Merck Manual

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Carbohydrate Intolerance


Atenodoro R. Ruiz, Jr.

, MD, The Medical City, Pasig City, Metro-Manila, Philippines

Last full review/revision Oct 2019| Content last modified Oct 2019
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Carbohydrate intolerance is the inability to digest certain carbohydrates due to a lack of one or more intestinal enzymes. Symptoms include diarrhea, abdominal distention, and flatulence. Diagnosis is clinical and by a hydrogen (H2) breath test. Treatment is removal of the causative disaccharide from the diet.

Carbohydrate intolerance is a malabsorption syndrome.


Disaccharides are normally split into monosaccharides by disaccharidases (eg, lactase, maltase, isomaltase, sucrase [invertase]) located in the brush border of small-bowel enterocytes. Undigested disaccharides cause an osmotic load that attracts water and electrolytes into the bowel, causing watery diarrhea. Bacterial fermentation of carbohydrates in the colon produces gases (hydrogen, carbon dioxide, and methane), resulting in excessive flatus, bloating and distention, and abdominal pain.


Enzyme deficiencies can be

  • Congenital (eg, rare deficiencies of lactase or sucrase-isomaltase)

  • Acquired (primary)

  • Secondary

Acquired lactase deficiency (primary adult hypolactasia) is the most common form of carbohydrate intolerance. Lactase levels are high in neonates, permitting digestion of milk; in most ethnic groups (80% of blacks and Hispanics, > 90% of Asians), the levels decrease in the post-weaning period rendering older children and adults unable to digest significant amounts of lactose. However, 80 to 85% of whites of Northwest European descent produce lactase throughout life and are thus able to digest milk and milk products. It is unclear why the normal state of > 75% of the world’s population should be labeled a “deficiency.”

Secondary lactase deficiency occurs in conditions that damage the small-bowel mucosa (eg, celiac disease, tropical sprue, acute intestinal infections [see Gastroenteritis]). In infants, temporary secondary disaccharidase deficiency may complicate enteric infections or abdominal surgery. Recovery from the underlying disease is followed by an increase in activity of the enzyme.

Symptoms and Signs

Symptoms and signs of carbohydrate intolerance are similar in all disaccharidase deficiencies. A child who cannot tolerate lactose develops diarrhea after ingesting significant amounts of milk and may not gain weight. An affected adult may have watery diarrhea, bloating, excessive flatus, nausea, borborygmi, and abdominal cramps after ingesting lactose. The patient often recognizes early in life that dairy causes gastrointestinal problems and avoids eating dairy products. Symptoms typically require ingestion of more than the equivalent of 250 to 375 mL (8 to 12 oz) of milk. Diarrhea may be severe enough to purge other nutrients before they can be absorbed. Symptoms may be similar to and can be confused with irritable bowel syndrome.

Pearls & Pitfalls

  • Most people with lactase deficiency can tolerate up to 250 to 375 mL (8 to 12 oz) of milk; symptoms that occur after consuming much smaller amounts may suggest another diagnosis.


  • Clinical diagnosis

  • Hydrogen breath test for confirmation

Lactose intolerance can usually be diagnosed with a careful history supported by dietary challenge. Patients usually have a history of diarrhea and/or gas after ingestion of milk and dairy foods; other symptoms, such as rash, wheezing, or other anaphylactic symptoms (particularly in infants and children), suggest a cow's milk allergy. Milk allergy is rare in adults and also may cause vomiting and symptoms of esophageal reflux, which are not manifestations of carbohydrate intolerance. The diagnosis is also suggested if the stool from chronic or intermittent diarrhea is acidic (pH < 6) and can be confirmed by an H2 breath test or a lactose tolerance test.

In the hydrogen breath test, 50 g of lactose is given orally and the hydrogen produced by bacterial metabolism of undigested lactose is measured with a breath meter at 2, 3, and 4 hours postingestion. Most affected patients have an increase in expired hydrogen of > 20 parts per million over baseline. Sensitivity and specificity are > 95%.

The lactose tolerance test is less sensitive, about 75%, although specificity is > 95%. Oral lactose (1.0 to 1.5 g/kg body weight) is given. Serum glucose is measured before ingestion and 60 and 120 minutes after. Lactose-intolerant patients develop diarrhea, abdominal bloating, and discomfort within 20 to 30 minutes, and their serum glucose levels do not rise to > 20 mg/dL (< 1.1 mmol/L) above baseline.


  • Dietary restriction

Carbohydrate malabsorption is readily controlled by avoiding dietary sugars that cannot be absorbed (ie, following a lactose-free diet in cases of lactase deficiency). However, because the degree of lactose malabsorption varies greatly, many patients can ingest up to 375 mL (18 g of lactose) of milk daily without symptoms. Yogurt is usually tolerated because it contains an appreciable amount of lactase produced by intrinsic Lactobacilli. Cheese contains lower amounts of lactose than milk and is often tolerated, depending on the amount ingested.

For symptomatic patients wishing to drink milk, lactose in milk can be predigested by the addition of a commercially prepared lactase, and pretreated milk is now available. Enzyme supplements should be an adjunct to, not a substitute for, dietary restriction. Lactose-intolerant patients must take calcium supplements (1200 to 1500 mg/day).

Key Points

  • Disaccharide deficiency (usually of lactase) can be acquired or, rarely, congenital.

  • Undigested disaccharides, such as lactose, create an osmotic load that causes diarrhea.

  • Intestinal bacteria metabolize some undigested disaccharides, producing gases that cause distention and flatus.

  • Confirm clinical diagnosis by doing a hydrogen breath test.

  • Dietary restriction is usually adequate treatment.

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NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
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