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Lactose


Lactose Intolerance

The lactose disaccharide is hydrolyzed to galactose and glucose in the intestine by an enzyme called lactase. Lactase is a ß-galactosidase enzyme and is found associated with the intestinal brush border membrane. 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. If an infant is breast-fed over a long period of time, the lactose intake may reach 30-40 g/day and exceed the lactase capacity. 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 malabsorption (lactose nondigestion) is the decreased hydrolysis of lactose in the intestine and may result from genetic defects, decreased lactase activity normally associated with advancing age, or damage to the intestinal lining. 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. Primary lactase nonpersistency (adult lactase deficiency) is common in mammalian species and occurs as the lactase activity naturally declines postweaning. Lactase persistence may be associated with a genetic mutation in certain ethnic groups.

Secondary lactase deficiency (acquired hypolactasia) is a temporary condition in which lactase activity is low due to damage to the intestinal lining. This damage may result from intestinal surgery or diarrheal diseases such as gastroenteritis, inflammatory bowel disease, cow's milk intolerance, or AIDS. This condition is temporary in that normal lactase activity may return upon normalization of intestinal function.

Galactosanemia is a human disease where galactose can not be utilized. An enzyme in galactose metabolism, galactose-1-phosphate uridyl transferase, is lacking and the person can not convert galactose back to glucose. Feeding milk to infants that have this disease results in increased size of the liver and spleen, increased incidence of cataracts, and feeble-mindedness. Removal of galactose from the diet results in rapid regression of the symptoms.


Lactose
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