The pH of the stool is low because of the presence of lactic acid formed by the bacterial fermentation of the unabsorbed carbohydrates. The presence of the lactic acid serves to maintain the diarrhoea as it acts as an irritant and increases intraluminal osmolality.
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