Lactose, which is the only sugar of animal origin, is the principal carbohydrate found in milk. Lactose is a disaccharide composed of two simpler monosaccharides -- galactose and glucose -- and it cannot be absorbed intact through your intestinal wall. Therefore, lactose must be split into its constituent monosaccharides by the enzyme lactase, which is found in the cells lining the interior of your intestine. Lactase is not present in all individuals -- in some ethnic groups it is distinctly lacking -- which causes problems with milk digestion.
Congenital Lactase Deficiency
Milk is the staple diet of all mammalian infants. When you were born, the cells in your small intestine were richly endowed with lactase, which permitted the degradation of lactose into molecules that were small enough to be transported across your intestinal wall and into your bloodstream. Almost all infants have sufficient lactase in their intestines. Lactase deficiency that is present at birth -- congenital lactase deficiency -- is a rare inherited condition that has only been reported in a few infants, according to a September 2006 review in "Pediatrics."
Primary Lactase Deficiency
Scientists at the University of Kansas School of Medicine report that all land mammals exhibit a dramatic fall in lactase levels after weaning. Worldwide, humans lose 90 to 95 percent of their intestinal lactase by early childhood, with a further steady decrease in lactase levels throughout life. This normal decline in your lactase level, called primary lactase deficiency, is the most common cause of lactose intolerance. While only 2 percent of northern Europeans develop primary lactase deficiency, 50 to 80 percent of Latinos, 60 to 80 percent of blacks and Ashkenazi Jews and nearly 100 percent of Asians and Native Americans eventually become lactose intolerant.
Secondary Lactase Deficiency
Under certain circumstances, you can develop symptoms of lactase deficiency even though you are usually capable of digesting milk and other dairy products. Viral gastroenteritis, or "stomach flu," can remove the lactase from your intestine and leave you intolerant to lactose. Likewise, intestinal injury, surgery, cancer chemotherapy or persistent diarrhea can impair your ability to digest lactose. Secondary lactase deficiency is usually temporary; as soon as the underlying condition resolves, your intestine once again produces lactase and you can return to consuming milk products.
Symptoms
If lactose cannot be digested due to lactase deficiency, it remains in your intestine and acts like a "chemical sponge," attracting fluids into the interior of your bowel. This increases the volume and fluidity of your stool, which accelerates its movement through your gastrointestinal tract. Once the undigested lactose reaches your colon, it becomes a substrate for bacterial fermentation, which results in the production of large volumes of gas. These events culminate in the flatulence, rumbling, cramping and diarrhea that are typical symptoms of lactose intolerance.
Considerations
Lactase deficiency, or lactose intolerance, is a common condition globally, with some ethnic groups being almost universally intolerant to milk products. In fact, lactase deficiency is normal among these groups. Avoidance of lactose-containing foods is the simple remedy for lactose intolerance. Lactase supplements, however, are commercially available if you are lactose intolerant but still wish to consume milk products. You can purchase milk that has been pretreated with lactase, or you can use lactase capsules in doses that vary according to the amount of dairy products consumed. Ask your doctor for advice in using lactase supplements.
References
- "Staying Healthy with Nutrition: Lactose"; Elson M. Haas, M.D.; 2006
- "Pediatrics"; Lactose Intolerance in Infants, Children, and Adolescents; M.B. Heyman; September 2006
- "American Family Physician"; Lactose Intolerance; D.L. Swagerty, et al.; May 2002
