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- Lactose intolerance is a syndrome of abdominal pain, bloating, and flatulence after the ingestion of lactose.
- Lactose malabsorption results from a reduction in lactase activity in the brush border of the small intestinal mucosa.
- Lactase activity peaks at birth then decreases after the first few months of life, declining continuously throughout life. 75% of adults worldwide exhibit a decline in lactase activity after birth. Only 50% of lactase activity is needed to digest lactose without causing symptoms of lactose intolerance.
- Congenital lactose intolerance: very rare
- Primary lactose intolerance: common in adults who develop low lactase levels after childhood
- Secondary lactose intolerance: inability to digest lactose caused by any condition injuring the intestinal mucosa (e.g., infectious enteritis, celiac disease, eosinophilic gastroenteritis, or inflammatory bowel disease) or a reduction of available mucosal surface (e.g., resection)
- Lactose malabsorption may be asymptomatic and is equally common in healthy patients and in those with functional bowel disorders.
- System(s) affected: endocrine/metabolic, gastrointestinal
- Primary lactose intolerance begins in late childhood.
- No consensus on whether young children (<5 years of age) should avoid lactose following diarrheal illness
- Lactose-free formulas are available.
- Exclude milk protein allergy.
- ≥50% of infants with acute or chronic diarrheal disease have lactose intolerance; particularly common with rotavirus infection
- Lactose intolerance is common with giardiasis, ascariasis, irritable bowel syndrome (IBS), tropical and nontropical sprue, and AIDS malabsorptive syndrome.
- In South America, Africa, and Asia, rates of lactose intolerance are >50%.
- In the United States, the prevalence is 15% among whites, 53% among Hispanics, and 80% among African Americans.
- In Europe, lactose intolerance varies from 15% in Scandinavian countries to 70% in Italy.
- Predominant age:
- Primary: teenage and adult
- Secondary: depends on underlying condition
- Predominant sex: male = female
Etiology and Pathophysiology
- Primary lactose intolerance: The normal decline in lactase activity in the intestinal mucosa is genetically determined and permanent after weaning from breast milk.
- Secondary lactose intolerance: associated with gastroenteritis in children; also associated with any gastrointestinal infection or inflammation of the small intestine with resultant lactose malabsorption in both adults and children
- In whites, lactase deficiency is associated with a single nucleotide polymorphism (SNP) consisting of a nucleotide switch of T for C 13910 bp on chromosome 2. This results in variants of CC-13910 (lactase nonpersistence) OR CT-13910/TT-13910 (lactase persistence) (1).
- SNP (C/T-13910) is associated with lactase persistence in northern Europeans.
- Other SNPs (G/C-14010, T/G-13915, and C/G-13907) have been linked to lactase persistence in some patients of African descent.
- Adult-onset lactase deficiency has wide geographic variation.
- Signs and symptoms usually do not become apparent until after age 6 to 7 years.
- Symptoms may not be apparent until adulthood, depending on dietary lactose intake and rate of decline of intestinal lactase activity.
- Lactase activity correlates with age, regardless of symptoms.
Lactose avoidance relieves symptoms. Patients can learn what level of lactose is tolerable in their diet.
Commonly Associated Conditions
- Tropical or nontropical sprue
- IBS or other functional bowel disorders
- Small intestinal bacterial overgrowth (SIBO)
- Celiac disease