Lactose Intolerance

Reviewed on 8/8/2022

What Facts Should I Know about Lactose Intolerance

Lactose Intolerance
Lactose intolerance is a common disorder caused by the inability to digest lactose, a carbohydrate found in milk and other dairy products.

What is the medical definition of lactose intolerance?

  • Lactose intolerance is a common disorder caused by the inability to digest lactose, a carbohydrate found in milk and milk products.
  • It typically causes symptoms of bloating, flatulence, diarrhea, and abdominal pain. Avoidance of milk and other dairy products alleviates most symptoms of lactose intolerance.
  • Lactose molecules cannot be directly absorbed by the body. Therefore, lactose has to be split into smaller molecules in order to be absorbed and transported across the wall of the intestines.
  • Normally, lactose is broken up by an enzyme (protein that expedites chemical reactions in the body) called lactase. This enzyme is located on the lining of the intestines (the brush border) and helps to break up lactose into its smaller carbohydrate components, glucose and galactose. These two smaller molecules are more easily absorbed by the body and used for metabolism.

How do you find out if you have lactose intolerance?

  • Lactose intolerance is caused by a deficiency of lactase in the intestinal wall. As a result, the entire lactose molecule travels undigested in the small and large intestines. The lactose molecules draw water into the intestines (by a process similar to osmosis). This results in faster transit through the intestines, thus making the process of digestion even more difficult.
  • Eventually, bacteria present in the large intestine (colon) begin to digest (ferment) the lactose molecule by utilizing their own lactase enzyme, producing hydrogen gas and smaller molecules as byproducts. The combination of these processes leads to the symptoms of lactose intolerance:

Can lactose intolerance get worse?

  • Lactase enzyme levels are highest after birth and gradually decline thereafter.

Lactose Intolerance Causes

Lactose intolerance is due to a basic deficiency of the lactase enzyme. This enzyme is normally found on the inner lining of the intestinal wall and splits the carbohydrate lactose into smaller components, glucose, and galactose. These breakdown products are then absorbed and transported from the intestinal wall for further digestion.

Lactose intolerance may occur because of a deficient or completely absent lactase enzyme level. The complete absence of lactase is a rare genetic condition where the gene responsible for producing the lactase enzyme is defective. This form of lactase deficiency results in a marked lactose intolerance from birth.

More commonly, lactose intolerance is an acquired condition, not present at birth. The intestinal wall may become damaged due to a variety of reasons, commonly from infections or certain medications. Some of the possible causes of lactase deficiency include:

The most common cause of lactose intolerance is the gradual loss of lactase after childhood. In this type of lactose intolerance, there is a genetically progressive decrease in the lactase enzyme levels. In populations with high prevalence rates of lactose intolerance, a gradual loss of lactase is more common and begins at an earlier age. In Asians and Asian Americans, the decline in lactase enzyme levels is more prevalent, followed by Native Americans, African Americans, and Hispanics.


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Lactose Intolerance Symptoms

It is important to recognize that deficiency in the lactase enzyme does not necessarily translate into lactose intolerance. Many people with mild degrees of lactase deficiency have no symptoms and are able to tolerate lactose ingestion. On the other hand, people with severe lactase deficiency may have symptoms even with a minimal amount of lactose ingestion.

The amount of lactose in the diet and the difference in the makeup of bacteria in the colon are other factors that determine the variability and severity of symptoms in some individuals.

Symptoms of lactose intolerance include:

  • bloating
  • abdominal pain
  • diarrhea
  • flatulence (passing gas)
  • nausea

The production of gas (flatus) is the result of the activity of bacteria in the large intestine (colon). As the large lactose molecule passes unchanged through the small intestines, it is metabolized by the bacteria that are normally present in the colon. As a result, certain gases, such as hydrogen, are produced and released from the rectum.

Additionally, a portion of the lactose reaching the colon does not get metabolized by the bacteria. Because these larger molecules are accompanied by an increase in the secretion of water through osmosis, this results in the passage of loose stools and diarrhea.

When to Seek Medical Care for Lactose Intolerance

Most people with lactose intolerance are able to manage their symptoms without any medical care. Generally, lactose intolerance symptoms are mild, intermittent in nature, self-limited, and not life-threatening.

However, in the presence of severe diarrhea, marked abdominal pain, fever, or other unusual and prolonged symptoms, a prompt visit to the doctor's office or the emergency room may be advisable to ensure that other more serious conditions are investigated.

Exams and Tests for Lactose Intolerance

Evaluation of lactose intolerance includes a careful medical history, review of symptoms, and physical examination.

Because symptoms are usually non-specific, another potential diagnosis also needs to be considered and excluded. These include:

Some of the commonly recommended methods for the evaluation of lactose intolerance will be discussed in this section.

Subjective Self-Testing

Elimination of dietary lactose is a subjective self-test commonly done by many people who think they may have lactose intolerance. This easy test is a self-evaluation to assess if symptoms resolve with avoidance of dietary lactose. The limitation of this test is that lactose may exist in many foods other than milk and milk products. Complete restriction of lactose products, therefore, is difficult. Consultation with a dietician or nutritionist may help to identify other lactose-containing, non-milk foods. Another limitation of the self-test is a possible placebo effect where people may think their symptoms are better, when in fact they are not.

Milk Tolerance Test

The milk tolerance test is a simple and relatively reliable test that can be performed to evaluate lactose intolerance. In this test, a person drinks a glass of milk in the morning after an overnight fast and then resumes fasting for the next 3-5 hours. If the typical symptoms of lactose intolerance occur within a few hours after the intake of milk, the person is likely to have lactose intolerance. If symptoms do not occur, lactose intolerance is unlikely. It is recommended that nonfat milk be used for this test to eliminate the possibility of symptoms resulting from fat intolerance.

Milk allergy is an uncommon condition that can present similarly, although this condition typically occurs almost always in infants and young children.

Lactose Tolerance Test

The lactose tolerance test is a more objective evaluation in assessing individuals with symptoms suggestive of lactose intolerance. This test involves an overnight fast and measurement of a fasting baseline blood glucose level in the morning. Then, 50 grams of lactose are ingested and blood glucose measurements are taken 60 minutes and 120 minutes later. The diagnosis of lactose intolerance is made if there is a less than 20 grams/deciliter (one-tenth of a gram) increase in the blood glucose level from baseline. If the glucose level increases by more than 20 grams/deciliter, it means that lactase activity in the intestines is sufficient to split lactose into glucose and galactose. This test is very specific, but not very sensitive, meaning a normal test does not exclude lactose intolerance. The presence of diabetes mellitus and bacterial overgrowth in the intestines can give normal results despite actual lactase deficiency. Additionally, abnormal emptying of food from the stomach (either too rapid or too slow) can cause abnormal test results.

Hydrogen Breath Test

The hydrogen breath test is the most reliable test and the method of choice for the assessment of lactose intolerance. This test takes advantage of hydrogen production by the bacteria in the colon when these bacteria metabolize lactose and produce hydrogen gas. Some of the gas is excreted as flatus and some are absorbed by the body where it is eventually exhaled through the lungs. The amount of exhaled hydrogen gas is measured. After an overnight fast, 25 grams of lactose (about 16 ounces of milk) are ingested. Hydrogen gas in the breath is measured before the ingestion, as a baseline value, and then every 30 minutes for three hours. An increase in breath hydrogen concentration of more than 20 parts per million from baseline is diagnostic of lactose intolerance and lactase deficiency. The amount of expired hydrogen gas can also help determine the severity of lactase deficiency. The hydrogen breath test also has its shortcomings. The results can be abnormal because of bacterial overgrowth in the intestines. It is also a long and tedious test.

Stool Acidity Test

The stool acidity test is sometimes performed in infants and young children suspected of having lactose intolerance. When lactose is split by bacteria in the colon, some acid (lactic acid) is produced as a result of the chemical reaction by the bacteria. In this test, the infant is given small amounts of lactose, and stool samples are collected for the measurement of acidity. Stool that is more acidic than normal may suggest lactase deficiency. This test is infrequently done due to its inferiority to the hydrogen breath test.

Biopsy of the Small Intestine

Biopsy of the small intestine is an invasive test that is rarely done for the evaluation of lactose intolerance. The biopsy is usually performed by endoscopy where a long tube is passed from the mouth and into the small intestine. Biopsies of the lining of the intestinal wall are taken and analyzed for lactase activity. This test is not available routinely except for research purposes in specialized centers. The results may also not be as reliable because the biopsied area of the intestine may have normal lactase activity compared to other areas of lactase deficiency that may be missed by the biopsy.

Imaging studies, such as X-rays and CT scans, are not generally recommended in the evaluation of lactose intolerance, although these studies may help eliminate other possible diagnoses.

Lactose Intolerance Treatment

  • The most effective way to treat the symptoms of lactose intolerance is to modify the diet.
  • Lactase supplements are also available, which help the digestive system break down products containing lactose.

Self-Care at Home for Lactose Intolerance

Dietary changes designed to reduce or eliminate lactose products are the simplest and most effective treatment for lactose intolerance. Foods that should be avoided by people with lactose intolerance are listed in the previous section and include milk, ice cream, yogurt, and cheese.

Replacement of milk with substitutes, such as soy milk and other soy products or rice milk is one option for people with lactose intolerance.

  • Lactase enzyme replacement in milk products is also a common recommendation for treating lactose intolerance.
  • Lactase supplements in pill or liquid (Lactaid) form are available and can be added to milk.
  • Other similar preparations include Lactase, LactAce, Dairy Ease®, and Lactrol.
  • Commercially available milk predigested with lactase is also available in most supermarkets (Lactaid milk).

Lactose Intolerance Diet

As mentioned previously, the most important part of treating lactose intolerance is dietary changes to limit the intake of foods containing lactose. Usually, complete elimination of lactose from the diet is not necessary, as most people with this condition can tolerate some amount of lactose in their diet depending on the degree of lactase deficiency.

The highest concentration of lactose per serving is found in milk and ice cream. Cheese generally contains less lactose per serving than milk and ice cream. It is also important to note that lactose may be found in many food products other than milk and dairy. Some of the common foods that may contain lactose are:

  • margarine,
  • some salad dressings,
  • bread and other baked goods,
  • breakfast cereals,
  • pancake, cookie, and biscuit mixes,
  • candy,
  • powdered coffee creamers,
  • dry milk, milk powder, nonfat milk powder, and
  • curds.

It is prudent to read the list of ingredients on food labels to find out if lactose is present in food products.

Occasionally, lactose may be found in certain medications. Intake of these medications may cause side effects similar to the symptoms of lactose intolerance. For example, some birth control pills may contain lactose as an ingredient.

Among milk products, yogurt may be better tolerated by people with lactose intolerance; this is because bacteria that are used in preparing yogurt contain lactase and are able to split lactose into glucose and galactose before yogurt is ingested.


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Lactose Intolerance Medication

Because milk and milk-containing products are the main sources of dietary calcium and vitamin D, people with lactose intolerance may be deficient in calcium and vitamin D. Calcium and vitamin D deficiency can lead to early osteoporosis and fragile bones. Therefore, calcium and vitamin D supplements are recommended for people with lactose intolerance.

Lactase enzyme supplements can be added to milk, as mentioned earlier, for people with lactose intolerance.

Lactose Intolerance Follow-up

Most people with lactose intolerance do not require routine follow-up with their doctors for this condition. Once the diagnosis is established, proper dietary changes, ingestion of milk substitutes, and proper supplementation usually result in improvement or resolution of the symptoms of lactose intolerance.

Most people with lactose intolerance are cared for by their primary care physicians, internists, and gastroenterologists. Consultation with a dietician or nutritionist is advisable in order to review different foods that may contain hidden lactose and to understand nutritional alternatives.

Prevention of Lactose Intolerance

Prevention of the symptoms of lactose intolerance is primarily focused on avoidance of dietary milk and milk-containing products. Some aspects of lactose intolerance may be genetically determined and not modifiable.

Some individuals with lactose intolerance are able to slowly increase the amount of lactose intake in their diet without producing symptoms. This adaptation is most likely due to changes in the metabolism of the bacteria in the colon and not because of the production of more lactase enzymes. The bacteria, for example, may produce less gas in order to adapt to a more acidic colonic environment induced by slowly introducing more lactose over time.

Lactose Intolerance Outlook

With proper dietary restriction and supplementation, lactose intolerance, in general, carries an excellent prognosis.

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Reviewed on 8/8/2022
Medically reviewed by Venkatachala Mohan, MD; Board Certified Internal Medicine with a subspecialty in Gastroenterology

REFERENCES:; "Lactose Intolerance."; "Lactose Intolerance."

Rosado, JL, Solomons, NW, Lisker, R, et al. Enzyme replacement therapy for primary adult lactase deficiency: Effective reduction of lactose malabsorption and milk intolerance by direct addition of beta-galactosidase to milk at mealtime. Gastroenterology 1984; 87:1077.