Primary lactase deficiency is an example of a defect in a specific brush border enzyme, lactase, which causes the malabsorption of the disaccharide lactose. Infants and young children in all populations and most white adults of North America and Europe normally have sufficient lactase to hydrolyze milk lactose to its constituents, glucose and galactose. However, most of the adult populations of the world, including blacks, Asians, South and Central Americans, and Inuits, are typically «lactase-deficient» On a worldwide scale, therefore, whether the presence or lack of lactase is normal or abnormal depends on the population under consideration.
People with lactase deficiency typically experience abdominal cramps and watery diarrhea within minutes after ingesting milk. These symptoms develop because the unhydrolyzed lactose is not absorbed and remains within the intestinal lumen, where it acts as an osmotic cathartic. As the lactose passes into the lower bowel, bacterial action converts it to lactic acid and carbon dioxide, which contribute to the catharsis and cramping. The clinical history of abdominal cramps and diarrhea after milk ingestion usually is sufficient evidence of lactase deficiency. If the diagnosis is uncertain, however, a lactose tolerance test can be performed.
Treatment consists of restricting milk and milk products. Some milk is available commercially in which the lactose has been hydrolyzed.
Because lipoproteins are necessary for the formation of the apoprotein that combines with triglycerides, cholesterol, and phospholipids within the intestinal absorptive cells to form chylomicrons, the lack of beta-lipoproteins results in an accumulation of fat within the enterocyte and consequent fat malabsorption.
Stool fat is increased, but the small-bowel x-ray series and the xylose tolerance test are normal. Serum cholesterol and triglycerides are low, and beta-lipoproteins are absent. A small-bowel biopsy is diagnostic, showing villous epithelial cells distended with fat, but the appearance is otherwise normal.
There is no specific treatment of the underlying disease. The fat malabsorption improves with restriction of dietary long-chain triglycerides and substitution of medium-chain triglycerides, which do not require chylomicron formation but rather are absorbed directly from the villous epithelial cells into the blood. Fat-soluble vitamins also are indicated.