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Lactose Intolerance 101

Updated: Oct 13, 2020

Lactose Intolerance

Lactose Intolerance is the small intestine’s inability to fully digest lactose, a sugar contained in milk/dairy products. The disorder is a common digestive issue and while typically harmless, its symptoms can be very uncomfortable.

Symptoms often include intestinal bloating and cramps, nausea, flatulence, and diarrhea.

When your body doesn’t have enough of the enzyme lactase, produced in the small intestine, this often results in lactose intolerance. Even low levels of lactase will allow you to digest milk products. But if your levels are too low you become lactose intolerant, leading to symptoms after you eat or drink dairy products.


Lactose, the main sugar found in milk and other dairy products, is broken down by the enzyme lactase, which is produced by the cells in the inner lining of the small intestine. Lactase separates lactose into two parts, glucose and galactose. These simple sugars get absorbed into the bloodstream. Without lactase, lactose cannot be digested or absorbed.

If you're lactase deficient, lactose passes into the colon instead of being broken down and absorbed. In the colon, bacteria interact with undigested lactose, producing the signs and symptoms of lactose intolerance.

Lactase levels are high in infants, allowing them to digest milk. However, in some ethnic groups, lactase levels decrease after weaning. These decreased levels result in older children and adults in these ethnic groups being unable to digest lactose. However, most people of Northwest European descent produce lactase into adulthood and are thus able to digest dairy products for life.

Temporary lactose intolerance can develop when the lining of the small intestine is damaged by a disorder, such as an autoimmune disease or intestinal infection. After recovery from these disorders, most people are able to digest lactose again.

Risk factors

Factors that can make you more prone to lactose intolerance include:

  • Premature birth: Babies born prematurely might have lower levels of lactase because the small intestine doesn't develop lactase-producing cells until late in the third trimester.

  • Increasing age: Lactose intolerance typically presents in adulthood. The condition is atypical in babies and young children.

  • Ethnicity: Lactose intolerance is most prevalent in people of American Indian, Asian, Hispanic, and African descent.

  • Diseases affecting the small intestine: Lactose intolerance can present after intestinal issues such as bacterial overgrowth, celiac disease and Crohn's disease.

  • Certain cancer treatments: If you've had surgery, radiation or chemotherapy for cancer in your GI or you have intestinal complications from these treatments, your risk of developing lactose intolerance increases.


High concentration of undigested lactose draws fluid into the small intestine, causing diarrhea. The lactose then moves into the colon, where it is fermented by bacteria, producing gases that cause bloating, stomach cramps and flatulence. Additional intolerance symptoms can involve nausea and occasionally vomiting.

An adult may experience bloating and cramps, nausea, flatulence, rumbling or burbling sounds in the bowel (borborygmi), diarrhea and an urgent sensation of a bowel movement. Symptoms usually begin between 30 minutes and 2 hours after you have consumed food containing lactose.

Sometimes, severe diarrhea prevents proper absorption of nutrients because they are eliminated from the body too rapidly. However, loose bowel movements that result from lactose intolerance due to insufficient amount of lactase are typically mild.

Lactose intolerance affecting the production of lactase that occurs after an illness, injury or surgery involving your small intestine can be more severe.


A diagnosis of lactose intolerance is made in consultation with your dietitian or physician, recognizing that your symptoms appear after you have ingested dairy products.

If a 3- to 4-week trial period of a dairy free diet clear up your symptoms, and symptoms return again after reintroducing dairy products into your diet, the diagnosis is confirmed.

Specific tests are necessary in rare cases, but in some people, diagnosis can be confirmed with a hydrogen breath test or lactose tolerance test.

To learn about dietary management of lactose intolerance click here.

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