MECHANISM OF DIARRHOEA IN CONGENITAL DISACCHARIDE MALABSORPTION

Intestinal malabsorption of lactose and sucrose, and the effect of these disaccharides on the movements of water and electrolytes were studied in two siblings with congenital lactose malabsorption (CLM) and one infant with congenital sucroseisomaltose malabsorption (CSIM), using the intestinal intubation technique. Sucrose in the CLM‐patients and lactose in the CSIM‐patient were absorbed already in the proximal part of the small intestine, whereas hardly any lactose in CLM and sucrose in CSIM disappeared from the small intestinal lumen. The unabsorbed disaccharides caused considerable movement of water and electrolytes into the intestinal lumen test fluid being diluted to 1 1/2–2 1/2 times in the duodenum and proximal jejunum. The contents of the small intestine were throughout isoosmotic with extracellular fluid and the unabsorbed disaccharides represented 25–30% of the osmotic activity. In the colon, the unabsorbed disaccharides disappeared in varying degree, they were split into monosaccharides and converted to lactate. Water, Na+ and Cl‐ were absorbed in the colon even in relative excess to the disappearance of the disaccharide and its splitting products, and in all cases the faecal volume was smaller than the volume of the test fluid. In a CLM‐patient a 3‐week period on lactose‐containing diet had no effect on the phenomena demonstrated in the small intestine, but disappearance of lactose and formation of lactate was greater in the colon, and amount of water less in the stools than before. It is suggested that the retention of water in the intestinal lumen through the osmotic activity of the unabsorbed disaccharide, together with the organism's tendency to Na+‐equilibrium between the luminal and extracellular fluid is the most important mechanism of diarrhoea in disaccharide malabsorption. Copyright © 1968, Wiley Blackwell. All rights reserved