Lactose Digestion And Lactose Intolerance

The enzyme lactase, located in the small intestine, catalyzes the hydrolysis of lactose into its monosaccharide con

stituents, D-glucose and D-galactose. Both are rapidly absorbed and enter the bloodstream.

lactase lactose -ยป D-glucose + D-galactose

Only monosaccharides can be absorbed from the small intestine. Therefore, if for any reason, ingested lactose is only partially hydrolyzed (digested) or is not hydrolyzed at all, a clinical syndrome results (8-10). Its symptoms are abdominal distention, cramps, flatulence (gas), and diarrhea. When there is an insufficient amount of lactase, some lactose is not absorbed from the small intestine and remains in the lumen. The presence of lactose draws fluid into the intestinal lumen by osmosis. It is this fluid that leads to the abdominal distention, cramps, and diarrhea. From the small intestine, the lactose passes into the large intestine (colon) where bacteria effect its anaerobic fermentation to short-chain fatty acids. Production of more molecules results in still greater retention of fluid. The acidic products of fermentation lower the pH and irritate the lining of the colon, leading to an increased movement of the contents. Diarrhea is caused by the retention of fluid and the increased movement of the intestinal contents. The gaseous products of fermentation (carbon dioxide, hydrogen, methane, and oxygen) cause bloating.

This syndrome resulting from an absence or deficiency of lactase is called lactose intolerance. Lactose intolerance is not usually seen in children until after about 6 years of age. At this point, the incidence of lactose-intolerant individuals begins to rise. There are varying degree of lactose intolerance. The difference in incidence among populations of different genetic backgrounds can be large. By 12 years of age, 45% of blacks develop the symptoms of lactose intolerance; among teenage blacks, the incidence climbs to 70% and by adulthood, 90% of the black population in the United States shows symptoms of lactose intolerance. Lactose intolerance is also high among Orientals. Among the whites of western European ancestry, the peak incidence in adulthood is about 15%.

Humans consume lactose in milk and other nonfer-mented dairy products. Cheese, cottage cheese, and most yogurts contain less lactose because some lactose is converted into lactic acid during fermentation and/or removed with the whey. Milk from which much of the lactose has been removed is available.

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