Lactose Intolerance

By Sonia Gulati. Medically reviewed by Tom Iarocci, MD. May 7th 2016

Lactose intolerance is the inability to digest lactose, a sugar found in milk and dairy products. It is caused by the insufficient production of the enzyme lactase, which degrades lactose in the digestive system.

While most people begin life digesting lactose just fine, about 75 percent of the world’s population goes on to lose this ability during maturation and adulthood. Lactose intolerance is common among adults in the United States and occurs more often in people of Native American, Asian, African, South American, and Southern European descent. Lactose intolerance primarily causes digestive problems such as abdominal pain, bloating and diarrhea. Although there is no cure for lactose intolerance, it can be effectively managed by minimizing lactose in the diet to a tolerable level. 


Symptoms of lactose intolerance can occur 30 minutes to 2 hours after consuming lactose. The severity of the symptoms depends on how much lactose was ingested and how much lactose a person can handle. Common symptoms of lactose intolerance include:

  • Abdominal pain
  • Bloating
  • Flatulence
  • Diarrhea
  • Nausea


Lactose intolerance stems from the small intestine's inability to produce an adequate amount of the enzyme, lactase. Usually, the cells that line the small intestine make lactase, which binds to lactose (a sugar found in milk and dairy products) and breaks it down into two simple sugars, glucose and galactose. These simple sugars are absorbed into the bloodstream. When an insufficient amount of lactase is produced, lactose moves undigested into the colon and interacts with the intestinal bacteria. This interaction causes the common symptoms associated with lactose intolerance.

Decreased levels of lactase is caused by:

  • Age: Most babies and infants produce ample amounts of lactase. After 2 to 12 years of age, at least two distinct groups of people start to emerge: those who continue to produce lactase actively, and those who do not, resulting in lactose intolerance. Even in those who
  • Genetics: In addition to the genetics of the common form of lactose intolerance due to inadequate lactase production, a complete deficiency of lactase can be inherited from your parents. This disorder is autosomal recessive, which means that both parents must pass on a mutated form of the lactase encoding gene. Infants with congenital lactose intolerance are unable to tolerate lactose found in breast milk. Therefore, these babies must be fed lactose-free infant formulas.
  • Illness or injury: Illness, surgery or injury to the small intestine can result in the decreased production of lactase. Furthermore, gastrointestinal diseases such as celiac disease, gastroenteritis and Crohn's disease can cause a short-term lactose deficiency and symptoms of lactose intolerance. Resolution or treatment of these underlying disorders may restore lactase levels.

Risk Factors

Factors that contribute to lactose intolerance include:

  • Age: Lactose intolerance becomes more common with age; this condition is uncommon in babies and young children.
  • Ethnicity: Lactose intolerance is more prevalent among African-Americans, Asians, Hispanics, Native Americans, and Southern Europe; Northern Europeans have a frequency of less than 5 percent, while some Southeast Asian groups are nearly 100 percent lactose intolerant.
  • Premature birth: Premature babies may have reduced levels of lactase, since production of this enzyme increases late in the third trimester.
  • Diseases affecting the small intestine: Small intestine problems that can cause lactose intolerance include bacterial overgrowth, celiac disease and Crohn's disease.
  • Radiation to the abdomen: Radiation therapy for abdominal cancer may put you at a greater risk for lactose intolerance.


In order to diagnose lactose intolerance, your doctor will initially perform a comprehensive physical exam and medical history. Additionally your doctor may order one or more of the following tests:

  • Lactose tolerance test: This test assesses your body's reaction to a liquid that contains high levels of lactose. Two hours after drinking this liquid, blood tests will be performed to measure the amount of glucose in the bloodstream. If glucose levels do not rise, it means that lactose is not being properly digested.
  • Hydrogen breath test: This test also assesses your body's reaction to high levels of lactose. After drinking a liquid that contains large amounts of lactose, the doctor will measure the amount of hydrogen in your breath at regular intervals. If the body is unable to digest lactose, then bacteria in the intestines are free to metabolize it, producing hydrogen and other gases that are released and subsequently exhaled. Therefore, detecting increased amounts of hydrogen in the breath indicates lactose intolerance.
  • Stool acidity test: This test is often administered to infants and children who can't tolerate being challenged with high levels of lactose. Undigested lactose ferments in the colon, creating lactic acid and other acids that can be detected in a stool sample.


Currently there is no cure for lactose tolerance, but it can be effectively managed by reducing your consumption of certain dairy products. Individuals vary in how much lactose they can handle, so it is a process of trial and error at first. Milk, yogurt, and ice cream tend to be the biggest offenders, but less frequently, individuals may have to exclude foods with even lower levels of lactose, including some types of cheese and processed foods.

Palatable substitutes for lactose-containing foods are now widely available, including soymilk and lactose free brands of milk, yogurt, and even ice cream.

Another option is to eat the dairy item, but take lactase supplements along with the item. Some individuals with lactose intolerance may find this approach effective while others still experience bothersome symptoms. The data are limited, but supplements and probiotics appear to be helpful to some people with lactose intolerance.

If you are avoiding dairy products, it is important to ensure that you are getting enough of the nutrients usually found in dairy milk, such as calcium. Therefore it is important to increase the intake of foods rich in calcium. These foods include:

  • Broccoli, okra, kale, collards, bok choy, and spinach.
  • Canned sardines, tuna, and salmon.
  • Calcium-fortified juices and cereals.
  • Soy products fortified with calcium

Next Steps

Currently, there is no fail-proof cure for lactose tolerance, but you suspect this condition in you or someone you love, initiate a process of dietary trial and error. Milk, yogurt and ice cream tend to be the biggest offenders, but less frequently, you may have to exclude foods with lower levels of lactose, including some types of cheeses and processed foods.

Palatable substitutes for lactose-containing foods are now widely available, including soy milk, rice milk and lactose-free brands of milk, yogurt and ice cream. Another option is to eat the dairy item and swallow a lactase supplement simultaneously.

If you are avoiding dairy products entirely, it’s important to ensure that you are getting adequate calcium, so be sure to eat more:

  • Okra, kale, collards, bok choy and spinach
  • Canned sardines, tuna and salmon (with small bones)
  • Calcium-fortified fruit juices and grain cereals
  • Soy products (such as tofu) fortified with calcium

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