Lactose intolerance is the insufficient concentration of lactase enzyme, leading to fermentation of malabsorbed lactose by intestinal bacteria with subsequent production of intestinal gas and various organic acids, manifesting clinically with diarrhea, abdominal pain, flatulence, or bloating after lactose intake. Lactose malabsorption occurs when a substantial amount of lactose is not absorbed in the intestine. Lactase deficiency is defined as brush-border lactase activity that is markedly reduced relative to the activity observed in infants.

Lactose malabsorption

Lactase deficiency

Milk intolerance

Carbohydrate malabsorption

• Nearly 50 million people in the U.S. have partial or complete lactose intolerance. There are racial differences, with <25% of White adults being lactose intolerant but >85% of Asian Americans and >60% of African Americans having some form of lactose intolerance.
• There are geographic variations: Highest in Asians (up to 90%), lowest in northern Europeans (approximately 10%), intermediate in southern Europeans and Middle Eastern populations (up to 40%).

• Abdominal tenderness and cramping, bloating, flatulence
• Diarrhea
• Symptoms are directly related to the osmotic pressure of substrate in the colon and occur ∼2 h after ingestion of lactose
• Physical examination: May be entirely within normal limits

• Before it can be absorbed, lactose is cleared to glucose and galactose by the enzyme lactase in the brush border of the small intestine. If the amount of lactase is marginal or its expression is left, lactose intolerance will result.
• Congenital lactase deficiency: Common in premature infants; rare in term infants and generally inherited as a chromosomal recessive trait.
• Secondary lactose intolerance: Usually a result of injury of the intestinal mucosa (Crohn disease, viral gastroenteritis, AIDS enteropathy, cryptosporidiosis, Whipple disease, sprue).
• Acquired primary lactase deficiency (adult-type hypolactasia OMIM #223100) is the most common form of lactase deficiency worldwide. The decline in lactase activity is a multifactorial process that is regulated at the gene transcription level and leads to decreased biosynthesis, retardation of intracellular transport, or maturation of the enzyme lactase-phlorizin hydrolase.

• Inflammatory bowel disease
• Irritable bowel syndrome
• Pancreatic insufficiency
• Nontropical and tropical sprue
• Cystic fibrosis
• Diverticular disease
• Bowel neoplasm
• Laxative abuse
• Celiac disease
• Parasitic disease (e.g., giardiasis)
• Viral or bacterial infections

• A detailed dietary history is essential in the evaluation of patients with suspected carbohydrate malabsorption.
• The diagnosis can usually be made on the basis of the history and improvement with dietary manipulation.
• Diagnostic workup may include confirming the diagnosis with hydrogen breath test and excluding other conditions listed in the differential diagnosis that may also coexist with lactase deficiency.

• Laboratory evaluation may not be necessary in patients with significant history.
• Lactose breath hydrogen test: A rise in breath hydrogen >20 ppm within 90 min of ingestion of 50 g of lactose is positive for lactase deficiency. This test is positive in 90% of patients with lactose malabsorption. Common causes of false-negative results are recent use of oral antibiotics or recent high colonic enema. Fig. E1 illustrates the role of symptoms in determining the clinical importance of lactose malabsorption.
• The lactose tolerance test is an older and less accurate testing modality (20% rate of false-positive and false-negative results). The patient is administered an oral dose of 1 to 1.5 g of lactose/kg body weight. Serial measurement of blood glucose level on an hourly basis for 3 h is then performed. The test is considered positive if the patient develops intestinal symptoms and the blood glucose level rises <20 mg/dl above the fasting baseline level.
• Diarrhea associated with lactase deficiency is osmotic in nature with an osmotic gap and a pH <6.5.

Imaging studies are generally not indicated. A small bowel series may be useful in patients with significant malabsorption.

Management consists of reducing lactose exposure by avoiding milk and milk-containing products or using milk in which the lactose has been prehydrolyzed with lactase. A lactose-free diet generally results in prompt resolution of symptoms. Lactose is primarily found in dairy products but may be present as an ingredient or component of common foods and beverages. Possible sources of lactose include breads, candies, cold cuts, dessert mixes, cream soups, bologna, commercial sauces and gravies, chocolate, drink mixes, salad dressings, and medications. Labels should be read carefully to identify sources of lactose.

• Addition of lactase enzyme supplement (Lactaid tablets, Dairy Ease) before the ingestion of milk products may prevent symptoms in some patients. However, it is not effective for all lactose-intolerant patients.
• Lactose-intolerant patients must ensure adequate calcium intake. Calcium supplementation is recommended to prevent osteoporosis.

Patient education regarding foods high in lactose, such as milk, cottage cheese, or ice cream, is recommended.

Clinical improvement with restriction or elimination of milk products

GI referral for endoscopic procedures if concomitant GI disorders are suspected

• There is great variability in signs and symptoms in patients with lactose intolerance depending on the degree of lactase deficiency. Most individuals with presumed lactose malabsorption can tolerate 12 to 15 g of lactose or up to 12 oz of milk daily without symptoms.
• Nondairy synthetic drinks (e.g., Coffee-Mate) and use of rice milk are generally well tolerated.

Lactose Intolerance (Patient Information)