Lactose Intolerance

Updated: Dec 04, 2019
Author: Praveen K Roy, MD, MSc; Chief Editor: Burt Cagir, MD, FACS 



Lactose intolerance is a common disorder and is due to the inability to digest lactose into its constituents, glucose and galactose, secondary to low levels of lactase enzyme in the brush border of the duodenum.[1] Lactase deficiency is the most common form of disaccharidase deficiency. Enzyme levels are the highest shortly after birth and decline with aging, despite continued intake of lactose. Within the animal world, nonhuman mammals usually lose the ability to digest lactose as they reach adulthood. Some populations of the human species, including those of Asian, South American, and African descent, have a propensity for developing lactase deficiency. By contrast, races descended from northern Europe or from the northwestern Indian subcontinent are likely to retain the ability to absorb lactose into adulthood.[2]

Symptoms of lactose intolerance include loose stools, abdominal bloating and pain, flatulence, nausea, and borborygmi.[3, 4, 5] A diagnosis or even the suggestion of lactose intolerance leads many people to avoid milk and/or to consume specially prepared food with digestive aids, adding to health care costs.


Lactose, a disaccharide, is present in milk and processed foods. Dietary lactose must be hydrolyzed to a monosaccharide in order to be absorbed by the small intestinal mucosa. A deficiency of intestinal lactase prevents hydrolysis of ingested lactose. The osmotic load of the unabsorbed lactose causes secretion of fluid and electrolytes until osmotic equilibrium is reached. Dilation of the intestine caused by the osmosis induces an acceleration of small intestinal transit, which increases the degree of maldigestion. Within the large intestine, free lactose is fermented by colonic bacteria to yield short-chain fatty acids and hydrogen gas.[6] The combined increase in fecal water, intestinal transit, and generated hydrogen gas accounts for the wide range of gastrointestinal symptoms.

An association between certain single nucleotide polymorphisms (C>T-13910 and G>A-22018) with lactose tolerance in a northeaster Brazilian population has been reported.[7] In Indo-Europe, lactase deficiency is associated with rs4982235 SNP (or -13910C>T), which may predispose affected individuals to lactose intolerance.[8]


Congenital lactose intolerance is inherited as an autosomal recessive trait and is very rare.[9, 10]

Primary lactose intolerance is due to low levels of lactase, which develop after childhood.

Secondary, or acquired, lactase deficiency may develop in a person with a healthy small intestine during episodes of acute illness. This occurs because of mucosal damage or from medications. Some causes of secondary lactase deficiency are as follows:

  • Acute gastroenteritis

  • Giardiasis

  • Ascariasis

  • Crohn disease

  • Celiac sprue

  • Tropical sprue

  • Gastrinoma

  • Radiation enteritis

  • Diabetic gastropathy

  • Carcinoid syndrome

  • Whipple syndrome

  • HIV enteropathy

  • Kwashiorkor

  • Chemotherapy


United States statistics

The prevalence of primary lactose intolerance varies according to race. As many as 25% of the white population (prevalence in those from southern European roots) is estimated to have lactose intolerance, while among black, Native American, and Asian American populations, the prevalence of lactose intolerance is estimated at 75-90%.[2]

International statistics

An estimated 70-75% off the world's population is lactose-deficient.[8, 11] Lactose intolerance is very common among Asian, South American, and African persons.

Lactose intolerance also appears to have a higher prevalence in patients with diarrhea-predominant irritable bowel syndrome (IBS-D) than healthy individuals.[12]

Race-, sex-, and age-related demographics

Persons of all races are affected by lactose intolerance, with a higher prevalence among Asian, African, and South American persons.

Males and females are equally affected by lactose intolerance. However, of adult women who are lactose intolerant, 44% regain the ability to digest lactose during pregnancy. This is probably due to slow intestinal transit and bacterial adaptation during pregnancy.

Few data are available regarding the prevalence of lactose intolerance in children aged 1-5 years; however, primary lactose intolerance in this group is estimated to be 0-17.9%, whereas it is a reported 0-19% for secondary lactose intolerance.[13]

Among adults, the age of presentation of lactose intolerance is 20-40 years.[9, 14]


The prognosis of patients with lactose intolerance is excellent with dietary restrictions. Morbidity/mortality include the following:

  • Lactose intolerance is not lethal.

  • Morbidity is low from lactose intolerance.

  • Osteopenia can be a complication of lactose intolerance.[15, 16, 17] Vitamin D deficiency appears to be associated with the LCT-13910C>T gene variant of lactose intolerance in white populations.[17]



History and Physical Examination


Patients with lactose intolerance may include history of abdominal fullness/bloating, nausea, abdominal pain, diarrhea, and flatulence.[18, 19] Symptomatic variability is dependent on the amount of ingested lactose, the residual lactose activity, and the small bowel transit time.[11] It appears that 12 g of lactose in one intake, with about 18 g of lactose over the course of a day, is tolerable for most lactose-intolerant individuals.[11]

The symptoms of irritable bowel syndrome (IBS) resemble those of lactose intolerance and can easily be confused. Some patients with IBS can also have lactose intolerance. Restriction of milk products in these patients may relieve the symptoms of IBS.[20, 21]

Physical examination

Physical examination findings very often are normal. Borborygmi may be present.



Diagnostic Considerations

Other conditions to consider in the differential diagnosis of lactose intolerance include the following:

  • Sucrase deficiency

  • Diabetic diarrhea

  • Disorders associated with secondary lactose intolerance (see Etiology)

Differential Diagnoses



Laboratory Studies

Lactose tolerance test

This test is rarely done in clinical practice.

Measure serial blood glucose levels after an oral lactose load. A fasting serum glucose level is obtained, after which 50 g of lactose is administered.[22] Measure the serum glucose level at 0, 60, and 120 minutes.

The lactose tolerance test has a sensitivity of 75% and a specificity of 96%.

False-negative results occur in the presence of diabetes and small bowel bacterial overgrowth. Abnormal gastrointestinal emptying can also affect the results of the lactose tolerance test.[23]

The diagnosis is confirmed if the serum glucose level fails to increase by 20 g/dL above the baseline.

Milk tolerance test

Administer 500 mL of milk and measure the blood glucose level.

An increase of less than 9 mg/dL indicates lactose malabsorption.[24]

Other Tests

Imaging studies

Imaging tests are not helpful in the diagnosis of primary lactose intolerance, but they may be helpful for excluding secondary causes.

Breath hydrogen test

This is the diagnostic test of choice.

Subjects are administered lactose after an overnight fast, after which expired air samples are collected before and at 30-minute intervals for 3 hours to assess hydrogen gas concentrations.

A rise in breath hydrogen concentration greater than 20 parts per million over the baseline after lactose ingestion suggests lactase deficiency.[22]

Dietary elimination

Resolution of symptoms with the elimination of lactose-containing food products and resumption of symptoms with their reintroduction are findings suggestive of lactose intolerance.[25, 26]


Small bowel biopsy

This is the criterion standard; however, it is invasive and rarely performed. A major advantage is that it provides definitive information.

Biopsy samples from the small bowel are assayed for lactase activity. The biopsy results may be normal if the deficiency is focal or patchy.

This is not readily available and is not usually necessary.



Medical Care

Dietary adjustment is the primary form of therapy for patients with lactose intolerance.[27, 28] Advise patients to reduce or restrict products containing lactose. Prehydrolyzed milk (LACTAID) is available and is effective. Yogurt and fermented products, such as cheeses, are better tolerated than regular milk. Soy-based milk or food products are well tolerated.

Commercially available lactase enzyme preparations (eg, LACTAID, Lactrase) are effective in reducing symptoms; however, they may not be effective in some patients, partially due to insufficient dosing.

Plant-based milk alternatives are emerging; however, although these products show some potential, technologic, palatability, and/or nutritional balance issues remain a concern.[29, 30]

Probiotics (DDS-1 strain of Lactobacillus) have been shown to improve symptoms.[31, 32]

Supplemental calcium and vitamin D should also be recommended.[15]

In secondary lactase deficiency, treatment is directed at the underlying cause.[33]


Consider consulting with a gastroenterologist and a nutritionist in the management of patients with suspected lactose intolerance.


Note the following dietary considerations in patients with lactose intolerance:

  • Patients should read labels on commercial products.

  • Patients should avoid or reduce intake of lactose-containing foods.

  • Most patients who are lactose intolerant can ingest as much as 240 mL of milk without an exacerbation of their symptoms.[34]

  • Whole milk and chocolate milk may be better tolerated than skim milk.[35]

  • Certain medications and foods contain hidden lactose, such as breads, margarine, salad dressings, and candies.



Medication Summary

The goals of pharmacotherapy are to reduce morbidity and to prevent complications.

Digestive enzymes

Class Summary

Provide necessary enzymes for lactose digestion.

Lactase enzymes (LACTAID, Dairy Ease, Lactrase)

For patients with lactase enzymatic deficiency. Prevent osmotic diarrhea in patients deficient in lactase enzyme who consume milk.


Questions & Answers