Lactose Intolerance

Table of Contents

1. What Is Lactose Intolerance?
2. Types of Lactose Intolerance
3. Symptoms
4. Who Gets It — Ancestry & Genetics
5. How It Is Diagnosed
6. Managing It: Diet & Enzymes
7. Calcium & Vitamin D Without Dairy
8. Lactose Intolerance vs Milk Allergy vs IBS
9. Key Points
10. Key Research Papers
11. Connections
12. Featured Videos

What Is Lactose Intolerance?

Lactose is the main sugar in milk. Chemically it is a disaccharide — two smaller sugars, glucose and galactose, joined together. Lactose cannot be absorbed as-is; the two halves must first be split apart by an enzyme called lactase, made by the cells lining the small intestine (Misselwitz et al. 2019). When enough lactase is present, lactose is broken down, the glucose and galactose are absorbed, and milk is digested without symptoms.

When there is not enough lactase, undigested lactose travels on into the large intestine (the colon). Undigested lactose traps water in the bowel by osmosis, and colonic bacteria ferment it, producing hydrogen, carbon dioxide, methane and short-chain fatty acids; one review notes that fermentation raises the osmotic load in the colon about eightfold, and that diarrhoea results when the lactose load exceeds what the bacteria can ferment or the colon can reabsorb (Misselwitz et al. 2019). That fluid and gas produce the bloating, cramping, rumbling, wind and diarrhoea of lactose intolerance.

Two terms are worth separating. Lactose malabsorption means the gut does not fully digest lactose. Lactose intolerance is malabsorption plus symptoms. Not everyone who malabsorbs lactose has symptoms: reviews list the lactose dose, the amount of lactase made, the intestinal bacteria, how fast the gut moves and how sensitive the gut is as factors that decide whether malabsorption causes symptoms (Misselwitz et al. 2013; Deng et al. 2015).

Lactose intolerance is not a milk allergy. Lactose intolerance is a digestive problem — too little of a sugar-splitting enzyme. A milk allergy is an immune reaction to one or more proteins in milk (NIDDK). The two have different causes and different levels of danger: lactose intolerance causes uncomfortable symptoms, while a serious allergic reaction to milk can be life-threatening (NIDDK). The BSACI guideline on cow's milk allergy states that it must be distinguished from primary lactose intolerance (Luyt et al. 2014). The two are compared side by side below.

Types of Lactose Intolerance

Lactose intolerance has several causes, and the cause bears on whether it is permanent, temporary, or present from birth.

1. Primary lactase deficiency (lactase non-persistence)

This is the genetically programmed kind and the most common cause of low lactase levels (NIDDK). In people with lactase non-persistence the small intestine makes less lactase after infancy, and levels fall further with age (NIDDK); one review describes the decrease as happening in the first two decades of life (Misselwitz et al. 2019). People who keep making lactase for life have a genetic trait called lactase persistence, described in the ancestry section. No treatment increases the amount of lactase the small intestine makes in lactase non-persistence (NIDDK).

2. Secondary (acquired) lactase deficiency

Here lactase falls because the lining of the small intestine — where the enzyme is made — has been injured or inflamed. A 2019 review lists infectious gastroenteritis, inflammatory bowel disease, celiac disease and systemic sclerosis as conditions that secondary lactose malabsorption can complicate, and names small intestinal bacterial overgrowth and rapid transit as other causes of malabsorption (Misselwitz et al. 2019). NIDDK also lists Crohn's disease (one form of inflammatory bowel disease) and treatments for other conditions — medicines, surgery or radiation therapy. Secondary malabsorption is often transitory: many people newly diagnosed with celiac disease recover the ability to digest lactose after 6–12 months on a gluten-free diet (Misselwitz et al. 2019), and NIDDK states that people may tolerate lactose again once the cause of the injury is treated.

3. Congenital lactase deficiency

This is a very rare genetic disorder in which the small intestine makes little or no lactase from birth (NIDDK; Misselwitz et al. 2019). One review counts about 40 reported cases, mainly in Finland (Deng et al. 2015). Another describes it as caused by recessive mutations, with symptoms developing rapidly once breastfeeding begins — often severe diarrhoea, acidosis and hypercalcaemia — and with all forms of milk withheld as the therapy (Szilagyi & Ishayek 2018).

4. Developmental lactase deficiency

In babies born prematurely, the small intestine may not make enough lactase for a short time after birth (NIDDK); lactase activity rises until about week 34 of pregnancy and peaks by birth (Deng et al. 2015). NIDDK states that the condition usually improves without treatment as the baby gets older.

Symptoms

NIDDK describes symptoms as starting within a few hours of eating or drinking something that contains lactose, and as mild or severe depending on how much lactose was taken (NIDDK). The symptoms listed by NIDDK and by reviews are:

• Bloating and a feeling of fullness or pressure in the belly
• Abdominal cramps or pain, often lower down
• Excess gas (flatulence) and audible stomach rumbling (borborygmi)
• Diarrhea, sometimes loose or watery and sometimes urgent
• Nausea, and sometimes vomiting
• Constipation, which one review lists as a less common presentation (Deng et al. 2015)

Symptoms are dose-dependent. A systematic review of 36 randomized trials found moderate-quality evidence that 12–15 g of lactose (about 1 cup of milk) is well tolerated by most adults with presumed lactose intolerance or malabsorption (Shaukat et al. 2010), and a 2019 review states that larger doses may be tolerated if taken with food or spread over the day (Misselwitz et al. 2019). In a 1995 double-blind crossover trial of 30 people who reported severe lactose intolerance, a cup (240 ml) of milk a day with breakfast produced minimal symptoms, no different from lactose-hydrolysed milk; the authors concluded that people who identify themselves as severely lactose intolerant may mistakenly attribute a variety of abdominal symptoms to lactose (Suarez et al. 1995). In the irritable bowel syndrome literature, weight loss and rectal bleeding are listed as alarm symptoms that call for investigation rather than a diagnosis from the history alone (Ford et al. 2020) — see Key Points.

Who Gets It — Ancestry & Genetics

Lactase persistence is described as a textbook example of gene–culture coevolution: an adaptation to the domestication of dairy animals and the drinking of their milk (Ségurel & Bon 2017). A 2017 review describes lactase non-persistence — the fall in lactase after childhood that underlies primary lactose malabsorption — as affecting 65–70% of the world's adults (Bayless et al. 2017), but that average hides large differences between populations. A 2017 meta-analysis (Storhaug et al.) that was widely quoted for a worldwide malabsorption figure of 68% received an expression of concern from its journal in 2024 and was retracted in January 2025, so none of its estimates is used on this page.

Earlier surveys summarised by Tishkoff et al. (2007) put lactase persistence (the ability to keep digesting lactose) at more than 90% of Swedes and Danes, about 50% of Spanish, French and pastoralist Arab populations, about 5–20% of West African agriculturalists and about 1% of Chinese people. Malabsorption is therefore the norm in populations such as China's and uncommon in Scandinavian populations. A worldwide comparison of phenotype and genotype data concluded that the known variants cannot yet explain how common persistence is in much of western and southern Africa, southeastern Europe, the Middle East and parts of central and southern Asia (Itan et al. 2010).

The ability to keep digesting lactose into adulthood — lactase persistence — is controlled not by changes in the lactase gene's coding sequence but by regulatory DNA nearby. In Finnish families a variant about 14 kb upstream of the lactase gene, C/T−13910, was completely associated with biochemically verified lactase non-persistence (Enattah et al. 2002), and in people of European ancestry persistence is nearly uniformly mediated by this variant (Misselwitz et al. 2019). Pastoralist populations in Tanzania, Kenya and Sudan carry different variants in the same region (G/C−14010, T/G−13915 and C/G−13907) that enhance lactase-gene transcription; the authors describe this as convergent evolution under strong selective pressure from animal domestication and adult milk drinking, with a selective sweep over roughly the past 7,000 years (Tishkoff et al. 2007). Another variant, −13915G, is listed for the Middle East (Misselwitz et al. 2019). Why persistence was so strongly selected for, when milk processing can lower its lactose content, is still described as an open question (Ségurel & Bon 2017).

Because lactase falls gradually, NIDDK notes that symptoms of lactase non-persistence may not begin until later childhood, the teen years, or early adulthood (NIDDK).

How It Is Diagnosed

NIDDK describes diagnosis as starting from symptoms, family and medical history and eating habits, sometimes with a physical exam and tests (NIDDK). A 2019 review lists five tests of lactose digestion, each with different strengths and limits (Misselwitz et al. 2019):

• Hydrogen breath test. After a measured drink of lactose, breath samples are collected over a few hours (NIDDK describes every 30 minutes). If lactose is malabsorbed, colonic bacteria ferment it and produce hydrogen, which is exhaled. A baseline below 20 parts per million (ppm) is required for a reliable test, and a rise of 20 ppm or more within 3 hours is diagnostic of malabsorption. The 2019 review calls it the test of choice for assessing both malabsorption and symptoms; false negatives occur in people whose gut bacteria do not produce hydrogen (some convert it to methane), and false positives with bacterial overgrowth or rapid transit (Misselwitz et al. 2019).
• Lactose tolerance test (blood glucose). Blood glucose is measured after a lactose drink; a rise of less than 1.1 mmol/L within 3 hours is the cut-off used for malabsorption (Deng et al. 2015). The 2019 review lists its best use as low-resource settings and epidemiology, with false results in disorders of glucose metabolism (Misselwitz et al. 2019).
• Dietary trial. NIDDK describes doctors asking people to stop milk and milk products for a period of time to see whether symptoms go away, with further tests if they do not.
• Genetic testing. A test on blood or a cheek swab detects the −13910 variant. It can identify genetic lactase non-persistence in people of European ancestry, but it cannot detect secondary malabsorption and can give false-negative results where other variants are common, as in people of African or Asian heritage (Misselwitz et al. 2019).
• Duodenal biopsy with a lactase assay. Measures lactase activity directly; the review lists its best use as when a gastroscopy is being done for other reasons, and notes that patchy lactase expression limits it (Misselwitz et al. 2019).

NIDDK lists irritable bowel syndrome, celiac disease, inflammatory bowel disease and small bowel bacterial overgrowth as conditions that can cause symptoms similar to lactose intolerance.

Managing It: Diet & Enzymes

The 2010 NIH consensus panel concluded that, in most cases, people with lactose intolerance do not need to eliminate dairy completely, and that many people with real or perceived lactose intolerance avoid dairy and take in too little calcium and vitamin D (Suchy et al. 2010, NIH statement). A 2019 review notes that, unlike celiac disease or food allergy, a strict lactose-free diet is not required, because people with lactose intolerance often tolerate up to 250 ml of milk (12 g lactose) without symptoms, and more with food (Misselwitz et al. 2019). Reviews describe the clinical results of treatment as modest, partly because lactose is only one of several poorly absorbed carbohydrates that cause symptoms the same way (Misselwitz et al. 2019).

Approaches described in the research

• Amount and timing. Most adults in the trials reviewed tolerated 12–15 g of lactose (about 1 cup of milk) at a time (Shaukat et al. 2010); taking lactose with meals or spreading it across the day is described as raising the amount tolerated (Misselwitz et al. 2019). One review notes that taking lactose with a meal, especially with fat, slows stomach emptying (Szilagyi & Ishayek 2018).
• Lower-lactose dairy foods. Representative values in a 2019 review: whole milk 4.7 g lactose per 100 g, fresh yoghurt 3.0 g, soft cheese such as camembert 0.3 g, butter 0.5 g (about 0.1 g per serving) and hard cheese such as cheddar or gruyère 0.1 g; the review notes that cheese is low in lactose, especially long-ripened cheese (Misselwitz et al. 2019).
• Cultured dairy. Yoghurt contains about half the lactose of unprocessed milk (Misselwitz et al. 2019), and NIDDK notes that some people find yoghurt and hard cheeses easier to tolerate than other milk products. Yoghurt and kefir carry live bacteria, some of which make the lactose-splitting enzyme, and one review notes that lactose-intolerant people in many parts of the world are observed to prefer yoghurt (Szilagyi & Ishayek 2018).
• Lactose-free dairy. These are milk products with lactase added, so the lactose is already split; they contain less than 0.1 g lactose per 100 g, taste sweeter, and are considered safe, although allergic reactions have been reported (Misselwitz et al. 2019). NIDDK describes lactose-free and lactose-reduced milk products as just as healthy as regular milk products.
• Lactase enzyme tablets or drops. Tablets are taken before eating dairy, or drops are added to milk beforehand (NIDDK). Lactase tablets improve both lactose digestion and symptoms, but the effects are modest (Misselwitz et al. 2019). NIDDK notes that some people, such as young children and pregnant and breastfeeding women, may not be able to use lactase products.
• Regular exposure (colonic adaptation). Lactase itself is not increased by eating lactose in humans (Szilagyi & Ishayek 2018), but regular intake may shift the colonic bacteria. Reviews describe symptoms lessening with regular lactose consumption, which may be partly colonic adaptation and partly a placebo effect (Szilagyi & Ishayek 2018); the 2010 systematic review found the trial evidence for colonic adaptation insufficient (Shaukat et al. 2010).
• Probiotics. The 2010 systematic review found the evidence for probiotics insufficient (Shaukat et al. 2010); a later systematic review cited in 2019 found an overall positive effect, but not consistently larger than lactase supplements and with limited study quality (Misselwitz et al. 2019).

Lactose in other foods and medicines

Lactose powder is a common additive in processed foods such as sausages, gravy, margarines, bread, sauces and many prepared meals (Misselwitz et al. 2019). NIDDK lists breads and baked goods, breakfast cereals, instant potatoes, soups, margarine, salad dressings, flavoured snack foods, processed meats, milk-based meal replacements and protein powders and bars, and non-dairy coffee creamers and whipped toppings. On ingredient lists, NIDDK states that the words milk, lactose, whey, curds, milk by-products, dry milk solids and nonfat dry milk powder mean the product contains lactose (NIDDK). Some prescription and over-the-counter medicines contain a small amount of lactose as a filler (NIDDK); in a controlled breath-test study, the amount in medicines (an average of 400 mg) did not produce a positive breath-hydrogen rise (Szilagyi & Ishayek 2018).

Calcium & Vitamin D Without Dairy

Milk and milk products are some of the main sources of calcium and vitamin D (NIDDK). The 2010 NIH consensus panel found that many people with real or perceived lactose intolerance avoid dairy and take in too little calcium and vitamin D, which may predispose them to decreased bone accrual, osteoporosis and other adverse outcomes (Suchy et al. 2010, NIH statement). A 2019 review notes that these nutrients can also be obtained from other foods (Misselwitz et al. 2019).

The Institute of Medicine's 2011 Recommended Dietary Allowances are 1,000 mg of calcium a day for adults aged 19–50 and men aged 51–70, and 1,200 mg for women aged 51–70 and everyone over 70; for vitamin D, 600 IU a day up to age 70 and 800 IU from 71 (Ross et al. 2011). Non-dairy sources named by NIDDK and in absorption studies include:

• Calcium. Fish with soft bones, such as canned sardines or salmon; broccoli and leafy green vegetables; oranges; almonds, Brazil nuts and dried beans; and foods with added calcium, such as some cereals and juices (NIDDK). Calcium absorption from kale, a low-oxalate green, was as good as or better than from milk in 11 women (Heaney & Weaver 1990), while in 13 adults absorption from spinach, whose oxalate binds its calcium, averaged 5.1% against 27.6% from milk (Heaney et al. 1988).
• Vitamin D. Eggs and certain fish such as salmon; some ready-to-eat cereals, orange juice and milk products have vitamin D added (NIDDK). Sunlight lets the body make vitamin D (NIDDK); the IOM allowances were set assuming minimal sun exposure, because skin synthesis varies widely and carries a skin-cancer risk (Ross et al. 2011).

NIDDK describes lactose-free and lactose-reduced milk products as just as healthy as regular milk products.

Lactose Intolerance vs Milk Allergy vs IBS

These three are often confused because they can cause overlapping abdominal symptoms, but their causes, risks and management differ.

Lactose intolerance and IBS frequently overlap. A 2015 review states that intolerance of the wider group of fermentable carbohydrates (FODMAPs) is present in at least half of people with IBS, and that this group needs restriction of more than lactose to improve symptoms (Deng et al. 2015). A 2019 review adds that people with visceral hypersensitivity linked to anxiety or IBS are at increased risk of lactose intolerance regardless of how well they digest lactose, and that intolerance of low–moderate lactose doses often indicates IBS (Misselwitz et al. 2019).

Key Points

A summary of what the sources above report:

• Tolerated amounts. Most adults with lactose malabsorption tolerate 12–15 g of lactose, about a cup of milk, at a time (Shaukat et al. 2010), and many people who believe they are lactose intolerant are not lactose malabsorbers (NIH statement 2010).
• Lower-lactose forms. Hard cheeses, butter and lactose-free milk contain little lactose, and yoghurt about half that of milk (Misselwitz et al. 2019); kefir is a fermented milk carrying live bacteria (Szilagyi & Ishayek 2018).
• With meals. Larger doses may be tolerated when taken with food or spread over the day (Misselwitz et al. 2019).
• Lactase enzyme tablets. They improve digestion and symptoms modestly (Misselwitz et al. 2019).
• Bone health. Dairy avoidance is linked to low calcium and vitamin D intake, which may predispose to decreased bone accrual and osteoporosis (NIH statement 2010); non-dairy calcium sources such as canned sardines are listed above.
• Labels and medicines. Ingredient words that mean lactose is present are listed above; the lactose in medicines is usually a small amount (NIDDK; Szilagyi & Ishayek 2018).
• Look-alike conditions. Celiac disease, inflammatory bowel disease, IBS and small bowel bacterial overgrowth can cause similar symptoms (NIDDK), and weight loss or rectal bleeding are listed as alarm symptoms that call for investigation (Ford et al. 2020). Decisions about diet, tests and treatment belong with a clinician.

Key Research Papers

1. Suchy FJ, Brannon PM, Carpenter TO, Fernandez JR, et al. National Institutes of Health Consensus Development Conference: Lactose Intolerance and Health. Annals of Internal Medicine. 2010;152(12):792-796. — doi:10.7326/0003-4819-152-12-201006150-00248
2. Shaukat A, Levitt MD, Taylor BC, MacDonald R, et al. Systematic Review: Effective Management Strategies for Lactose Intolerance. Annals of Internal Medicine. 2010;152(12):797-803. — doi:10.7326/0003-4819-152-12-201006150-00241
3. Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut. 2019;68(11):2080-2091. — doi:10.1136/gutjnl-2019-318404
4. Storhaug CL, Fosse SK, Fadnes LT. Country, regional, and global estimates for lactose malabsorption in adults: a systematic review and meta-analysis. The Lancet Gastroenterology & Hepatology. 2017;2(10):738-746. — doi:10.1016/S2468-1253(17)30154-1 — Retracted; no figure on this page rests on this paper (retraction notice: PubMed PMID: 39674208; expression of concern, 2024: PubMed PMID: 39127069)
5. The Editors of The Lancet Gastroenterology & Hepatology. Expression of concern — Country, regional, and global estimates for lactose malabsorption in adults: a systematic review and meta-analysis. The Lancet Gastroenterology & Hepatology. 2024;9(9):784. — PubMed PMID: 39127069
6. The Editors of The Lancet Gastroenterology & Hepatology. Retraction — Country, regional, and global estimates for lactose malabsorption in adults: a systematic review and meta-analysis. The Lancet Gastroenterology & Hepatology. 2025;10(1):13. — PubMed PMID: 39674208
7. Enattah NS, Sahi T, Savilahti E, Terwilliger JD, et al. Identification of a variant associated with adult-type hypolactasia. Nature Genetics. 2002;30(2):233-237. — doi:10.1038/ng826
8. Tishkoff SA, Reed FA, Ranciaro A, Voight BF, et al. Convergent adaptation of human lactase persistence in Africa and Europe. Nature Genetics. 2007;39(1):31-40. — doi:10.1038/ng1946
9. Itan Y, Jones BL, Ingram CJ, Swallow DM, et al. A worldwide correlation of lactase persistence phenotype and genotypes. BMC Evolutionary Biology. 2010;10:36. — doi:10.1186/1471-2148-10-36
10. Suarez FL, Savaiano DA, Levitt MD. A Comparison of Symptoms after the Consumption of Milk or Lactose-Hydrolyzed Milk by People with Self-Reported Severe Lactose Intolerance. New England Journal of Medicine. 1995;333(1):1-4. — doi:10.1056/NEJM199507063330101
11. Deng Y, Misselwitz B, Dai N, Fox M. Lactose Intolerance in Adults: Biological Mechanism and Dietary Management. Nutrients. 2015;7(9):8020-8035. — doi:10.3390/nu7095380
12. Misselwitz B, Pohl D, Frühauf H, Fried M, et al. Lactose malabsorption and intolerance: pathogenesis, diagnosis and treatment. United European Gastroenterology Journal. 2013;1(3):151-159. — doi:10.1177/2050640613484463
13. Szilagyi A, Ishayek N. Lactose Intolerance, Dairy Avoidance, and Treatment Options. Nutrients. 2018;10(12):1994. — doi:10.3390/nu10121994
14. Bayless TM, Brown E, Paige DM. Lactase Non-persistence and Lactose Intolerance. Current Gastroenterology Reports. 2017;19(5):23. — doi:10.1007/s11894-017-0558-9
15. Suchy FJ, Brannon PM, Carpenter TO, Fernandez JR, et al. NIH consensus development conference statement: Lactose intolerance and health. NIH Consensus and State-of-the-Science Statements. 2010;27(2):1-27. — PubMed PMID: 20186234
16. Ross AC, Manson JE, Abrams SA, Aloia JF, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. Journal of Clinical Endocrinology & Metabolism. 2011;96(1):53-58. — doi:10.1210/jc.2010-2704
17. Heaney RP, Weaver CM, Recker RR. Calcium absorbability from spinach. American Journal of Clinical Nutrition. 1988;47(4):707-709. — doi:10.1093/ajcn/47.4.707
18. Heaney RP, Weaver CM. Calcium absorption from kale. American Journal of Clinical Nutrition. 1990;51(4):656-657. — doi:10.1093/ajcn/51.4.656
19. Luyt D, Ball H, Makwana N, Green MR, et al. BSACI guideline for the diagnosis and management of cow's milk allergy. Clinical & Experimental Allergy. 2014;44(5):642-672. — doi:10.1111/cea.12302
20. Ford AC, Sperber AD, Corsetti M, Camilleri M. Irritable bowel syndrome. The Lancet. 2020;396(10263):1675-1688. — doi:10.1016/S0140-6736(20)31548-8
21. Ségurel L, Bon C. On the evolution of lactase persistence in humans. Annual Review of Genomics and Human Genetics. 2017;18:297-319. — doi:10.1146/annurev-genom-091416-035340

PubMed Topic Searches

These links open live PubMed searches for the listed keywords — results update as new studies are indexed.

1. Lactose intolerance — PubMed search
2. Lactase persistence genetics — PubMed search
3. Hydrogen breath test for lactose — PubMed search
4. Lactase enzyme supplements — PubMed search
5. Lactose malabsorption diagnosis — PubMed search
6. Lactose-free diet, calcium & bone — PubMed search
7. Lactose intolerance & IBS — PubMed search

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Connections

• Gastroenterology
• Irritable Bowel Syndrome
• Celiac Disease
• Inflammatory Bowel Disease
• Crohn's Disease
• SIBO
• Food Intolerance
• Food Allergy
• Bloating
• Osteoporosis
• Milk
• Yogurt
• Kefir
• Probiotics
• Calcium
• Vitamin D3
• Milk and Lactose Intolerance — lactase persistence, dose thresholds, and which dairy foods stay tolerable.

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