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Lactose intolerance and malabsorption: Clinical manifestations, diagnosis, and management

Lactose intolerance and malabsorption: Clinical manifestations, diagnosis, and management
Heinz F Hammer, MD
Christoph Högenauer, MD
Section Editor:
Lawrence S Friedman, MD
Deputy Editor:
Shilpa Grover, MD, MPH, AGAF
Literature review current through: Dec 2022. | This topic last updated: Jan 11, 2022.

INTRODUCTION — Intolerance to lactose-containing foods is common [1]. Symptoms of lactose intolerance are abdominal pain, flatulence, nausea, bloating, and diarrhea after ingestion of milk or milk-containing products. These symptoms may be associated with lactose malabsorption, which results from low levels of small intestinal lactase. However, these symptoms may also occur after ingestion of lactose or lactose-containing foods without demonstrable malabsorption [2]. This topic will review the causes, clinical manifestations, diagnosis, and management of lactose intolerance and malabsorption. Our recommendations are largely consistent with the European Guideline, with a focus on definition and diagnostic evaluation of lactose malabsorption and lactose intolerance [3]. Other causes of malabsorption are discussed in detail, separately. (See "Overview of nutrient absorption and etiopathogenesis of malabsorption" and "Approach to the adult patient with suspected malabsorption" and "Overview of the treatment of malabsorption in adults".)


Lactose intolerance – A clinical syndrome in which ingestion of lactose or lactose-containing food causes symptoms (abdominal pain, bloating, flatulence, nausea, diarrhea). Lactose intolerance may or may not be associated with lactose malabsorption [4].

Lactose malabsorption – Failure of the small bowel to absorb ingested lactose due to lactase deficiency. Lactose malabsorption can occur with or without symptoms of lactose intolerance.

EPIDEMIOLOGY — Lactase deficiency has been described in most regions of the world and in people with diverse ethnic backgrounds. The prevalence of lactase deficiency varies across racial and ethnic groups, with the lowest prevalence in Europeans and European Americans and higher prevalence in African Americans, Asian Americans, and Native Americans. In Africa, some ethnic groups have high rates of lactase deficiency (South Nigerian, Hausa, Bantu), while others have low rates (Hima, Tutsi, nomadic Fulani). The prevalence of lactase deficiency is low in children younger than six years and increases with age [5,6].

PATHOPHYSIOLOGY — Among infants, carbohydrates (primarily lactose) account for 35 to 55 percent of daily calories ingested. Lactose intake falls as weaning foods are introduced and eventually approaches the level ingested by adults. The disaccharide lactose is hydrolyzed by intestinal lactase to the monosaccharides glucose and galactose. Lactase is located on the microvillus membrane of the intestinal adsorptive cells [7]. Uptake of the monosaccharides is accomplished by the sodium-dependent glucose carrier [8].

Lactose that is not absorbed by the small bowel is passed into the colon. In individuals with low lactase activity, up to 75 percent of lactose passes unabsorbed through the small intestine toward the cecum, depending on the ingested amount of lactose [9]. In the colon, lactose is converted to short-chain fatty acids and hydrogen gas by the intestinal bacteria. If large amounts of lactose are malabsorbed, or if conditions that increase intestinal sensitivity (eg, irritable bowel syndrome) are present with moderate amounts of malabsorbed lactose, the accumulation of lactose and its fermentation products cause symptoms of intolerance [10]. In persons with low small intestinal lactase activity, colonic bacterial flora can adapt to persistent lactose ingestion and thereby contribute to a reduction in the incidence and severity of symptoms following a lactose load [11,12].


Primary lactose malabsorption — The most common cause of primary lactose malabsorption is acquired primary lactase deficiency (adult-type hypolactasia, lactase nonpersistence). In patients with lactase deficiency, the intestinal brush border lactase enzyme activity is lower than that of normal individuals. Less common causes of primary lactose malabsorption include congenital lactase deficiency and developmental lactase deficiency.

Acquired primary lactase deficiency is the genetically regulated reduction of lactase enzyme production as an individual ages. The majority of the world's population develop low intestinal lactase levels at preschool age. This characteristic is most frequent in Asian and African populations; in contrast, the majority of White populations of Northern European descent maintain lactase activity into adulthood [13]. Genetic analysis has established that this lactase persistence in adulthood is inherited as an autosomal dominant trait. Multiple single nucleotide polymorphisms (SNPs) in both the coding region and the regulatory region of the lactase gene on 2q21 have been identified. Lactase persistence is due to a gain-of-function mutation on chromosome 2, 13.9 kb upstream of the lactase gene (LCT 13910) [14]. This variant creates a new binding site for the transcription factor that promotes persistent lactase expression after infancy [15-18]. Wildtype CC genotype is associated with acquired primary lactase deficiency, whereas TC and TT genotypes cause lactase persistence. Heterozygotes with a TC genotype may have higher hydrogen levels after lactose ingestion, indicating partial malabsorption, than those with a TT genotype [19]. This intermediate phenotype may be clinically relevant during lactase challenge or in intestinal diseases. Epigenetic regulation of the lactase gene seems to be of critical importance. Promotor methylation is low after birth but increases in the presence of LCT-13910:C but not LCT 13910:T [20]. In southern parts of Africa and the Middle East, different mutations in the same genetic region are responsible for lactase persistence (table 1). The assessment of the presence of the T allele at -13.9 kb is of diagnostic value for lactase persistence or nonpersistence in homogeneous genetic groups such as those of Northern European origin. However, it is not of value in African populations and is of limited value in the heterogeneous United States populations [21]. (See 'Other tests with limited roles' below.)

Molecular studies have generally found a positive correlation between lactase levels and lactase mRNA expression [22-24]. This indicates that molecular regulation of this enzyme is at the level of gene transcription. In some subjects, however, there is a dissociation between lactase mRNA and lactase activity, suggesting that posttranscriptional factors also contribute to the decline in lactase biosynthesis, perhaps due to a partial block in transport of the enzyme from endoplasmic reticulum to the Golgi apparatus [24,25].

Congenital lactase deficiency — Congenital lactase deficiency is a rare autosomal recessive disorder. Affected infants have diarrhea from birth and have been reported to have hypercalcemia and nephrocalcinosis [26]. The largest number of reported cases worldwide has been described in the Finnish population. The disorder is characterized by the absence of lactase activity in the small intestine, with normal histologic findings and normal levels of other disaccharidases. Genetic analysis has demonstrated mutations in the coding region of the LCT gene in affected patients from Finland [27].

Developmental lactase deficiency — Developmental lactose malabsorption results from low lactase levels and is a consequence of prematurity. Lactase activity in the fetus increases late in gestation; thus, premature infants born at 28 to 32 weeks of gestation have reduced lactase activity [28]. If these infants are otherwise healthy, their colons can salvage the unabsorbed carbohydrates, preventing malnutrition and diarrhea. (See 'Pathophysiology' above.)

Secondary lactose malabsorption — Lactose malabsorption may be secondary to intestinal diseases that affect large areas of the mucosal surface and which result in a decrease of digestive capacity, with lactose digestion being predominantly affected due to lower reserve capacities as compared with other disaccharidases. Small intestinal infection or inflammation that causes flattening of the villi or damage to the intestinal epithelium can result in lactose malabsorption (table 2) [29,30]. The lactase enzyme is usually the first disaccharidase to be affected, presumably because of its distal location on the villus.


Clinical manifestations — Patients with lactose malabsorption may be asymptomatic. In patients with malabsorption who are symptomatic, the symptoms of lactose intolerance include abdominal pain, bloating, flatulence, nausea, and diarrhea within a few hours after ingestion of a lactose-containing meal [31-33]. The abdominal pain may be cramping in nature and is often localized to the periumbilical area or lower quadrants. In children, the stools may be bulky, frothy, and watery. In adults, diarrhea is usually not the predominant symptom. Borborygmi may be audible on physical examination and to the patient. Isolated lactose malabsorption should not lead to weight loss or nutritional deficiencies.  

There is considerable variability in the severity of symptoms among patients with lactose intolerance [31]. This is related to the amount of lactose ingested, other food components in a lactose-containing meal, and visceral hypersensitivity [2]. Individuals vary in their sensitivity to the abdominal distension caused by gas or the influx of water into the lumen of the small intestine due to the presence of undigested lactose [34]. In patients with concurrent irritable bowel syndrome (IBS), other conditions known to be associated with an increase in IBS symptoms (eg, life events or emotional stress, phase of the menstrual cycle) may also be associated with periods of increased intolerance of ingested lactose. (See "Clinical manifestations and diagnosis of irritable bowel syndrome in adults", section on 'Clinical manifestations'.)

Laboratory findings — Patients with diarrhea due to lactose malabsorption have a stool osmotic gap of >125 mOsm/kg due to the presence of unabsorbed carbohydrates within the intestinal lumen and a stool pH <6 due to bacterial fermentation of lactose in the colon. However, these findings are not specific for lactose malabsorption and also occur in other etiologies of carbohydrate-induced diarrhea, such as short bowel syndrome, villous atrophy, or pancreatic insufficiency [35,36]. (See "Approach to the adult with chronic diarrhea in resource-abundant settings", section on 'Laboratory evaluation'.)


When to suspect lactose malabsorption — The diagnosis of lactose malabsorption should be suspected in patients with symptoms of abdominal pain, bloating, flatulence, nausea, or diarrhea occurring within a few hours after significant lactose ingestion (>2 servings of dairy per day or >1 serving in a single dose that is not associated with a meal) and resolution after five to seven days of avoidance of lactose-containing foods [37]. In adults, diarrhea is an infrequent symptom.

Patients with severe symptoms, or alarm symptoms and/or signs, require endoscopic evaluation and/or imaging to rule out other organic disorders prior to performance of breath tests. Alarm symptoms include:

Age of onset after age 50 years

Rectal bleeding or melena

Nocturnal pain or diarrhea

Progressive abdominal pain

Unexplained weight loss, fever, or other systemic symptoms

Laboratory abnormalities (iron deficiency anemia, elevated C-reactive protein or fecal calprotectin)

Family history of inflammatory bowel disease (IBD) or colorectal cancer

The evaluation of patients with diarrhea and alarm features is discussed in detail separately. (See "Approach to the adult with chronic diarrhea in resource-abundant settings", section on 'Patients with alarm features'.)

Malabsorption testing by hydrogen breath test

Test protocol — For the hydrogen breath test, a lactose solution is ingested by the patient and serial breath samples are collected to measure hydrogen levels. In adults, 25 to 50 g lactose should be used. In children, there is no consensus on the dose of lactose, ranging from 0.5 to 2 g/kg lactose suspended in water, up to a maximum of 25 to 50 g of lactose [4].

Cigarette smoking or physical exercise sufficient to produce hyperventilation should be avoided for two hours prior to testing as it can decrease test accuracy.

Complex carbohydrates (eg, bread, pasta, fiber) and dairy should be avoided for 12 hours prior to testing.

Antibiotics should be avoided four weeks prior to testing.

Colonic cleansing for endoscopic or surgical procedures should be avoided for at least two weeks before the test [4].

The recommended test duration is three to five hours but may conclude early if positive diagnosis for malabsorption and intolerance is confirmed. The standard measurement interval to assess malabsorption and intolerance is 30 minutes. Longer intervals up to 60 minutes may be adequate to assess malabsorption. Shorter intervals of 10 to 15 minutes may be required to provide evidence of intolerance. Minimum reporting criteria of a breath test used to detect lactose maldigestion or malabsorption should include a statement whether there is evidence of maldigestion or malabsorption and of intolerance [4].

False-negative results can be seen after the recent use of antibiotics, in patients with lung disease, or in up to 20 percent of subjects who are hydrogen nonexcretors. False-positive results may be seen with small bowel bacterial overgrowth; these patients usually have an early rise in breath hydrogen concentration (earlier than 60 minutes after ingestion of lactose).

The detection rate of carbohydrate malabsorption is not significantly affected by additional measurement of methane or of carbon dioxide [38]. The potential increase in test accuracy due to these additional measurements must be weighed against higher costs of equipment and potentially more complicated breath collection [4].

Symptom assessment — The assessment of symptoms of lactose intolerance manifesting after carbohydrate ingestion is an integral part of the test [4,39,40]. Symptom correlation is essential as therapy is only indicated to alleviate symptoms in patients with symptomatic lactose malabsorption; patients with asymptomatic malabsorption do not require treatment. In addition, in asymptomatic lactose malabsorption, the symptoms that had prompted the diagnostic evaluation cannot be assigned to lactose malabsorption, and further diagnostic evaluation of these symptoms is indicated. Symptoms should be assessed using a validated symptom assessment tool [4,39,40].

In patients with lactose intolerance, a strong correlation has been demonstrated between the appearance of hydrogen in breath and the development of symptoms. However, the relation between concentration of gas in breath and the severity of symptoms was weak [41]. It has been demonstrated that anxiety, visceral hypersensitivity, and high levels of gas production on breath tests increase the severity of symptoms after ingestion of lactose [42]. Data such as these suggest that patients with irritable bowel syndrome (IBS) develop symptoms when exposed to lactose [43].

Interpretation of results — A hydrogen cut-off >20 parts per million (ppm) increase above baseline at a single time point during the test indicates lactose maldigestion or malabsorption.

The combination of carbohydrate breath test with symptom assessment during the test allows for the determination of four different entities after a carbohydrate load [4]:

Lactose maldigestion/malabsorption with symptoms. (See 'Patients with lactose malabsorption and symptoms of lactose intolerance' below.)

Maldigestion/malabsorption alone. (See 'Patients with lactose malabsorption without symptom correlation' below.)

Symptoms alone – Abdominal symptoms may arise after carbohydrate ingestion without objective evidence of malabsorption on breath test. This may be due to other mechanisms not related to malabsorption, such as a nocebo-effect (ie, patients' expectation of symptoms) or visceral hypersensitivity to distension of the gastrointestinal tract by the test meal (eg, functional dyspepsia, IBS), or food allergy (especially if symptoms occur after whole milk or other food).

Neither lactose maldigestion/malabsorption nor symptoms.

Diagnostic accuracy — The hydrogen breath test is noninvasive and, at a diagnostic cut-off of 20 ppm, has a sensitivity and specificity of 78 and 98 percent, respectively [44]. The sensitivity is limited by the fact that a proportion of patients have a colonic bacterial flora that uses hydrogen to produce methane. Up to 20 percent of patients evaluated for lactose malabsorption are "hydrogen nonexcretors" [45]. The sensitivity gap due to hydrogen nonexcretion can be addressed by simultaneous measurement of serum glucose by performing a lactose tolerance test. (See 'Lactose tolerance test' below.)

Lactose tolerance test — The lactose tolerance test has largely been replaced by the lactose breath hydrogen test. The lactose tolerance test measures lactose absorption through serum glucose levels following lactose ingestion. The lactose tolerance test depends on glucose metabolism and, in adults, the sensitivity of the lactose tolerance test is lower than the hydrogen breath test. The lactose tolerance does, however, fill part of the gap in the sensitivity of the breath test, which is due to hydrogen nonexcretion [45].

The lactose tolerance test requires repeated measurements of glucose in blood. Following oral administration of a 50 g test dose in adults (or 2 g/kg in children), blood glucose levels are monitored at 0, 60, and 120 minutes. An increase in blood glucose by less than 20 mg/dL (1.1 mmol/L) plus the development of symptoms is diagnostic for lactose malabsorption and intolerance. False-negative results may occur in patients with diabetes and false positives in bacterial overgrowth of the small intestine. Abnormal gastric emptying also can lead to spurious results; the blood glucose may be relatively higher with rapid emptying and depressed with delayed gastric emptying.

Other tests with limited roles — These tests cannot detect whether symptoms are caused by lactase deficiency.

Small bowel biopsy — A jejunal biopsy allows for an assessment of lactase enzyme activity. It can also help to distinguish between primary and secondary lactase deficiency by evaluation of small bowel histology. While lactase activity assay is considered the gold standard for lactase deficiency, this is rarely performed due to the need to perform an upper endoscopy and the availability of noninvasive diagnostic tests. In addition, low lactase activity induced by intestinal injury may be missed if the lesion is focal or patchy, and it does not allow symptom assessment of lactose intolerance. Lactase activity may also be lower in the proximal small intestine where most biopsy samples are obtained.

Genetic testing — Genetic testing only detects primary lactase deficiency and can only be used in selected populations. A polymorphism, 13.9 kb upstream from the 5' end of the lactase transcriptional start site, has been shown to be associated with lactase persistence in populations of European ancestry [46]. In a meta-analysis of 19 studies that included populations of European ancestry, the C/C (-13910) genotype had a sensitivity and specificity for acquired primary lactase deficiency of 88 and 90 percent, respectively [47]. People with the genotype T/C (-13910) can be lactase persistent or nonpersistent [21,48]. This test can be used in populations where the -13910 T/C polymorphism has been associated with lactase persistence (table 1) [13,18]. (See 'Primary lactose malabsorption' above.)

Gaxilose test — This gaxilose test is a noninvasive test for lactose malabsorption that is based on a rise in D-xylose levels in the serum and urine after a synthetic lactose analog. The test involves oral administration of 0.45 g of 4-galactosylxylose (gaxilose), which is then cleaved by intestinal lactase to galactose and D-xylose. D-xylose is passively absorbed from the small intestine. For this test, repeated urine samples are taken for up to five hours. The diagnostic performance of the gaxilose test compares well to small bowel biopsies or genetic testing, and in one study, had both a sensitivity and specificity of 92 percent [49]. The test has limited availability.

Differential diagnosis — Symptoms of lactose intolerance are not specific [33,37,41]. The differential diagnosis includes intolerance to other components of milk or IBS. However, symptoms similar to lactose intolerance can be due to lactose malabsorption or due to intolerance to other carbohydrates, such as FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols) or proteins (gluten, casein). (See "Approach to the adult with acute diarrhea in resource-rich settings".)

Small intestinal bacterial overgrowth – Small intestinal bacterial overgrowth may be associated with increased fermentation of dietary carbohydrates in the small bowel that leads to symptoms suggestive of lactose intolerance. The diagnosis should be suspected from the clinical history and from a very early peak of breath hydrogen during a carbohydrate breath test. This peak generally occurs 15 to 30 minutes after ingestion of the test substrate and is often accompanied by symptoms. (See "Small intestinal bacterial overgrowth: Clinical manifestations and diagnosis", section on 'Carbohydrate breath test'.)

Carbohydrate intolerance – Other incomplete absorbed carbohydrates (eg, sorbitol, sucrose, maltose, mannitol, xylitol, fructose, FODMAPs) or therapeutic inhibitors of carbohydrate absorption (eg, acarbose) can cause symptoms of carbohydrate intolerance. The diagnosis is suspected based on dietary history. Breath testing and symptom measurement can identify specific forms of carbohydrate intolerance (such as fructose) for which specific dietary recommendations or food supplements may be adequate [50]. (See "Approach to the adult patient with suspected malabsorption", section on 'Malabsorption of specific carbohydrates'.)


Patients with lactose malabsorption and symptoms of lactose intolerance — The goal of management in symptomatic lactose malabsorption is to eliminate symptoms while maintaining calcium intake and vitamin D intake.

Treatment of the underlying disease — In patients with secondary lactose malabsorption, successful treatment of the primary disorder can lead to restoration of lactase activity. However, lactose intolerance may persist for months after healing starts and lags behind the return of normal intestinal morphology. (See 'Secondary lactose malabsorption' above.)

Dietary management

Dietary lactose restriction — Patients should be advised that restriction of their intake of lactose to the equivalent of two cups of milk (or its lactose equivalent in cheese and other lactose-containing food items) daily, taken in divided doses, is often sufficient to treat symptoms of intolerance [51]. The likelihood of reporting symptoms and the severity of symptoms in individuals with lactose malabsorption depends on the dose of lactose. Doses of lactose exceeding 10 grams are typically required to induce appreciable symptoms [52]. Individuals with lactose intolerance can tolerate dairy products in limited quantities [1,33,51,53,54]. Some dairy products are naturally lower in lactose content, and there is an increasing availability of dairy products processed to reduce lactose content. Individuals may start with more strict dietary restriction and then work up to their individually tolerated limit of ingestion of lactose-containing food. Routine daily consumption of lactose-containing products may be better tolerated than intermittent consumption. By far the highest concentration of lactose per serving is present in milk and ice cream (table 3), but the high fat content and attendant decrease in gastric emptying rate may allow ice cream ingestion without symptoms. Cheeses generally contain lower quantities of lactose. Live culture yogurt, which contains endogenous beta-galactosidase, is an alternative source of calories and calcium and may be well tolerated by many lactose intolerant patients. However, yogurts that contain lactose added back after fermentation may produce symptoms. (See 'Lactase enzyme supplementation' below.)

Calcium and vitamin D supplementation — Vitamin D levels should be monitored in patients who limit dairy intake. Patients who avoid dairy products and have an inadequate intake of calcium and vitamin D should be encouraged to increase their consumption of calcium-rich foods or consider using calcium and vitamin D supplementation. The recommended daily calcium intake for adolescents and young adults is 1200 mg per day. Recommendations in adults depend upon sex, menopausal status in females, and the presence of osteoporosis. Calcium supplementation in excess of 500 mg per day should be given in divided doses [38]. (See "Calcium and vitamin D supplementation in osteoporosis".)

Lactase enzyme supplementation — Commercially available lactase enzyme preparations are bacterial or yeast beta-galactosidases and are available as preparations that can be taken orally with lactose-containing food. Liquid lactase preparations can be added to milk (14 drops/quart), which is then refrigerated overnight before use. The resulting hydrolysis of lactose produces a sweeter taste than milk containing lactose [39]. Commercially available predigested dairy products are also available. Patients who have lactose intolerance should follow the instructions on the label and adjust both the dose and the lactose load to tolerance.

Lactase enzyme preparations cannot completely hydrolyze all dietary lactose, and the results achieved in individual patients are variable [55]. Studies that have evaluated the effectiveness of enzyme replacement therapy on symptoms of lactose intolerance have demonstrated conflicting results. This is mainly due to limitations in study inclusion criteria, which have largely relied on detection of malabsorption, rather than intolerance, and did not use validated measures to assess symptoms [55]. With the development of validated symptom assessment tools for the detection of symptoms of carbohydrate intolerance, these therapeutic studies need to be repeated in better defined patient groups [39,40].

No role for avoidance of lactose-containing medications — Lactose-containing medications do not need to be avoided [56]. The amount of lactose in tablets, where it is used as an additive, is so small that there is no reason to assume that this would result in symptoms. Advertisements promoting "lactose-free drugs" lack a scientific basis with regard to abdominal symptoms and may serve to promote misunderstanding about the dose-effect relationship between lactose dose and development of symptoms.

Patients with lactose malabsorption without symptom correlation — In patients with evidence of lactose malabsorption on testing but without concurrent symptoms of lactose intolerance during testing, the symptoms that had prompted the diagnostic evaluation cannot be assigned to lactose malabsorption. These patients require additional evaluation to determine the etiology of their symptoms. (See "Approach to the adult with chronic diarrhea in resource-abundant settings", section on 'Initial evaluation' and "Evaluation of the adult with abdominal pain".)

PATIENTS WITH PERSISTENT SYMPTOMS — Patients with symptoms that persist despite a lactose-restricted diet should undergo an evaluation to rule out other diseases. The evaluation of chronic diarrhea and abdominal pain are discussed in detail separately. (See "Approach to the adult with chronic diarrhea in resource-abundant settings", section on 'Initial evaluation' and "Evaluation of the adult with abdominal pain".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5thto 6thgrade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10thto 12thgrade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (see "Patient education: Gas and bloating (The Basics)" and "Patient education: Lactose intolerance (The Basics)")

Beyond the Basics topics (see "Patient education: Gas and bloating (Beyond the Basics)")


Terminology – Lactose intolerance is a clinical syndrome in which ingestion of lactose or lactose-containing food causes symptoms (abdominal pain, bloating, flatulence, nausea, diarrhea). Lactose intolerance may or may not be associated with lactose malabsorption. Lactose malabsorption is characterized by failure of the small bowel to absorb ingested lactose due to lactase deficiency. Lactose malabsorption can occur with or without symptoms of lactose intolerance. (See 'Definitions' above.)

Pathophysiology – Lactose that is not absorbed by the small bowel is passed rapidly into the colon (lactose malabsorption). In the colon, lactose is converted by bacteria to hydrogen gas and short-chain fatty acids (acetate, butyrate, and propionate). If sufficient lactose is malabsorbed, these fermentation products and unfermented lactose may cause symptoms of lactose intolerance. (See 'Pathophysiology' above.)

Etiology – The most common cause of primary lactose malabsorption is lactase deficiency due to enzyme nonpersistence. Lactose malabsorption may be secondary to underlying intestinal diseases (table 2). (See 'Etiology of lactose malabsorption' above.)

Clinical features – Patients with lactose malabsorption may be asymptomatic. Symptoms of lactose intolerance include abdominal pain, bloating, flatulence, nausea, and diarrhea within a few hours after ingestion of a lactose-containing meal. In children, the stools may be bulky, frothy, and watery. (See 'Clinical features' above.)

Diagnostic evaluation – Lactose intolerance should be considered in patients with bloating, flatulence, nausea, diarrhea, and abdominal pain that occur with significant lactose ingestion (>2 servings of dairy per day or >1 serving in a single dose that is not associated with a meal) and resolve after five to seven days of avoidance of lactose-containing foods.

Patients with severe symptoms or alarm symptoms require endoscopic evaluation and/or imaging to rule out other etiologies prior to evaluation for lactose malabsorption.

A lactose hydrogen breath test with simultaneous assessment of symptoms can be used to confirm lactose malabsorption and intolerance. This may be useful in patients who have a low probability of being lactase nonpersistent based on ethnicity (eg, White patients). (See 'Diagnostic evaluation' above and "Approach to the adult with chronic diarrhea in resource-abundant settings" and "Evaluation of the adult with abdominal pain".)


The goal of management in symptomatic lactose malabsorption is to eliminate symptoms while maintaining calcium intake and vitamin D intake.

Patients with symptomatic lactose malabsorption should be advised that complete elimination of dietary lactose is not required and that restriction of their intake of lactose is often sufficient to treat symptoms of lactose intolerance (table 3).

Patients may use lactose-reduced products or lactase supplements. However, lactase enzyme preparations cannot completely hydrolyze all dietary lactose, and the results achieved in individual patients are variable.

Patients with inadequate dietary calcium intake may require calcium supplementation.

Vitamin D levels should be monitored in patients who avoid dairy intake. (See 'Management' above.)

ACKNOWLEDGMENT — The editorial staff at UpToDate would like to acknowledge Robert Montgomery, PhD, Richard Grand, MD, and Hans Büller, MD, who contributed to an earlier version.

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