Digestive Health

Lactose Intolerance: Why Some People Cannot Digest Milk Products

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By Apollo 24/7, Published on- 06 September 2021, Updated on - 05 December 2022

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Lactose intolerance is a digestive problem that is characterized by the inability to digest lactose, a type of natural sugar commonly found in dairy products such as milk and yogurt. In this condition, the small intestine is unable to produce enough of an enzyme called lactase. The enzyme helps break lactose down into simple sugars which are absorbed into the bloodstream and used for energy. Without enough lactase, the undigested lactose ends up in the colon (large intestine) where it interacts with the bacteria and causes discomforting digestive symptoms. The condition is different from milk allergy, which is an immune disorder.

Symptoms of lactose intolerance

When people with lactose intolerance consume foods or drinks that contain lactose, they may experience the following symptoms:

  • Diarrhea
  • Nausea
  • Bloating
  • Gas or flatulence
  • Abdominal cramps

In most cases, the symptoms appear within one or two hours after consuming lactose-containing dairy products. The severity of the symptoms usually depends on the amount of lactose consumed and the relative lactose tolerance of the person.

Causes of lactose intolerance

The human body digests lactose with the help of a digestive enzyme called lactase. The enzyme breaks down lactose into two simple sugars - glucose and galactose. These simple sugars are easily absorbed into the bloodstream through the intestinal lining.

In some individuals, the small intestine cannot produce enough lactase. The deficiency of lactase enzyme causes lactose malabsorption in such people. In lactose malabsorption, the undigested lactose from food moves into the colon (large intestine) instead of being processed and absorbed. The bacteria in the colon break down and ferment the undigested lactose. This fermentation leads to the production of various fluids and gases, which cause the signs and symptoms associated with lactose intolerance.

Types of lactose intolerance

Depending on the underlying factors troubling the production of lactase enzyme, lactose intolerance may be temporary or permanent. Some of the major types of lactose intolerance are:

  • Primary lactose intolerance

This is the most common type of lactose intolerance. People with primary lactose intolerance are usually able to produce enough lactase during infanthood and early childhood. However, they experience a decline in lactase production as they age. The drop in lactase levels with age makes it difficult for such people to digest milk and other lactose-containing food products. The decrease in lactase production is usually attributed to the replacement of milk with a more diverse diet during childhood. Primary lactose intolerance is commonly observed in people of Asian, African, and Hispanic descent.

  • Secondary lactose intolerance

In this form of lactose intolerance, the inflammation in the lining of the small intestine causes a decline in lactase production. Secondary lactose intolerance is mostly caused by an injury or surgery involving the small intestine. It can also be triggered by intestinal diseases such as celiac disease, bacterial overgrowth, and inflammatory bowel disease (Crohn’s disease and ulcerative colitis). In most cases of secondary lactose intolerance, lactase production can be restored by treating the underlying medical condition.

  • Congenital or developmental lactose intolerance

In very rare cases, it is possible for infants to be born with lactose intolerance. In congenital lactose intolerance, a defective gene from both parents results in the complete absence of lactase in the infant. Such babies may experience diarrhea as soon as they are introduced to breast milk or a formula containing lactose. Diarrhea can cause dehydration and trigger loss of electrolytes. Unless treated early on, the condition can be life-threatening. It is usually treated by administering a lactose-free infant formula to the baby.

There are also cases in which lactose intolerance occurs in babies who are born prematurely. In such babies, the small intestine is not able to produce enough lactase for a short time after birth. This results in lactose intolerance because of a low lactase level. This type of lactose intolerance is called developmental lactose intolerance.

Risk factors of lactose intolerance

The following factors are associated with a higher likelihood of lactose intolerance:

  • Ethnicity: People of African, Asian, Hispanic, and American Indian heritage are more likely to have lactose intolerance than individuals belonging to other ethnicities.
  • Increasing age: The risk of lactose intolerance increases with age. The condition is less common in infants and young kids.
  • Medical conditions that affect the small intestine: Bacterial overgrowth, celiac disease, and inflammatory bowel disease (Crohn's disease and ulcerative colitis) are some of the small intestine problems that can lead to lactose intolerance.
  • Preterm birth: Premature babies can have low levels of lactase. The low levels of lactase can be attributed to the absence or low count of lactase-producing cells in the small intestine. Usually, the lactase-producing cells in the small intestine aren’t formed until later in the third trimester of pregnancy.
  • Cancer treatments: The risk of lactose intolerance is higher for people who are undergoing radiation therapy or chemotherapy for stomach or intestinal cancers.


People who experience diarrhea, nausea, bloating, and stomach cramps after consuming milk products may be asked by their physicians to undergo certain tests for lactose intolerance. Some of the tests that may be conducted to diagnose the condition include:

  • Lactose intolerance test

It is a blood test that measures the digestive system’s reaction to high levels of lactose. A person undergoes blood tests nearly two hours after drinking a liquid that contains high lactose levels. The test measures the amount of glucose in the bloodstream after drinking the liquid.  If the glucose level doesn't increase significantly, it implies that the body is not able to digest and absorb the lactose-filled drink. In such a case, the person may be diagnosed with lactose intolerance

  • Hydrogen breath test

The test is used to measure the amount of hydrogen exhaled after consuming a liquid that contains high levels of lactose. An individual with lactose intolerance is unable to digest and absorb lactose (lactose malabsorption). The undigested lactose moves into the colon (large intestine) where it is fermented by the bacteria to release hydrogen and other gases. The hydrogen and other gases are absorbed into the bloodstream and eventually exhaled. During the hydrogen test, the individual is asked to breathe out into a balloon-type container every 30 minutes for a few hours. If he breathes out a higher-than-normal amount of hydrogen during the test, then lactose intolerance may be diagnosed.

  • Stool acidity test

The test is meant to diagnose lactose intolerance in infants and kids. It evaluates the amount of lactic acid in their stool. If the child is not able to digest the lactose, then his stool will have higher than normal levels of lactic acid, glucose, and other fatty acids.

Treatment and management

Currently, there is no treatment to help the body produce more lactase. It is possible to restore the body’s ability to digest lactose by treating the underlying medical condition causing lactose intolerance. However, the process may take some time. There are also other treatments that focus on lowering or eliminating dairy products from the diet.

Nevertheless, it is important to note that most people with lactose intolerance can consume 1/2 cup of milk without experiencing any symptoms. Having small amounts of dairy products along with regular meals can help slow the digestive process and may lessen the severity of symptoms. Moreover, not all dairy products contain high levels of lactose. Those who have high lactose intolerance can also opt for lactose-free milk products. Such people can also consider the intake of over-the-counter lactase enzymes before consuming dairy products. The supplement will help break down the lactose without causing symptoms.

Low intake of milk and other dairy products can make people with lactose intolerance deficient in calcium, vitamin D, and protein. To overcome or avoid these deficiencies, intake of lactose-free alternatives such as green leafy vegetables, calcium-fortified foods, soy milk, rice milk, nuts, etc. that are rich in these essential nutrients is highly recommended. Consumption of probiotic supplements should also be considered to maintain a healthy digestive system.


Lactose intolerance is a common digestive disorder. It is characterized by the inability to produce enough lactase enzyme that is needed to break down and digest lactose. People who have this condition usually experience symptoms such as diarrhea, bloating, nausea, and stomach cramps after consuming milk or any other dairy products rich in lactose. It is usually managed by limiting the intake of lactose-containing foods. Nevertheless, most people with lactose intolerance can consume low amounts of dairy products without experiencing symptoms. It is important to remember that low intake of dairy products can lead to a deficiency of calcium, protein, and vitamin D. To avoid nutrient deficiency, it is recommended to consume lactose-free alternatives that are rich in these essential nutrients. People with lactose intolerance should consult a qualified dietician to learn more about adopting a healthy and balanced diet.

One can also try lactase enzymes to manage lactose intolerance.


Digestive Health


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