Lactose, Fructose, and Sucrose Intolerance

Lactose intolerance occurs when there is a deficiency of lactase in the brush border of the small intestine. When a person with lactase deficiency ingests lactose, the poorly digested lactose is fermented in the small and large intestines, resulting in abdominal bloating, discomfort, or diarrhea ( Fig. 63.1 ).

Open full size image

Lactose Intolerance.

Lactose, a disaccharide sugar found in milk, is a structural combination of glucose and galactose. Holzel described defective lactose absorption, which resulted in infant disease in 1959. Later, the clinical picture was also noted in adults.

The incidence of lactose intolerance varies throughout the world. Rates are 85% to 100% in persons of African and Asian descent, 40% to 90% in those of Mediterranean descent, and 5% to 20% in persons of English and Nordic descent. Initially the great ethnic and racial differences were thought to occur because of adaptation, but now genetic differences are believed to be the probable cause.

When lactose is digested, it is normally hydrolyzed in the small intestine. The lactase level appears to fall below 3 U/g tissue for intestinal protein in symptomatic lactase-deficient patients. When the intestinal microflora ferment the lactose, they produce tannic acids, lactic acid, carbon dioxide, and hydrogen. In addition, lactose itself has an osmotic effect in the colon.