The lactose disaccharide is hydrolyzed to galactose and glucose in the intestine by an enzyme called lactase . Lactase activity is limited primarily to the suckling period and disappears after weaning. In humans, the lactaase activity is slow to develop in the infant, and the enzymatic activity is slower in its hydrolytic function. Human milk also has relatively high lactose. Undigested lactose may then support growth of undesirable intestinal flora, as well as draw water into the intestine causing diarrhea.
Several conditions associated with lactose intolerance exist.
Lactose intolerance is the clinical symptoms associated with lactose malabsorption, including diarrhea, bloating and flatulence, abdominal pain, and gaseous accumulation in the intestine in response to a dose of lactose. These symptoms arise from the large osmotic effect of the undigested lactose which results in fluid being drawn into the intestinal lumen. Congenital lactase deficiency is a rare genetic condition where lactase activity is decreased or absent at birth and remains low throughout life. Lactase persistence may be associated with a genetic mutation in certain ethnic groups. This condition is temorary in that normal lactase activity may return upon normalization of intestinal function.