Lactose Intolerance

Basics

Lactose intolerance is defined as the inability to digest the ingested disaccharide lactose, secondary to a deficiency of the intrinsic enzyme lactase, resulting in clinical symptoms.
Lactase deficiency is an intrinsic low level of lactase production which may or may not be associated with clinical symptoms.
Lactose is a disaccharide composed of glucose and galactose.
Lactose is important as a source of energy; it is the major carbohydrate in human and other mammalian milks; promotes the absorption of calcium, phosphorus, and iron; and has a probiotic effect on the gut flora.
Four types of lactase deficiency
- Congenital lactase deficiency
  - Extremely rare
  - Presents during the newborn period
  - Will cause severe diarrhea and failure to thrive and risk the newborn’s life
- Primary lactase deficiency (adult-type hypolactasia)
  - Due to relative or absolute absence of lactase
  - Develops during childhood at different ages in different racial groups
  - Most common cause of lactose intolerance
- Secondary lactase deficiency
  - Results from small bowel injury (acute gastroenteritis, persistent diarrhea, small bowel bacterial overgrowth, chemotherapy)
  - Can present at any age, more common in infancy
- Developmental lactase deficiency
  - A relative lactase deficiency observed in premature infants <34 weeks’ gestation

~70% of the world’s population is prone to developing primary lactase deficiency by adulthood.
- The prevalence of primary lactase deficiency in northern Europeans, who have a diet rich in dairy, is 2%.
- In Hispanic populations, the prevalence of primary lactase deficiency is 50–80%.
- In Ashkenazi Jewish and African American populations, the prevalence is 60–80%.
- In Asian populations, the prevalence of primary lactase deficiency is nearly 100%.
Nearly 20% of children <5 years from Hispanic, Asian, or African American descent have lactase deficiency and lactose malabsorption.
Caucasian children usually do not develop symptoms until after 5 years of age.

Posttranslational regulatory mechanisms in primary lactase deficiency or adult-type hypolactasia
Correlation between the genetic polymorphism of mRNA and persistence of lactase activity with early loss at 1 to 2 years in Thai children and late loss at 10 to 20 years in Finnish children

Symptoms depend on the amount of lactose ingested.
Malabsorbed lactose creates an osmotic load that draws fluid and electrolytes into the bowel lumen, leading to an osmotic diarrhea.
Nonabsorbed lactose acts as a substrate for intestinal bacteria.
In the colon, bacteria metabolize lactose, producing volatile fatty acids and gases leading to flatulence, bowel distension, pain, and low pH.

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