Lactose Intolerance

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Overview

• What is lactose?

  Lactose is a natural sugar found in milk and dairy products such as cheese and yogurt. After eating this sugar, it passes through the stomach into the small intestine where it is digested. A digestive enzyme called lactase breaks down lactose into two simple sugars: glucose and galactose. These simple sugars are absorbed in the small intestine and are taken up into the bloodstream where they act as nutrients.

  Lactose is commonly found in milk and dairy products such as cream, ice cream, yogurt, and cheese. It can also be found in bread and baked goods, processed breakfast cereals, instant potatoes, some soups, non-kosher lunch meats, candies, dressings and mixes for pancakes and biscuits. Lactose is the main sugar found in breast milk and standard infant formulas and serves as a baby’s primary dietary sugar.

Symptoms

• What are the symptoms of lactose intolerance?

  In those who are lactose intolerant, lactose is not properly digested in the small intestine. Instead, it passes undigested into the colon (large intestine). Upon reaching the colon, lactose is broken down by the normal colon bacteria in a process called fermentation. This breakdown results in the production of carbon dioxide and hydrogen gases, organic acids, and other molecules that cause the following symptoms of lactose intolerance:

  • Abdominal distension called bloating
  • Abdominal cramping or pain
  • Excess burping
  • Loud bowel sounds
  • Excess gas
  • Diarrhea with watery or explosive bowel movements
  • Sudden urge to have a bowel movement

  The symptoms of lactose intolerance can start during childhood or adolescence and tend to get worse with age. Many people do not develop symptoms until later in life. The symptoms of lactose intolerance worsen when larger amounts of lactose are eaten during a meal.

  Although eating lactose-containing foods will result in discomfort for someone who is lactose intolerant, they are usually not at risk of developing serious disease. However, babies who are born with primary lactase deficiency or children with secondary lactase deficiency may have more serious health issues from consuming lactose. These are discussed below.

Causes

• What causes lactose intolerance?

  Primary Lactase Deficiency: This condition is rare. It occurs when babies are born with reduced activity or absence of the enzyme lactase. Babies inherit this condition by getting one gene that causes this problem from each of their parents. This can occur even when both parents have no problem digesting lactose. These babies require a specialized formula with another type of sugar such as sucrose (present in table sugar), which they can digest.

  Secondary Lactase Deficiency: This is a temporary lactose intolerance caused by damage to the lining of the small intestine from infection or other gastrointestinal conditions. This commonly occurs in infants and young children but can happen in adults as well. Once these conditions are treated, the small intestine heals and lactase deficiency will usually improve. The symptoms of lactose intolerance typically resolve within three to four weeks after the lining of the small intestine returns to normal.

  The most common cause of temporary lactose intolerance is an infection of the gastrointestinal tract that damages the small intestine. Many typical gastrointestinal infections can result in lactose intolerance. In infants and young children, infection with Rotavirus or Giardia are common causes.

  Secondary lactase deficiency can also be due to medical conditions that affect the small intestine. One such condition is Celiac disease. This is an autoimmune disease directed against the gluten protein found in wheat, rye, barley, and other grains. Crohn's disease, an inflammatory condition that can affect any part of the gastrointestinal tract, can lead to secondary lactase deficiency as well.

  Acquired Lactase Deficiency: Many individuals acquire lactose intolerance as they get older. It is estimated that one half of adults in the United States have acquired lactase deficiency. This condition is due to a normal age-related decline in the amount of the lactase enzyme present in the small intestine. Although lactose is an important part of the diet in infants and young children it represents only 10% of the carbohydrate (sugar) intake in adults. Individuals who are lactose intolerant may develop symptoms with even small amounts of this sugar in their diet.

  Lactose intolerance occurs more frequently in certain families. One of the most important factors in developing lactose intolerance is an individual’s ethnic background. Approximately 15% of adult Caucasians and 85% of adult African Americans in the United States are lactose intolerant. The rate of lactose intolerance is also high in individuals of Asian descent, Hispanic descent, Native Americans, and Jewish individuals.

Diagnosis

• How is lactose intolerance diagnosed?

  Lactose intolerance is diagnosed by a simple test called a hydrogen breath test. After an overnight fast before the test, an individual breathes into a bag and then drinks a specified amount of milk sugar as a syrup. In adults this corresponds to the amount of milk sugar in a quart of milk. Subsequent breath samples are taken for up to three hours. The breath that they exhale into the bag is analyzed to determine its hydrogen content. During the test individuals who are lactose intolerant will have an increase in the amount of hydrogen that they exhale. If the values for hydrogen increase above a certain value, the diagnosis of lactose intolerance is made. Patients who are lactose intolerant may also develop their typical symptoms during the test.

  In those who cannot undergo testing, lactose intolerance is diagnosed after a trial of a lactose free diet. All lactose is removed from the diet for two weeks to see if symptoms improve. Lactose can then be added back while monitoring for any worsening symptoms.

Treatment

• How is lactose intolerance treated?

  The best treatment of lactose intolerance is dietary modification to avoid lactose containing foods. Individuals who are lactose intolerant should meet with a dietician to review the sources of lactose in their diet. Some reduction in lactose intake is usually required.

  Lactase supplements are also available even without a prescription. These supplements help break down the lactose sugar, reducing the symptoms of lactose intolerance. This type of supplement is taken throughout the day whenever lactose is ingested. Some individuals will be less intolerant of lactose than others and will be able to tolerate larger amounts of lactose in the diet. Alternatives to milk for lactose intolerant individuals include products such as soy milk. If an individual restricts their milk/dairy intake, it is important to ensure adequate supplementation of calcium and Vitamin D in the diet. This is especially important for children and women.

  Recommended daily calcium intakes:
  1-3 years of age	500 mg
  4-8 years of age	800 mg
  9-24 years of age	1300 mg
  Age 25 and above	800-1000 mg
  Pregnant and nursing women	1200 mg

Author(s) and Publication Date(s)

Marsha H. Kay, MD, The Cleveland Clinic, Cleveland, OH, and Vasundhara Tolia, MD, Children's Hospital of Michigan, Detroit, MI – Published September 2004.

Marsha H. Kay, MD, The Cleveland Clinic, Cleveland, OH, and Anthony F. Porto, MD, MPH, Yale University/Greenwich Hospital, Greenwich, CT – Updated December 2012.

Ryan K. Fawley, MD, Naval Medical Center, San Diego, CA – Updated October 2021.

Christopher Schmoyer, MD, Penn Presbyterian Medical Center, Philadelphia, PA – Updated April 2026.

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