Lactose Intolerance

Article Author:
Talia Malik
Article Editor:
Kiran Panuganti
10/27/2018 12:31:41 PM
PubMed Link:
Lactose Intolerance


Lactose intolerance is a clinical syndrome that manifests with characteristic signs and symptoms upon the consumption of food substances containing lactose, a disaccharide. Normally upon the consumption of lactose, it is hydrolyzed into glucose and galactose by lactase enzyme, which is found in the small intestinal brush border. Deficiency of lactase due to primary or secondary causes results in clinical symptoms. Disease severity varies among individuals. Lactose is present in dairy, milk products, and mammalian milk.[1] It is also sometimes referred to as lactose malabsorption.


Lactase enzyme deficiency can occur in individuals, and they have lower levels of this enzyme which results in failure to hydrolyze lactose into absorbable glucose and galactose components. There are 4 main causes of lactase deficiency.

Primary Lactase Deficiency

It is the most common cause of lactase deficiency, also known as lactose non-persistence. There is a gradual decline in lactase enzyme activity with increasing age. Enzyme activity begins to decline in infancy and symptoms manifest in adolescence or early adulthood. It is a hereditary cause of lactase deficiency.

Secondary Lactase Deficiency

Injury to intestinal mucosa due to several infectious, inflammatory or other diseases can cause secondary lactase deficiency. Common causes include:

  • Gastroenteritis
  • Celiac disease
  • Crohn disease
  • Ulcerative colitis
  • Chemotherapy
  • Antibiotics

Congenital Lactase Deficiency

There is a decrease or absence of lactase enzyme activity since birth due to autosomal recessive inheritance. It manifests in the newborn after ingestion of milk. It is a rare cause of the deficiency. 

Developmental Lactase Deficiency

It is seen in premature infants born at 28 to 37 weeks of gestation.[2] The intestine of the infant is underdeveloped resulting in an inability to hydrolyze lactose. This condition improves with increasing age due to the maturation of intestine, which results in adequate lactase activity.


Lactose intolerance is a common disease; however, it is rare in children younger than 5 years of age. It is most often seen in adolescents and young adults. On average, 65% of the world's population is lactose intolerant.[3] The prevalence of lactose intolerance is variable among different ethnicities. It is most common in African Americans, Hispanics/Latinos, and Asians, while least prevalent in people of European descent.[4] Ethnic groups with a higher prevalence of lactose intolerance also are more likely to have lactose non-persistence.[5][6]


Lactase enzyme is located in the brush border of the small intestinal mucosa.  Deficiency of lactase results in presence of unabsorbed lactose within the bowel. This results in an influx of fluid into the bowel lumen resulting in osmotic diarrhea. Colonic bacteria ferment the unabsorbed lactose producing gas(hydrogen, carbon dioxide, and methane) which hydrolyzes lactose into monosaccharides. This results in an additional influx of fluid within the lumen. The overall effect of these mechanisms results in various abdominal signs and symptoms.


Microscopic findings of the small intestine vary depending on the cause of lactase deficiency. In primary lactase deficiency, the mucosa appears normal. Lactase activity can be measured to assess the severity of the disease. In secondary lactase deficiency, the mucosa may be abnormal depending upon the underlying cause. It is helpful in determining the secondary causes of disease such as celiac disease.

History and Physical

Signs and symptoms of lactose intolerance manifest 30 minutes to 1 to 2 hours after ingestion of milk containing (dairy) products. The severity of symptoms depends upon the amount of lactose consumed and the severity of the disease. Common signs and symptoms may include:

  • Diarrhea
  • Abdominal bloating
  • Abdominal Pain
  • Nausea and vomiting
  • Fullness
  • Flatulence

Less commonly it can present with headache, muscle pain,  joint pain, mouth ulcers, urinary symptoms and loss of concentration.[7][8]


Lactose intolerance is evaluated by getting a careful history, performing the physical exam and medical tests.


It is important to take a past medical, family and dietary history to determine the cause of lactose intolerance and exclude secondary causes.

Physical Exam

Assess for presence of abdominal bloating, tenderness, and pain.

Medical Tests

  1. Hydrogen breath test: This test measures the hydrogen content of breath after ingestion of lactose. The test is positive for lactose malabsorption if post lactose breath hydrogen value rises >20ppm compared with baseline. 
  2. Stool acidity test: Unabsorbed lactose is fermented by colonic bacteria into lactic acid which lowers the stool pH.
  3. Lactose tolerance test: This test determines the absorption of lactose after ingestion of a lactose-containing liquid. Blood samples are obtained every 2 hours to check for blood glucose levels. Failure of blood glucose levels to rise may indicate lactose intolerance.
  4. Small bowel biopsy: This test is rarely performed as it is invasive. It is only indicated to rule out secondary causes of lactose intolerance.

Treatment / Management

Management of lactose intolerance consists of dietary modification, lactase supplementation and the treatment of an underlying condition in people with secondary lactase deficiency.

Dietary Modification

Lactase containing milk products and calcium supplements are recommended. Limiting dietary intake of lactose by avoiding intake of lactose-containing products improves the symptoms of the disease. Following products contain lactose and therefore must be avoided:

  • Soft and processed cheese
  • Buttermilk
  • Cream
  • Milk
  • Ice cream
  • Sour cream
  • Whey
  • Yogurt
  • Pancakes and waffles
  • Mashed potatoes
  • Butter
  • Margarine
  • Custard and pudding

 Lactase Supplements

Lactase enzyme supplements contain lactase which breaks down lactose in milk and milk containing products. They are available as lactase enzyme tablets or drops.

Differential Diagnosis

  • Irritable bowel syndrome
  • Celiac disease
  • Tropical sprue
  • Cystic fibrosis
  • Ulcerative colitis
  • Diverticular disease
  • Intestinal Neoplasm or polyp
  • Excessive ingestion of laxatives
  • Viral gastroenteritis
  • Bacterial infection
  • Giardiasis


Lactose intolerance has an excellent prognosis. Most patients have a considerable improvement in signs and symptoms with dietary modification alone.


  • Osteopenia
  • Osteoporosis
  • Malnutrition
  • Weight loss


Once a diagnosis of lactose intolerance is made, consult with a gastroenterologist and dietician. 

Pearls and Other Issues

People with lactose intolerance can check the ingredients on food labels for lactose on food products to avoid their use.

Some people with lactose intolerance can tolerate some amounts of milk and milk containing products and may not need to avoid them completely.

Lactose intolerance is commonly confused with milk allergy. Lactose intolerance is a gastrointestinal disorder while milk allergy is an autoimmune reaction against certain milk proteins. Milk allergy is life-threatening and presents early in infancy while Lactose intolerance usually presents in adolescence or early adulthood. 

Milk is rich in calcium, and vitamin D. Prolong avoidance of milk in people with lactose intolerance can result in calcium and vitamin D deficiency.