lactose (disaccharide of β-D-galactose & β-D-glucose) is normally split by lactase.
|Classification and external resources|
|Patient UK||Lactose intolerance|
Lactose intolerance is the inability of adults to digest lactose, a sugar found in milk and to a lesser extent dairy products, causing side effects. It is due to a lactase deficiency, or hypolactasia. Congenital lactase deficiency prevents babies from drinking even human milk.
Lactose intolerant individuals have insufficient levels of lactase, an enzyme that catalyzes the hydrolysis of lactose into glucose and galactose, in their digestive system. In most cases, this causes symptoms which may include abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi (rumbling stomach), or vomiting after consuming significant amounts of lactose. It is common for patients with inflammatory bowel disease to experience gastrointestinal symptoms after lactose ingestion, although the prevalence of lactase deficiency in this population has not been well studied.
Most mammals normally cease to produce lactase, becoming lactose intolerant, after weaning, but some human populations have developed lactase persistence, in which lactase production continues into adulthood. Research reveals intolerance is more common globally than lactase persistence, and that the variation is genetic. A study of 21 African American girls showed that 75% had some decrease in lactase activity during adolescence. The frequency of lactose intolerance ranges from 5% in Northern European to more than 90% in some African and Asian countries. This distribution is now thought to have been caused by recent natural selection favoring lactase-persistent individuals in cultures in which dairy products are available as a food source. Although populations in Europe, India, Arabia, and Africa were first thought to have high frequencies of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently.
- 1 Terminology
- 2 Symptoms
- 3 Causes
- 4 Diagnosis
- 5 Management
- 6 Lactase persistence
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
"Lactose intolerance" primarily refers to a syndrome having one or more symptoms upon the consumption of food substances containing lactose. Individuals may be lactose intolerant to varying degrees, depending on the severity of these symptoms. "Lactose malabsorption" refers to the physiological concomitant of lactase deficiency (i.e., the body does not have sufficient lactase capacity to digest the amount of lactose ingested). A medical condition with similar symptoms is fructose malabsorption.
Lactase deficiency has a number of causes and is classified as one of three types:
- Primary lactase deficiency is genetic, only affects adults, and is caused by the absence of a lactase persistence allele. It is the most common cause of lactose intolerance, as a majority of the world's population lacks these alleles.
- Secondary, acquired, or transient lactase deficiency is caused by an injury to the small intestine, usually during infancy, from acute gastroenteritis, diarrhea, chemotherapy, intestinal parasites, or other environmental causes.
- Congenital lactase deficiency is a very rare, autosomal recessive genetic disorder that prevents lactase expression from birth. It is particularly common in Finland. People with congenital lactase deficiency cannot digest lactose from birth, so cannot digest breast milk.
Lactose intolerance is not an allergy, because it is not an immune response, but rather a problem with digestion caused by lactase deficiency. Milk allergy is a separate condition, with distinct symptoms that occur when the presence of milk proteins trigger an immune reaction.
The principal symptom of lactose intolerance is an adverse reaction to products containing lactose (primarily milk), including abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi, and vomiting (particularly in adolescents). These appear one-half to two hours after consumption. The severity of symptoms typically increases with the amount of lactose consumed; most lactose-intolerant people can tolerate a certain level of lactose in their diets without ill effects.
Nutritional implications of CLD
Congenital lactase deficiency (CLD), where the production of lactase is inhibited from birth, can be dangerous in any society because of infants' initial reliance on human breast milk for nutrition until they are weaned onto other foods. Before the 20th century, babies born with CLD often did not survive, but death rates decreased with soybean-derived infant formulas and manufactured lactose-free dairy products. Beyond infancy, individuals affected by CLD usually have the same nutritional concerns as any lactose-intolerant adult.
Lactose intolerance is a consequence of lactase deficiency, which may be genetic or environmentally induced. In either case, symptoms are caused by insufficient levels of lactase in the lining of the duodenum. Lactose, a disaccharide molecule found in milk and dairy products, cannot be directly absorbed through the wall of the small intestine into the bloodstream, so, in the absence of lactase, passes intact into the colon. Bacteria in the colon can metabolise lactose, and the resulting fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane) that causes the various abdominal symptoms. The unabsorbed sugars and fermentation products also raise the osmotic pressure of the colon, causing an increased flow of water into the bowels (diarrhea). The LCT gene provides the instructions for making lactase. The specific DNA sequence in the MCM6 gene helps control whether the LCT gene is turned on or off. Possibly years ago, some humans developed a mutation in the MCM6 gene that keeps the LCT gene turned on even after breast feeding is stopped. People who are lactose intolerant do not have this mutation. The LCT and MCM6 genes are both located on the long arm (q) of chromosome 2 in region 21. The locus can be expressed as 2q21. The lactase deficiency also could be linked to certain heritages. Seventy-five percent of all African American, Jewish, Mexican American, and Native American adults are lactose intolerant.
Different alleles for lactase persistence have developed at least three times in East African populations, with persistence extending from 26% in Tanzania to 88% in the Beja pastoralist population in Sudan.
To assess lactose intolerance, intestinal function is challenged by ingesting more dairy products than can be readily digested. Clinical symptoms typically appear within 30 minutes, but may take up to two hours, depending on other foods and activities. Substantial variability in response (symptoms of nausea, cramping, bloating, diarrhea, and flatulence) is to be expected, as the extent and severity of lactose intolerance varies among individuals.
Lactose intolerance is distinct from milk allergy, an immune response to cow's milk proteins. They may be distinguished in diagnosis by giving lactose-free milk, producing no symptoms in the case of lactose intolerance, but the same reaction as to normal milk in the presence of a milk allergy. A person can have both conditions. If positive confirmation is necessary, four tests are available.
Hydrogen breath test
In a hydrogen breath test, the most accurate lactose intolerance test, after an overnight fast, 25 grams of lactose (in a solution with water) are swallowed. If the lactose cannot be digested, enteric bacteria metabolize it and produce hydrogen, which, along with methane, if produced, can be detected on the patient's breath by a clinical gas chromatograph or compact solid-state detector. The test takes about 2.5 hours to complete. If the hydrogen levels in the patient's breath are high, they may have lactose intolerance. This test is not usually done on babies and very young children, because it can cause severe diarrhea.
In conjunction, measuring blood glucose level every 10 to 15 minutes after ingestion will show a "flat curve" in individuals with lactose malabsorption, while the lactase persistent will have a significant "top", with a typical elevation of 50% to 100%, within one to two hours. However, due to the need for frequent blood sampling, this approach has been largely replaced by breath testing.
After an overnight fast, blood is drawn and then 50 grams of lactose (in aqueous solution) are swallowed. Blood is then drawn again at the 30-minute, 1-hour, 2-hour, and 3-hour marks. If the lactose cannot be digested, blood glucose levels will rise by less than 20 mg/dl.
Stool acidity test
This test can be used to diagnose lactose intolerance in infants, for whom other forms of testing are risky or impractical. The infant is given lactose to drink. If the individual is tolerant, the lactose is digested and absorbed in the small intestine; otherwise, it is not digested and absorbed, and it reaches the colon. The bacteria in the colon, mixed with the lactose, cause acidity in stools. Stools passed after the ingestion of the lactose are tested for level of acidity. If the stools are acidic, the infant is intolerant to lactose. Stool pH in lactose intolerance is less than 5.5.
An intestinal biopsy can confirm lactase deficiency following discovery of elevated hydrogen in the hydrogen breath test. Modern techniques have enabled a bedside test, identifying presence of lactase enzyme on upper gastrointestinal endoscopy instruments. However, for research applications such as mRNA measurements, a specialist laboratory is required.
Stool sugar chromatography
Chromatography can be used to separate and identify undigested sugars present in faeces. Although lactose may be detected in the faeces of people with lactose intolerance, this test is not considered reliable enough to conclusively diagnose or exclude lactose intolerance.
It is used in primary lactose intolerance. Lactase activity persistence in adults is associated with two polymorphisms: C/T 13910 and G/A 22018 located in the MCM6 gene. These polymorphisms may be detected by molecular biology techniques at the DNA extracted from blood or saliva samples; genetic kits specific for this diagnosis are available. The procedure consists of extracting and amplifying DNA from the sample, following with a hybridation protocol in a strip. Colored bands are obtained as final result, and depending on the different combination, it would be possible to determine whether the patient is lactose intolerant. This test allows a noninvasive definitive diagnostic.
Lactose intolerance is not considered a condition that requires treatment in societies where the diet contains relatively little dairy. However, those living among societies that are largely lactose-tolerant may find lactose intolerance troublesome. Although no way to reinstate lactase production had been found, some individuals have reported their intolerance varies over time, depending on health status and pregnancy. About 44% of lactose-intolerant women regain the ability to digest lactose during pregnancy. This might be caused by slow intestinal transit and intestinal flora changes during pregnancy. Lactose intolerance can also be managed by ingesting live yogurt cultures containing lactobacilli that are able to digest the lactose in other dairy products. This may explain why many South Asians, though genetically lactose intolerant, are able to consume large quantities of milk without many symptoms of lactose intolerance, since consuming live yougurt cultures is very common among the South Asian population. Lactose intolerance is not usually an absolute condition: the reduction in lactase production, and the amount of lactose that can therefore be tolerated, varies from person to person. Since lactose intolerance poses no further threat to a person's health, the condition is managed by minimizing the occurrence and severity of symptoms. Berdanier and Hargrove recognise four general principles in dealing with lactose intolerance — avoidance of dietary lactose, substitution to maintain nutrient intake, regulation of calcium intake and use of enzyme substitute.
Avoiding lactose-containing products
Since each individual's tolerance to lactose varies, according to the U.S. National Institutes of Health, "Dietary control of lactose intolerance depends on people learning through trial and error how much lactose they can handle." Label reading is essential, as commercial terminology varies according to language and region.
Lactose is present in two large food categories: conventional dairy products, and as a food additive (casein, caseinate, whey), which may contain traces of lactose.
Lactose is a water-soluble substance. Fat content and the curdling process affect tolerance of foods. After the curdling process, lactose is found in the water-based portion (along with whey and casein), but not in the fat-based portion. Dairy products that are "reduced-fat" or "fat-free" generally have slightly higher lactose content. Low-fat dairy foods also often have various dairy derivatives added, such as milk solids, increasing the lactose content.
Human milk has a high lactose content, around 9%. Unprocessed cow milk is about 4.7% lactose. Unprocessed milk from other bovids contains a similar fraction of lactose (goat milk 4.7%, buffalo 4.86%, yak 4.93%, sheep 4.6%)
The butter-making process separates the majority of milk's water components from the fat components. Lactose, being a water-soluble molecule, will largely be removed, but will still be present in small quantities in the butter unless it is also fermented to produce cultured butter. Clarified butter, however, contains very little lactose and is safe for most lactose-intolerant people.
People can be more tolerant of traditionally made yogurt than milk, because it contains lactase produced by the bacterial cultures used to make the yogurt. Frozen yogurt will contain similarly reduced lactose levels.
Traditionally made hard cheeses, and soft-ripened cheeses may create less reaction than the equivalent amount of milk because of the processes involved. Fermentation and higher fat content contribute to lesser amounts of lactose. Traditionally made Emmental or Cheddar might contain 10% of the lactose found in whole milk. In addition, the ageing methods of traditional cheeses (sometimes over two years) reduce their lactose content to practically nothing. Commercial cheeses, however, are often manufactured by processes that do not have the same lactose-reducing properties. Ageing of some cheeses is governed by regulations; in other cases, no quantitative indication of degree of ageing and concomitant lactose reduction is given, and lactose content is not usually indicated on labels.
- Sour cream
If made in the traditional way, this may be tolerable, but most modern brands add milk solids.
- Examples of lactose levels in foods
As industry standardization has not been established concerning lactose content analysis methods (nonhydrated form or the monohydrated form), and considering that dairy content varies greatly according to labeling practices, geography, and manufacturing processes, lactose numbers may not be very reliable. The following table contains a guide to the typical lactose levels found in various foods.
Dairy product Serving size Lactose content Percentage Milk, regular 250 ml 12 g 4.80% Milk, reduced fat 250 ml 13 g 5.20% Yogurt, plain, regular 200 g 9 g 4.50% Yogurt, plain, low-fat 200 g 12 g 6.00% Cheddar cheese 30 g 0.02 g 0.07% Cottage cheese 30 g 0.1 g 0.33% Butter 1 tsp (5.9ml) 0.03 g 0.51% Ice cream 50 g 3 g 6.00%
Lactose in nondairy products
Lactose (also present when labels state lactoserum, whey, milk solids, modified milk ingredients, etc.) is a commercial food additive used for its texture, flavor, and adhesive qualities, and is found in foods such as processed meats (sausages/hot dogs, sliced meats, pâtés), gravy stock powder, margarines, sliced breads, breakfast cereals, potato chips, processed foods, medications, prepared meals, meal replacements (powders and bars), protein supplements (powders and bars), and even beers in the milk stout style. Some barbecue sauces and liquid cheeses used in fast-food restaurants may also contain lactose.
Kosher products labeled pareve or fleishig are free of milk. However, if a "D" (for "dairy") is present next to the circled "K", "U", or other hechsher, the food product likely contains milk solids, although it may also simply indicate the product was produced on equipment shared with other products containing milk derivatives.
Plant-based "milks" and derivatives such as soy milk, rice milk, almond milk, coconut milk, hazelnut milk, oat milk, hemp milk, and peanut milk are inherently lactose-free. Low-lactose and lactose-free versions of foods are often available to replace dairy-based foods for those with lactose intolerance.
Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by fungi of the genus Aspergillus. The enzyme, β-galactosidase, is available in tablet form in a variety of doses, in many countries without a prescription. It functions well only in high-acid environments, such as that found in the human gut due to the addition of gastric juices from the stomach. Unfortunately, too much acid can denature it, so it should not be taken on an empty stomach. Also, the enzyme is ineffective if it does not reach the small intestine by the time the problematic food does. Lactose-sensitive individuals can experiment with both timing and dosage to fit their particular needs.
While essentially the same process as normal intestinal lactose digestion, direct treatment of milk employs a different variety of industrially produced lactase. This enzyme, produced by yeast from the genus Kluyveromyces, takes much longer to act, must be thoroughly mixed throughout the product, and is destroyed by even mildly acidic environments. Its main use is in producing the lactose-free or lactose-reduced dairy products sold in supermarkets.
Rehabituation to dairy products
For healthy individuals with secondary lactose intolerance, it may be possible in some cases for the bacteria in the large intestine to adapt to an altered diet and break down small quantities of lactose more effectively by habitually consuming small amounts of dairy products several times a day over a period of time. This is not suitable for people who have an underlying or chronic illness which may have damaged the intestinal tract in a way which prevents the lactase enzyme from being expressed.
Environmental factors—more specifically, the consumption of lactose—may "play a more important role than genetic factors in the etio-pathogenesis of milk intolerance",.
Lactase persistence is the phenotype associated with various autosomal dominant alleles prolonging the activity of lactase beyond infancy; conversely, lactase nonpersistence is the phenotype associated with primary lactase deficiency. Among mammals, lactase persistence is unique to humans — it evolved relatively recently (in the last 10,000 years) among some populations, and the majority of people worldwide remain lactase nonpersistent. For this reason, lactase persistence is of some interest to the fields of anthropology, human genetics, and archeology, which typically use the genetically derived persistence/non-persistence terminology.
Recognition of the extent and genetic basis of lactose intolerance is relatively recent. Though its symptoms were described as early as Hippocrates (460-370 BC), until the 1960s, the prevailing assumption in the medical community was that tolerance was the norm and intolerance was either the result of milk allergy, an intestinal pathogen, or else was psychosomatic (it being recognised that some cultures did not practice dairying, and people from those cultures often reacted badly to consuming milk). Two reasons were given for this perception. Firstly, many Western countries have a predominantly European heritage, so have low frequencies of lactose intolerance, and have an extensive cultural history of dairying. Therefore, tolerance actually was the norm in most of the societies investigated by medical researchers at that point. Secondly, within even these societies, lactose intolerance tends to be under-reported: genetically lactase nonpersistent individuals can tolerate varying quantities of lactose before showing symptoms, and their symptoms differ in severity. Most are able to digest a small quantity of milk, for example in tea or coffee, without suffering any adverse effects. Fermented dairy products, such as cheese, also contain dramatically less lactose than plain milk. Therefore, in societies where tolerance is the norm, many people who consume only small amounts of dairy or have only mild symptoms, may be unaware that they cannot digest lactose. Eventually, however, lactose intolerance was recognised in the United States to be correlated with race. Subsequent research revealed intolerance was more common globally than lactase persistence, and that the variation was genetic.
- Food intolerance
- Gluten intolerance
- Lactose content of foods
- Soy cheese
- Soy milk and Plant milk
- Sucrose intolerance
- "Lactose Intolerance". National Digestive Diseases Information Clearinghouse (NDDIC). NIDDK. Retrieved 29 November 2011.
- Vesa TH, Marteau P, Korpela R (2000) Lactose intolerance. J Am Coll Nutr 19:165S–175S
- Joachim G (1999). "The relationship between habits of food consumption and reported reactions to food in people with inflammatory bowel disease--testing the limits". Nutrition and health 13 (2): 69–83. doi:10.1177/026010609901300203. PMID 10453452.
- Swallow DM (2003). "Genetics Oflactasepersistence Andlactoseintolerance". Annual Review of Genetics 37: 197–219. doi:10.1146/annurev.genet.37.110801.143820. PMID 14616060.
- Pribila BA, Hertzler SR, Martin BR, Weaver CM, Savaiano DA (2000). "Improved lactose digestion and intolerance among African-American adolescent girls fed a dairy-rich diet". Journal of the American Dietetic Association 100 (5): 524–8; quiz 529–30. doi:10.1016/S0002-8223(00)00162-0. PMID 10812376. Retrieved 2009-02-03.
Approximately 75% of the world's population loses the ability to completely digest a physiological dose of lactose after infancy
- A.C. Bulhões, H.A.S. Goldani, F.S. Oliveira, U.S. Matte, R.B. Mazzuca1 and T.R. Silveira (2007). "Correlation between lactose absorption and the C/T-13910 and G/A-22018 mutations of the lactase-phlorizin hydrolase (LCT) gene in adult-type hypolactasia". Brazilian Journal of Medical and Biological Research 40 (11): 1441–1446.
Adult-type hypolactasia is characterized by a fall of lactase activity levels to 5 to 10% of birth levels occurring during childhood and adolescence. The condition affects more than 75% of the population worldwide, with regional frequencies ranging from nearly 5% in northern Europe to more than 90% in some Asian and African countries
- Beja-Pereira A et al. Gene-culture coevolution between cattle milk protein genes and human lactase genes" Nat Genet 2003; 35: 311−313.
- Ingram CJ, Mulcare CA, Itan Y, Thomas MG, Swallow DM. 2009. Lactose digestion and the evolutionary genetics of lactase persistence" Hum Genet 2009 Jan;124(6) 579-91. Epub 2008 Nov 26.
- Heyman MB (2006). "Lactose Intolerance in Infants, Children, and Adolescents". Pediatrics 118 (3): 1279–1286. doi:10.1542/peds.2006-1721. PMID 16951027.
- Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Järvelä I (2002). "Identification of a variant associated with adult-type hypolactasia". Nat. Genet. 30 (2): 233–7. doi:10.1038/ng826. PMID 11788828.
- Swallow DM (2003). "Genetics of lactase persistence and lactose intolerance". Annual Review of Genetics 37: 197–219. doi:10.1146/annurev.genet.37.110801.143820. PMID 14616060.
- Swagerty DL, Walling AD, Klein RM (2002). "Lactose intolerance". Am Fam Physician 65 (9): 1845–50. PMID 12018807.
- Lawson, Margaret; Bentley, Donald; Lifschitz, Carlos (2002). Pediatric gastroenterology and clinical nutrition. London: Remedica. p. 109. ISBN 978-1-901346-43-5.
- Pediatric Lactose Intolerance at eMedicine
- Behrendt M, Keiser M, Hoch M, Naim HY (2009). "Impaired trafficking and subcellular localization of a mutant lactase associated with congenital lactase deficiency". Gastroenterology 136 (7): 2295–303. doi:10.1053/j.gastro.2009.01.041. PMID 19208354.
- Savaiano DA, Levitt MD (1987). "Milk intolerance and microbe-containing dairy foods". J. Dairy Sci. 70 (2): 397–406. doi:10.3168/jds.S0022-0302(87)80023-1. PMID 3553256.
- Mądry, E.; Krasińska, B.; Woźniewicz, M. G.; Drzymała-Czyż, S. A.; Bobkowski, W.; Torlińska, T.; Walkowiak, J. A. (2011). "Tolerance of different dairy products in subjects with symptomatic lactose malabsorption due to adult type hypolactasia". Gastroenterology Review 5: 310. doi:10.5114/pg.2011.25381.
- name="Sinden, A.A 1991 Emedicine|PED|1270|Lactose Intolerance" Guandalini S, Frye R, Rivera-Hernández D, Miller L, Borowitz S
- Lactose intolerance~overview at eMedicine
- "Lactose Intolerance". Johns Hopkins Health Library. Retrieved 2014-02-18.
- Tishkoff et al. 2006.
- R. Bowen (December 28, 2006). "Lactose Intolerance (Lactase Non-Persistence)". Pathophysiology of the Digestive System. Colorado State University.
- Olivier CE, Lorena SL, Pavan CR, dos Santos RA, dos Santos Lima RP, Pinto DG, da Silva MD, de Lima Zollner R (2012). "Is it just lactose intolerance?". Allergy and asthma proceedings : the official journal of regional and state allergy societies 33 (5): 432–6. doi:10.2500/aap.2012.33.3584. PMID 23026186.
- "Lactose tolerance tests". 3 May 2011.
- National Digestive Diseases Information Clearinghouse (March 2006). "Lactose Intolerance — How is lactose intolerance diagnosed?". National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.
- "Stool Acidity Test". Jay W. Marks, M.D. Retrieved 2011-05-20.
- Hargrove, James L.; Berdanier, Carolyn D. (1993). Nutrition and gene expression. Boca Raton: CRC Press. ISBN 0-8493-6961-4.
- Kuokkanen M, Myllyniemi M, Vauhkonen M, Helske T, Kääriäinen I, Karesvuori S, Linnala A, Härkönen M, Järvelä I, Sipponen P (2006). "A biopsy-based quick test in the diagnosis of duodenal hypolactasia in upper gastrointestinal endoscopy". Endoscopy 38 (7): 708–12. doi:10.1055/s-2006-925354. PMID 16761211.
- Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Järvelä I (2002). "Identification of a variant associated with adult-type hypolactasia". Nature Genetics 30 (2): 233–7. doi:10.1038/ng826. PMID 11788828.
- Lactose Intolerance at eMedicine Roy, Barakat, Nwakakwa, Shojamanesh, Khurana, July 5, 2006
- Composition of Human, Cow, and Goat Milks — Goat Milk — goatworld.com
- Peeva (2001). "Composition of buffalo milk. Sources of specific effects on the separate components". Bulg. J. Agric. Sci. 7: 329–35.
- Example of cheese regulations: "West Country Farmhouse Cheddar" must be aged for 9 months
- Reger, Combs, Coulter and Koch (February 1, 1951). "A Comparison of Dry Sweet Cream Buttermilk and Non-Fat Dry Milk Solids in Breadmaking". Journal of Dairy Science 34 (2): 136–44. doi:10.3168/jds.S0022-0302(51)91682-7.
- Goat Milk Composition
- "General guidelines for milk allergy". Oregon Health & Science University.
- "Margarine Regulations".
- "Enriched White Bread in Canada". The Canadian Celiac Association.
- Riggs, Lloyd K; Beaty, Annabel; Johnson, Arnold H (December 1946). "Influence of Nonfat Dry Milk Solids on the Nutritive Value of Bread". Journal of Dairy Science 29 (12): 821–9. doi:10.3168/jds.S0022-0302(46)92546-5.
- "Bartek, food additive company" (PDF).
- Montalto M, Curigliano V, Santoro L, Vastola M, Cammarota G, Manna R, Gasbarrini A, Gasbarrini G (2006). "Management and treatment of lactose malabsorption". World J. Gastroenterol. 12 (2): 187–91. PMID 16482616.
- He M, Yang Y, Bian L, Cui H (1999). "[Effect of exogenous lactase on the absorption of lactose and its intolerance symptoms]". Wei Sheng Yan Jiu (in Chinese) 28 (5): 309–11. PMID 12712706.
- O'Connell S, Walsh G (2006). "Physicochemical characteristics of commercial lactases relevant to their application in the alleviation of lactose intolerance". Appl. Biochem. Biotechnol. 134 (2): 179–91. doi:10.1385/ABAB:134:2:179. PMID 16943638.
- Lactose intolerant? Drink more milk Steve Tally
- Yoshida Y, Sasaki G, Goto S, Yanagiya S, Takashina K (1975). "Studies on the etiology of milk intolerance in Japanese adults". Gastroenterol. Jpn. 10 (1): 29–34. PMID 1234085.
- Curry, Andrew (31 July 2013). "Archaeology: The milk revolution". Nature.
- Tishkoff SA, Reed FA, Ranciaro A, Voight BF, Babbitt CC, Silverman JS, Powell K, Mortensen HM, Hirbo JB, Osman M, Ibrahim M, Omar SA, Lema G, Nyambo TB, Ghori J, Bumpstead S, Pritchard JK, Wray GA, Deloukas P (10 December 2006). "Convergent adaptation of human lactase persistence in Africa and Europe". Nature Genetics 39 (1): 31–40. doi:10.1038/ng1946. PMC 2672153. PMID 17159977. Retrieved 18 April 2013.
- Wilson J (December 2005). "Milk Intolerance: Lactose Intolerance and Cow's Milk Protein Allergy". Newborn and Infant Nursing Reviews 5 (4): 203–7. doi:10.1053/j.nainr.2005.08.004.
- Auricchio S, Rubino A, Landolt M, Semenza G, Prader A (1963). "Isolated intestinal lactase deficiency in the adult". Lancet 2 (7303): 324–6. doi:10.1016/s0140-6736(63)92991-x. PMID 14044269.
- Simoons FJ (December 1969). "Primary adult lactose intolerance and the milking habit: a problem in biological and cultural interrelations. I. Review of the medical research". Am J Dig Dis 14 (12): 819–36. doi:10.1007/bf02233204. PMID 4902756.
- Itan Y, Jones BL, Ingram CJ, Swallow DM, Thomas MG (2010). "A worldwide correlation of lactase persistence phenotype and genotypes". BMC Evolutionary Biology 10: 36. doi:10.1186/1471-2148-10-36. PMC 2834688. PMID 20144208.
- Bayless TM, Rosensweig NS (1966). "A racial difference in incidence of lactase deficiency. A survey of milk intolerance and lactase deficiency in healthy adult males". JAMA 197 (12): 968–72. doi:10.1001/jama.1966.03110120074017. PMID 5953213.
- Welsh JD, Rohrer V, Knudsen KB, Paustian FF (1967). "Isolated Lactase Deficiency: Correlation of Laboratory Studies and Clinical Data.". Archives of Internal Medlit 120 (3): 261–269. doi:10.1001/archinte.1967.00300030003003.
- Huang SS, Bayless TM (1968). "Milk and lactose intolerance in healthy Orientals". Science 160 (3823): 83–4. doi:10.1126/science.160.3823.83-a. PMID 5694356.
- Cook GC, Kajubi SK (1966). "Tribal incidence of lactase deficiency in Uganda". Lancet 1 (7440): 725–9. PMID 4159716.
- Jersky J, Kinsley RH (1967). "Lactase Deficiency in the South African Bantu". South African Medical Journal 41 (Dec): 1194–1196.
- Bolin TD, Crane GG, Davis AE (1968). "Lactose intolerance in various ethnic groups in South-East Asia". Australas Ann Med 17 (4): 300–6. PMID 5701921.
- Flatz G, Saengudom C, Sanguanbhokhai T (1969). "Lactose intolerance in Thailand". Nature 221 (5182): 758–9. doi:10.1038/221758b0. PMID 5818369.
- Elliott RB, Maxwell GM, Vawser N (1967). "Lactose maldigestion in Australian Aboriginal children". Med. J. Aust. 1 (2): 46–9. PMID 6016903.
- Flatz G, Rotthauwe HW (1971). "Evidence against nutritional adaption of tolerance to lactose". Humangenetik 13 (2): 118–25. doi:10.1007/bf00295793. PMID 5114667.
- Durand, P. (1959). "Lactosurie et saccharosurie". In Ed. E. Rossi, E. Gautier, and J. W. Weber. Paediat. IV. Carbohydrate Metabolism in Children. Basel. pp. 496–502.
- Holzel A, Schwarz V, Sutcliffe KW (1959). "Defective lactose absorption causing malnutrition in infancy". Lancet 1 (7083): 1126–8. doi:10.1016/S0140-6736(59)90710-X. PMID 13665980.
- Carroccio A, Montalto G, Cavera G, Notarbatolo A (1998). "Lactose intolerance and self-reported milk intolerance: relationship with lactose maldigestion and nutrient intake. Lactase Deficiency Study Group". J Am Coll Nutr 17 (6): 631–6. doi:10.1080/07315724.1998.10718813. PMID 9853544.
- McGee, Harold (2004). "Milk after infancy: dealing with lactose". On food and cooking: the science and lore of the kitchen. New York: Scribner. pp. 14–15. ISBN 0-684-80001-2.
- Rusynyk RA, Still CD (2001). "Lactose intolerance" (PDF). J Am Osteopath Assoc 101 (4 Suppl Pt 1): S10–2. PMID 11392211.
- Human Culture, an Evolutionary Force
- Wiki on living without dairy products
- Lactose Tolerance Tests - Lab Tests Online
- Lactose Intolerance information at Allergy UK