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JAMA Patient Page
October 22/29, 2019

Lactose Intolerance

Author Affiliations
  • 1Northwestern Memorial Hospital, Chicago, Illinois
  • 2Associate Editor, JAMA
JAMA. 2019;322(16):1620. doi:10.1001/jama.2019.9608

Lactose intolerance describes a group of symptoms caused by problems digesting and absorbing lactose.

Lactose is the main sugar found in dairy products. For lactose to be absorbed by the body, it must be broken down in the gut by a protein called lactase. If there is a shortage of lactase, lactose cannot be broken down, which can cause symptoms of lactose intolerance, such as abdominal discomfort, bloating or gas, cramping, and diarrhea.

Babies are usually born with enough lactase to digest the lactose in breast milk. However, by age 3, about two-thirds of the world’s population stops making high levels of lactase, leading to varying degrees of poor lactose absorption. Some populations, such as people of northern European descent, continue making enough lactase throughout their lives (lactase persistence), while other populations, such as Asians and certain African populations, do not. Factors that can affect the severity of symptoms include a person’s metabolism as well as gut microbiome, the collection of healthy bacteria living in the gut that helps digest foods. Sometimes, new or worsening lactose intolerance can occur as a result of other diseases such as celiac disease, small intestinal bacterial overgrowth, or viral or parasitic infections.


Diagnosis of lactose intolerance is usually made based on symptoms and testing. If someone experiences typical symptoms about an hour or 2 after eating dairy products and it can be confirmed that the symptoms are caused by dairy rather than other foods, lactose intolerance can be presumed. The diagnosis can be confirmed by several tests, such as a hydrogen breath test, which measures the level of hydrogen gas in a patient’s breath as a sign of bacterial breakdown of undigested lactose, or a lactose tolerance test, which measures a patient’s blood sugar (glucose) levels after they are given a dose of lactose. A patient who does not properly absorb lactose will not have as much of an increase in glucose, one of the by-products of lactose breakdown.


The mainstay of treating lactose intolerance is reducing lactose-containing foods in the diet. There is no universal amount of lactose that causes symptoms. Most clinicians and researchers agree that 12 to 15 g of lactose (about the amount in 1 to 2 cups of milk) is generally tolerated by people with lactose intolerance. Dairy products with the highest levels of lactose include milk and ice cream. Yogurt and cheeses, especially hard cheeses, contain much lower amounts. Eating small quantities of these products, especially with other products without lactose, can help reduce symptoms. Lactose-reduced products, such as lactose-reduced milk, are available. Patients are often advised to not completely eliminate dairy products, since they are important sources of calcium and vitamin D.

Another option is lactase replacement pills, which are taken immediately before consuming dairy products. These help the gut break down lactose, allowing for better absorption.

Colonic adaptation is a treatment currently being explored by researchers. A patient is instructed to slowly introduce more and more lactose into the diet. Although the body’s production of lactase will not increase, the gut may be able to become more tolerant to lactose by modifying its microbiome.

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Conflict of Interest Disclosures: None reported.

Source: Micic D, Rao VL, Rubin DT. Clinical approach to lactose intolerance [published online September 26, 2019]. JAMA. doi:10.1001/jama.2019.14740