Lactose intolerance pathophysiology: Difference between revisions

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{{Inflammatory bowel disease}}
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== Lactase Biology ==
{{CMG}}; {{AE}} {{MA}}
==Overview==
It is thought that lactose intolerance is the result of [[lactose]] [[malabsorption]] caused by low levels of [[Small intestine|small intestinal]] [[lactase]]. [[Lactose]] is metabolized by the [[Intestine|intestinal]] [[lactase]] to [[galactose]] and [[glucose]] in villous [[Enterocyte|enterocytes]]. In the [[Colon (anatomy)|colon]], unabsorbed [[lactose]] is converted to hydrogen and [[short chain fatty acid]]<nowiki/>s such as [[acetate]], [[butyrate]] and [[propionate]] by [[Intestine|intestinal]] [[bacteria]] and creates symtoms of lactose intolerance. Lactose intolerance is inherited in an [[autosomal recessive]] pattern. Acquired primary lactase deficiency is associated with a CC [[genotype]] at -13.9 kb upstream of the [[lactase]] gene. On gross and microscopic pathology, there are no characteristic findings of lactose intolerance.


The gene is expressed and the enzyme synthesized if at least one of the two genes are present. Only when both gene expressions are affected is lactase enzyme synthesis reduced, which in turn reduces lactose digestion.<ref name=soy>[http://www.soynutrition.com/SoyHealth/SoyLactoseIntolerance.aspx] Soy Nutrition</ref>. Lactose tolerance(lactase persistence) is the [[dominant allele]]. Lactose intolerance is an [[autosomal recessive trait]].
==Pathophysiology==


The normal [[mammal]]ian condition is for the young of a species to experience reduced [[lactose]] (milk sugar) production at the end of the [[weaning]] period (a species-specific length of time). In non dairy consuming societies, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely. However, certain human populations have a [[mutation]] on [[chromosome]] 2 which results in a bypass of the common shutdown in lactase production, making it possible for members of these populations to continue consumption of fresh milk and other dairy products throughout their lives.
===Pathogenesis===
* [[Lactose]] is a [[disaccharide]] of [[glucose]] and [[galactose]] in ruminant and human milk.  
** Human milk contains ~7% [[lactose]]  
** Ruminant’s milk contains ~5% [[lactose]]  


Pathological lactose intolerance can occur due to [[Coeliac disease]], which damages the villi in the small intestine that produce lactase. This lactose intolerance is temporary. Lactose intolerance associated with coeliac disease ceases after the patient has been on a [[gluten-free diet]] long enough for the villi to recover.
*It is thought that lactose intolerance is the result of [[lactose]] [[malabsorption]] caused by low levels of [[Small intestine|small intestinal]] [[lactase]] ( lactase-phlorizin hydrolase, or LPH''')'''<ref name="pmid26404364">{{cite journal |vauthors=Silanikove N, Leitner G, Merin U |title=The Interrelationships between Lactose Intolerance and the Modern Dairy Industry: Global Perspectives in Evolutional and Historical Backgrounds |journal=Nutrients |volume=7 |issue=9 |pages=7312–31 |year=2015 |pmid=26404364 |pmc=4586535 |doi=10.3390/nu7095340 |url=}}</ref> 
*The most important causes of low level of [[Small intestine|small intestinal]] [[lactase]] are:
**[[Mucous membrane|Mucosal]] injury 
**Reduced genetic expression of the [[enzyme]] lactase-phlorizin hydrolase 
*[[Lactose]] is metabolized by [[Intestine|intestinal]] [[lactase]] to [[galactose]] and [[glucose]] in the villous [[Enterocyte|enterocytes]] and then uptaken by [[Sodium-glucose transport proteins|Na+/glucose cotransporter]] ([[Sodium-glucose transport proteins|SGLT1]]). <ref name="pmid1702075">{{cite journal |vauthors=Maiuri L, Raia V, Potter J, Swallow D, Ho MW, Fiocca R, Finzi G, Cornaggia M, Capella C, Quaroni A |title=Mosaic pattern of lactase expression by villous enterocytes in human adult-type hypolactasia |journal=Gastroenterology |volume=100 |issue=2 |pages=359–69 |year=1991 |pmid=1702075 |doi= |url=}}</ref><ref name="pmid8563765">{{cite journal |vauthors=Martín MG, Turk E, Lostao MP, Kerner C, Wright EM |title=Defects in Na+/glucose cotransporter (SGLT1) trafficking and function cause glucose-galactose malabsorption |journal=Nat. Genet. |volume=12 |issue=2 |pages=216–20 |year=1996 |pmid=8563765 |doi=10.1038/ng0296-216 |url=}}</ref>   
*In the [[Colon (anatomy)|colon]], unabsorbed [[lactose]] is converted to hydrogen and [[short chain fatty acid]]<nowiki/>s such as [[acetate]], [[butyrate]] and [[propionate]] by [[Intestine|intestinal]] bacteria and creates symptoms of lactose intolerance.    
*Milk drinkers have greater [[lactase]] activity compared to non-drinkers. <ref name="pmid1234085">{{cite journal |vauthors=Yoshida Y, Sasaki G, Goto S, Yanagiya S, Takashina K |title=Studies on the etiology of milk intolerance in Japanese adults |journal=Gastroenterol. Jpn. |volume=10 |issue=1 |pages=29–34 |year=1975 |pmid=1234085 |doi= |url=}}</ref>   


Certain people who report problems with consuming lactose are not actually lactose intolerant. In a study of 323 Sicilian adults, Carroccio et al. (1998) found only 4% were both lactose intolerant and lactose maldigesters, while 32.2% were lactose maldigesters but did not test as lactose intolerant. However, Burgio et al. (1984) found that 72% of 100 Sicilians were lactose intolerant in their study and 106 of 208 northern Italians (i.e., 51%) were lactose intolerant.
==Genetics==
*Lactose intolerance is transmitted in an [[autosomal recessive]] pattern.<ref name="pmid11788828">{{cite journal |vauthors=Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Järvelä I |title=Identification of a variant associated with adult-type hypolactasia |journal=Nat. Genet. |volume=30 |issue=2 |pages=233–7 |year=2002 |pmid=11788828 |doi=10.1038/ng826 |url=}}</ref>
*Persistence of intestinal [[lactase]] until adulthood is inherited as an [[autosomal dominant]] manner.<ref name="pmid3140651">{{cite journal |vauthors=Scrimshaw NS, Murray EB |title=The acceptability of milk and milk products in populations with a high prevalence of lactose intolerance |journal=Am. J. Clin. Nutr. |volume=48 |issue=4 Suppl |pages=1079–159 |year=1988 |pmid=3140651 |doi= |url=}}</ref>
*[[Gene|Genes]] involved in the pathogenesis of lactose intolerance include [[polymorphism]] of the [[MCM6]] ( minichromosome maintenance complex component 6), gene located upstream from the  gene lactase-phlorizin hydrolase (LPH) on the long arm (q) of [[chromosome]] 2 in region 21 (2q21). Lactase persistence is strongly related with the presence of the T allele of the [[Single nucleotide polymorphism|single nucleotide polymorphisms]] (SNP ) located at -13.9 kb upstream of the [[lactase]] gene. This allele regulates [[lactase]] [[Messenger RNA|mRNA]].<ref name="pmid12692047">{{cite journal |vauthors=Kuokkanen M, Enattah NS, Oksanen A, Savilahti E, Orpana A, Järvelä I |title=Transcriptional regulation of the lactase-phlorizin hydrolase gene by polymorphisms associated with adult-type hypolactasia |journal=Gut |volume=52 |issue=5 |pages=647–52 |year=2003 |pmid=12692047 |pmc=1773659 |doi= |url=}}</ref><ref name="pmid26550699">{{cite journal |vauthors=Buzás GM |title=[Lactose intolerance: past and present. Part 1] |language=Hungarian |journal=Orv Hetil |volume=156 |issue=38 |pages=1532–9 |year=2015 |pmid=26550699 |doi=10.1556/650.2015.30261 |url=}}</ref>
*Acquired primary lactase deficiency is associated with a CC [[genotype]] at -13.9 kb and lactase persistence is related to TT [[genotype]].<ref name="pmid15479673">{{cite journal |vauthors=Rasinperä H, Savilahti E, Enattah NS, Kuokkanen M, Tötterman N, Lindahl H, Järvelä I, Kolho KL |title=A genetic test which can be used to diagnose adult-type hypolactasia in children |journal=Gut |volume=53 |issue=11 |pages=1571–6 |year=2004 |pmid=15479673 |pmc=1774274 |doi=10.1136/gut.2004.040048 |url=}}</ref>


==Lactose intolerance by group==
==Gross Pathology==
[[Image:LacIntol-World2.png|left|thumb|200px|Lactose Intolerance by Region (African countries are only a rough guess)]]
*On gross pathology, there are no characteristic findings for lactose intolerance.


Most Japanese can consume 200 ml (8 fl oz) of milk without severe symptoms (McGee 2004; Swagerty et al, 2002).<ref name="Yoshida">[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1234085&dopt=Abstract ''Studies on the etiology of milk intolerance in Japanese adults'', Yoshida Y, Sasaki G, Goto S, Yanagiya S, Takashina K, Gastroenterol Jpn.;10(1):29–34, 1975]</ref>
==Microscopic Pathology==
*On microscopic histopathological analysis, there are no characteristic findings for lactose intolerance.


{| class="wikitable"
==References==
| '''Human groups'''
{{Reflist|2}}
| '''Individuals Examined'''
| '''Percent Intolerant'''
| '''[[Allele frequency]]'''
|-
|Dutch || N/A || 1%<ref name=AdvHG>''Genetics of lactose digestion in humans.'', Flatz, G. , Advances in Human Genetics, 1987</ref> || N/A
|-
|Swedes || N/A || 2%<ref name=SA>''Lactose and Lactase'', Norman Kretchmer, Scientific American, October, 1972</ref> || 0.14
|-
||Europeans in Australia ||160 || 4%<ref name=SA/> || 0.20
|-
|White people of Northern European and Scandinavian descent||N/A||5%<ref name=variant/><ref name=UCD/>||N/A
|-
|Danes || N/A || 5%<ref>[[Anne Charlotte Jäger]], [http://www.dskb.dk/index.jsp?id=7353&gr=7446 "Laktose-intolerans: Gentest for laktose-intolerans - hurtig og billig diagnostik"], [[DSKB-NYT]], no. 1, February 2006.</ref> || N/A
|-
|Demographics of the United Kingdom|British||5–15%<ref name=probiotic>[[Michael de Vrese]], Anna Stegelmann, Bernd Richter, Susanne Fenselau, Christiane Laue and Jürgen Schrezenmeir,[http://www.ajcn.org/cgi/content/full/73/2/421S "Probiotics—compensation for lactase insufficiency"], ''[[American Journal of Clinical Nutrition]]'', Vol. 73, No. 2, 421S-429s, February 2001.</ref> ||N/A
|-
|Swiss || N/A || 10%<ref name=SA/> || 0.316
|-
|White Americans || 245 || 12%<ref name=SA/> || 0.346
|-
|Tuareg || N/A || 13%<ref name=probiotic/> || N/A
|-
| Germans || N/A || 15%<ref name=probiotic/>  || N/A
|-
| Austrians || N/A || 15–20%<ref name=probiotic/>  || N/A
|-
|Eastern Slavs (Russians, Belarusians, Ukrainians) || N/A || 15%<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15991859&dopt=Abstract Prevalence of the lactase deficiency among the population of the northwestern region of Russia]</ref> || N/A
|-
|Northern French||N/A||17%<ref name=probiotic/> ||N/A
|-
|Finns || 134 || 18%<ref name=SA/> || 0.424
|-
|Central Italians||  65  || 19%<ref name=Italian/> ||N/A
|-
|Indian Children||N/A||20%<ref name=variant>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11788828&query_hl=14&itool=pubmed_docsum ''Identification of a variant associated with adult-type hypolactasia'', Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Jarvela I, Nat Genet. 2002 Feb;30(2):233–7]</ref><ref name=UCD>[http://nutrigenomics.ucdavis.edu/nutrigenomics/index.cfm?objectid=968814F6-65B3-C1E7-0C7007B71CC9959A ''Lactose Intolerance: The Molecular Explanation'', UC Davis Nutritional Genomics website]</ref>
||N/A
|-
|African Tutsi || N/A || 20%<ref name=SA/> || 0.447
|-
|African Fulani || N/A || 23%<ref name=SA/> || 0.48
|-
|Bedouins || N/A||25%<ref name=probiotic/> ||N/A
|-
|Northern Indians ||N/A||27%<ref name=Indian>''Lactose intolerance in North and South Indians'', Tandon RK, Joshi YK, Singh DS, Narendranathan M, Balakrishnan V, Lal K., Am J Clin Nutr 1981;35:943–6, 1981.</ref>||N/A
|-
|African American Children||N/A||45%<ref name=variant/>||N/A
|-
|Indian Adults||150||50%<ref name=variant/><ref name=UCD/><ref>[http://www.indianjgastro.com/article.asp?issn=0254-8860;year=2004;volume=23;issue=2;spage=78;epage=78;aulast=Rana ''Lactose malabsorption in apparently healthy adults in northern India, assessed using lactose hydrogen breath test'', Rana SV, Bhasin DK, Naik N, Indian Journal of Gastroenterology, Volume 23, Issue 2, p. 78, 2004]</ref>||N/A
|-
|Southern Italians|| 51  || 41%<ref name=Italian/> || N/A
|-
|Saami (in Russia and Finland) || N/A || 25–60%<ref>A. Kozlov, D. Lisitsyn, "Hypolactasia in Saami subpopulations of Russia and Finland", ''[[Anthropologischer Anzeiger]]'', 55(3-4):281–287, 1997.</ref> || N/A
|-
|Northern Italians|| 89  || 52%<ref name=Italian>[http://www.ajcn.org/cgi/content/abstract/45/4/748''Primary adult lactose malabsorption in Italy: regional differences in prevalence and relationship to lactose intolerance and milk consumption'', LT Cavalli-Sforza, A Strata, A Barone and L Cucurachi, American Journal of Clinical Nutrition, Vol 45, 748–754, 1987]</ref>  || N/A
|-
|North American Hispanics ||N/A||53%<ref name=probiotic/>  ||N/A
|-
|Balkans|| N/A||55%<ref name=probiotic/> ||N/A
|-
|Mexican American Males||N/A||55%<ref name=variant/><ref name=UCD/>||N/A
|-
|Cretans||N/A||56%<ref name=variant/>||N/A
|-
|African Maasai || 21 || 62%<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=581925&dopt=Abstract    ''Lactose malabsorption among Masai children of East Africa'', RT Jackson, MC Latham, Am J Clin Nutr. 1979 Apr;32(4):779–82.]</ref>
|| N/A
|-
| Southern French||N/A||65%<ref name=probiotic/> ||N/A
|-
|Greek Cypriots||N/A||66%<ref name=variant/><ref name=UCD/>||N/A
|-
|North American Jews||N/A||68.8%<ref name=variant/><ref name=UCD/>||N/A
|-
|Southern Indians ||N/A||70%<ref name=Indian/>||N/A
|-
|Sicilians ||100||71%<ref name=Burgio>[http://www.ajcn.org/cgi/reprint/39/1/100.pdf ''Prevalence of primary adult lactose malabsorption and awareness of milk intolerance in Italy'', G Roberto Burgio, Gebhard Flatz, Cristiana Barbera, Rosario Patan, Attilio Boner, Cinzia Cajozzo, and Sibylle D Flaiz, The American Journal of Clinical Nutrition 39: pp 100–104, January 1984.]</ref><ref>[http://www.jacn.org/cgi/content/full/19/suppl_2/165S ''Lactose Intolerance'', Tuula H. Vesa, Philippe Marteau, and Riitta Korpela, Journal of the American College of Nutrition, Vol. 19, No. 90002, 165S-175S (2000)]</ref>||N/A
|-
|South Americans||N/A||65–75%<ref name=probiotic/> ||N/A
|-
|Rural Mexicans||N/A||73.8%<ref name=variant/><ref name=UCD/>||N/A
|-
|African Americans || 20 || 75%<ref name=SA/> || 0.87
|-
|Kazakhs from northwest Xinjiang || 195 || 76.4% <ref name=China/>||
|-
|Lebanese||75||78%<ref>[http://www.ajcn.org/cgi/reprint/32/10/1994.pdf ''Lactose intolerance in the Lebanese population and in “Mediterranean lymphoma”'', Salah M. Nasrallah, The American Journal of Clinical Nutrition 32 , pp. 1994–1996, October, 1979.]</ref>||N/A
|-
|Central Asians||N/A||80%<ref name=probiotic/>  ||N/A
|-
|Alaskan Eskimo||N/A||80%<ref name=variant/><ref name=UCD/>||N/A
|-
|Australian Aborigines || 44 || 85%<ref name=SA/> || 0.922
|-
|Inner Mongolians||198||87.9%<ref name=China>[http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=6235167&dopt=Abstract''Prevalence of primary adult lactose malabsorption in three populations of northern China''], Wang YG, Yan YS, Xu JJ, Du RF, Flatz SD, Kühnau W, Flatz G., Hum Genet. 1984;67(1):103-6.</ref>||
|-
|African Bantu || 59 || 89%<ref name=SA/> || 0.943
|-
|Asian Americans||N/A||90%<ref name=variant/><ref name=UCD/>||N/A
|-
|Northeastern Han Chinese || 248 || 92.3%<ref name=China/>||
|-
|Chinese || 71 || 93%<ref name=SA/> || 0.964
|-
|Southeast Asians||N/A||98%<ref name=variant/><ref name=UCD/>||N/A
|-
|Thais || 134 || 98%<ref name=SA/> || 0.99
|-
|Native Americans || 24 || 100%<ref name=SA/> || 1.00
|}
 
The [[statistical significance]] varies greatly depending on number of people sampled.
 
Lactose intolerance levels also increase with age. At ages 2 - 3 yrs., 6 yrs., and 9 - 10 yrs., the amount of lactose intolerance is, respectively:
 
* 6% to 15% in white Americans and northern Europeans
* 18%, 30%, and 47% in Mexican Americans
* 25%, 45%, and 60% in black South Africans
* approximately 30%, 80%, and 85% in Chinese and Japanese
* 30–55%, 90%, and >90% in Mestizos of Peru<ref name=Sahi> ''Genetics and epidemiology of adult-type hypolactasia'', Sahi T., Scand J Gastroenterol Suppl ;29:202:7–20, 1994</ref><ref>[http://www.ajcn.org/cgi/content/abstract/29/1/19?ijkey=10c44e0ee3932cb3ee9d514dd65488c8284bc803&keytype2=tf_ipsecsha  ''Lactose malabsorption in Mexican-American children'', Woteki CE, Weser E, Young EA, Am J Clin Nutr;29:19–24, 1976]</ref>
 
Chinese and Japanese populations typically lose between 80 and 90 percent of their ability to digest lactose within three to four years of weaning.
 
Ashkenazi Jews can keep 20 - 30 percent of their ability to digest lactose for many years.<ref name=Sahi><ref>''Genetics of lactose digestion in humans'', Flatz G., Adv Hum Genet ;16:1 - 77, 1987</ref> <ref>''Genetics of lactase persistence and lactose intolerance'', Swallow DM., Annu Rev Genet ;37:197 - 219, 2003.</ref> Of the 10% of the Northern European population that develops lactose intolerance, the development of lactose intolerance is a gradual process spread out over as many as 20 years.<ref>[http://pmj.bmj.com/cgi/content/full/81/953/167 ''Systemic lactose intolerance: a new perspective on an old problem'', S B Matthews, J P Waud, A G Roberts and A K Campbell, Postgraduate Medical Journal;81:167 - 173, 2005.]</ref>


==References==
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mahda Alihashemi M.D. [2]

Overview

It is thought that lactose intolerance is the result of lactose malabsorption caused by low levels of small intestinal lactase. Lactose is metabolized by the intestinal lactase to galactose and glucose in villous enterocytes. In the colon, unabsorbed lactose is converted to hydrogen and short chain fatty acids such as acetate, butyrate and propionate by intestinal bacteria and creates symtoms of lactose intolerance. Lactose intolerance is inherited in an autosomal recessive pattern. Acquired primary lactase deficiency is associated with a CC genotype at -13.9 kb upstream of the lactase gene. On gross and microscopic pathology, there are no characteristic findings of lactose intolerance.

Pathophysiology

Pathogenesis

Genetics

  • Lactose intolerance is transmitted in an autosomal recessive pattern.[5]
  • Persistence of intestinal lactase until adulthood is inherited as an autosomal dominant manner.[6]
  • Genes involved in the pathogenesis of lactose intolerance include polymorphism of the MCM6 ( minichromosome maintenance complex component 6), gene located upstream from the gene lactase-phlorizin hydrolase (LPH) on the long arm (q) of chromosome 2 in region 21 (2q21). Lactase persistence is strongly related with the presence of the T allele of the single nucleotide polymorphisms (SNP ) located at -13.9 kb upstream of the lactase gene. This allele regulates lactase mRNA.[7][8]
  • Acquired primary lactase deficiency is associated with a CC genotype at -13.9 kb and lactase persistence is related to TT genotype.[9]

Gross Pathology

  • On gross pathology, there are no characteristic findings for lactose intolerance.

Microscopic Pathology

  • On microscopic histopathological analysis, there are no characteristic findings for lactose intolerance.

References

  1. ↑ Silanikove N, Leitner G, Merin U (2015). "The Interrelationships between Lactose Intolerance and the Modern Dairy Industry: Global Perspectives in Evolutional and Historical Backgrounds". Nutrients. 7 (9): 7312–31. doi:10.3390/nu7095340. PMC 4586535. PMID 26404364.
  2. ↑ Maiuri L, Raia V, Potter J, Swallow D, Ho MW, Fiocca R, Finzi G, Cornaggia M, Capella C, Quaroni A (1991). "Mosaic pattern of lactase expression by villous enterocytes in human adult-type hypolactasia". Gastroenterology. 100 (2): 359–69. PMID 1702075.
  3. ↑ Martín MG, Turk E, Lostao MP, Kerner C, Wright EM (1996). "Defects in Na+/glucose cotransporter (SGLT1) trafficking and function cause glucose-galactose malabsorption". Nat. Genet. 12 (2): 216–20. doi:10.1038/ng0296-216. PMID 8563765.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ Scrimshaw NS, Murray EB (1988). "The acceptability of milk and milk products in populations with a high prevalence of lactose intolerance". Am. J. Clin. Nutr. 48 (4 Suppl): 1079–159. PMID 3140651.
  7. ↑ Kuokkanen M, Enattah NS, Oksanen A, Savilahti E, Orpana A, Järvelä I (2003). "Transcriptional regulation of the lactase-phlorizin hydrolase gene by polymorphisms associated with adult-type hypolactasia". Gut. 52 (5): 647–52. PMC 1773659. PMID 12692047.
  8. ↑ Buzás GM (2015). "[Lactose intolerance: past and present. Part 1]". Orv Hetil (in Hungarian). 156 (38): 1532–9. doi:10.1556/650.2015.30261. PMID 26550699.
  9. ↑ Rasinperä H, Savilahti E, Enattah NS, Kuokkanen M, Tötterman N, Lindahl H, Järvelä I, Kolho KL (2004). "A genetic test which can be used to diagnose adult-type hypolactasia in children". Gut. 53 (11): 1571–6. doi:10.1136/gut.2004.040048. PMC 1774274. PMID 15479673.

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