Lactose Intolerance

D M Paige, Johns Hopkins Bloomberg School of

Public Health, Baltimore, MD, USA

© 2005 Elsevier Ltd. All rights reserved.

Lactose maldigestion and intolerance result from an inability to digest varying amounts of the milk sugar lactose. This is a result of an inadequate amount of the genetically regulated milk sugar enzyme lactase. The most common reason for lactose maldigestion is a decline of lactase activity with increasing age. Lactose maldigestion may also occur secondary to intestinal tract infection and diarrhea. A rare form of alactasia, an absence of the milk sugar enzyme, can occur at birth. The symptoms associated with lactose maldigestion are a result of the incomplete hydrolysis, or splitting, of the disaccharide lactose into its absorbable monosaccharide components, glucose and galactose. Lactose maldigestion may result in abdominal bloating and/or pain, flatulence, loose stools, and diarrhea, singly or in combination.

The symptoms associated with lactose maldigestion result in lactose intolerance. The most common form of lactose maldigestion and lactose intolerance, as observed in the majority of the world's adult population, is due to genetically determined low lactase levels. Lactase deficiency due to genetic non-persistence is reported in approximately 70% of the world's adult population.

The prevalence is lowest in people of Northern European descent (15%) and highest in many Asian populations (near 100%). The prevalence of lactase deficiency in individuals of African descent is approximately 70-80%. Similar levels are reported for Latinos and those of Eastern European and South American ancestry. Not all individuals with a reduced level of the enzyme lactase exhibit symptoms with the ingestion of dietary lactose. The presence or absence of symptoms varies with the amount and type of food consumed, intestinal transit time, and level of residual intestinal lactase. Individuals with low lactase levels may tolerate a moderate intake of lactose. Lactase deficiency can generally be identified by a breath hydrogen test measuring the level of undigested lactose reaching the colon. Bacterial fermentation of the undigested lactose is responsible for the volume of breath hydrogen production. A lactose tolerance test measuring blood sugar rise has also been used. Individuals experiencing discomfort with lactose ingestion can elect to consume commercially hydrolyzed milk that is readily available, milk substitutes, or alternative food sources equally rich in calcium.

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