Lactose intolerance
Lactose intolerance is osmotic diarrhea, bloating, and flatulence caused by deficiency of the brush-border enzyme lactase, most often the primary age-related decline seen in people of Asian, African, or Native American descent. Diagnosis rests on symptom resolution with lactose elimination and recurrence on rechallenge, confirmed when needed by a hydrogen breath test, and treatment is dietary lactose restriction with lactase enzyme replacement and calcium and vitamin D intake.
- Topic reviewThe full article, organized the way the exam asks it.
- ABIM-format MCQsVignettes with an explanation for every wrong answer.
- Key factsThe numbers and thresholds worth memorizing, up front.
- AI tutorMedBrain answers the question you're stuck on.
What this article covers
- Overview
Definition: lactose intolerance is the clinical syndrome of bloating, flatulence, cramps, and diarrhea that follows ingestion of lactose-containing dairy in a person with insufficient brush-border lactase activity to digest it.
- Epidemiology
Primary (adult-type) lactase decline is the dominant form worldwide: a genetically programmed loss of lactase-persistence that follows weaning, becoming clinically apparent in adolescence or adulthood.
- Risk factors
Asian, African, or Native American ancestry: the strongest predictor, because absence of the lactase-persistence allele is common in these populations.
- Pathophysiology
Normal digestion requires brush-border lactase: the enzyme lactase, expressed on the small intestinal brush border, splits dietary lactose into its absorbable monosaccharides glucose and galactose.
- Clinical features
Symptoms follow dairy ingestion by roughly 30 minutes to 2 hours: bloating, abdominal cramps, and flatulence are the earliest and most consistent complaints.
- Differential diagnosis
Celiac disease: intermittent abdominal pain, diarrhea, and malabsorption tied to gluten rather than dairy specifically, with possible weight loss and anemia; confirmed by tissue transglutaminase…
- Diagnosis
A clinical diagnosis in most patients: lactose intolerance is confirmed when symptoms resolve on a trial of dietary lactose exclusion (or with administration of lactase enzymes with meals) and recur…
- Management
First step, dietary lactose restriction: reduce or eliminate lactose-containing dairy (milk, custard, ice cream, soft cheeses) as part of a low-lactose or lactose-free diet; most patients tolerate…
- Complications
Inadequate calcium and vitamin D intake: unsupervised, prolonged dairy avoidance without dietary substitution or supplementation risks low bone mineral density over time.
- Prognosis
Excellent with dietary management: symptoms resolve promptly, typically within days, once dietary lactose is reduced to a tolerated level or covered with lactase enzyme replacement.
- Prevention
Individualize the lactose threshold rather than eliminating all dairy: severity is dose-dependent, and titrating intake to the individual's own tolerated amount of dairy prevents unnecessary total avoidance.
Read the whole article
Subscribing to Internal Medicine opens every article, the full question bank and MedBrain, on the web, iPhone and Android.
Related articles
- Celiac disease
Celiac disease is an immune-mediated small-intestinal enteropathy triggered by dietary gluten in genetically predisposed individuals, presenting with malabsorptive diarrhea, iron-deficiency anemia, or dermatitis herpetiformis. Diagnosis requires IgA tissue transglutaminase serology with a total IgA level while the patient still eats gluten, confirmed by duodenal biopsy showing villous atrophy, and the only treatment is a strict lifelong gluten-free diet.
- Irritable bowel syndrome (IBS)
Irritable bowel syndrome is a functional disorder of gut-brain interaction diagnosed clinically by Rome IV criteria, recurrent abdominal pain related to defecation, stool frequency, or stool form, without alarm features. Management is subtyped by stool pattern, with fiber and dietary change first, then IBS-C secretagogues or IBS-D antidiarrheals and antibiotics, and low-dose tricyclics or antispasmodics for pain.
- Small intestinal bacterial overgrowth
Small intestinal bacterial overgrowth is excessive bacterial colonization of the proximal small bowel, driven by stasis from strictures, blind loops, or dysmotility, or by loss of gastric-acid and immune defenses, producing bloating, diarrhea, and fat and B12 malabsorption. Diagnosis rests on a carbohydrate breath test or jejunal aspirate culture, and treatment corrects the underlying cause while a course of rifaximin or another antibiotic reduces the bacterial burden.