Editing Lactose (section)

==Metabolism==
{{See also|Lactose intolerance|Lactase persistence}}

Infant [[mammal]]s nurse on their mothers to drink milk, which is rich in lactose. The [[Intestinal villus|intestinal villi]] secrete the [[enzyme]] [[lactase]] (β-D-galactosidase) to digest it. This enzyme cleaves the lactose molecule into its two subunits, the simple [[sugar]]s glucose and galactose, which can be absorbed. Since lactose occurs mostly in milk, in most mammals, the production of lactase gradually decreases with maturity due to weaning; the removal of lactose from the diet removes the metabolic pressure to continue to produce lactase for its digestion.<ref>{{Citation |last1=Balaresque |first1=P. |title=Chapter Nine - Human Phenotypic Diversity: An Evolutionary Perspective |date=2016-01-01 |journal=Current Topics in Developmental Biology |volume=119 |pages=349–390 |editor-last=Orgogozo |editor-first=Virginie |url=https://linkinghub.elsevier.com/retrieve/pii/S0070215316300011 |access-date=2025-02-15 |series=Genes and Evolution |publisher=Academic Press |doi=10.1016/bs.ctdb.2016.02.001 |last2=King |first2=T. E.|pmid=27282030 }}</ref><ref>{{Cite journal |last1=Gerbault |first1=Pascale |last2=Liebert |first2=Anke |last3=Itan |first3=Yuval |last4=Powell |first4=Adam |last5=Currat |first5=Mathias |last6=Burger |first6=Joachim |last7=Swallow |first7=Dallas M. |last8=Thomas |first8=Mark G. |date=2011-03-27 |title=Evolution of lactase persistence: an example of human niche construction |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=366 |issue=1566 |pages=863–877 |doi=10.1098/rstb.2010.0268 |issn=0962-8436 |pmc=3048992 |pmid=21320900}}</ref>

Many people with ancestry in [[Europe]], [[West Asia]], [[South Asia]], the [[Sahel]] belt in [[West Africa]], [[East Africa]] and a few other parts of [[Central Africa]] maintain lactase production into adulthood due to selection for genes that continue lactase production. In many of these areas, milk from mammals such as [[cattle]], [[goats]], and [[sheep]] is used as a large source of food. It was in these regions that genes for lifelong lactase production first [[evolution|evolved]].<ref>{{Cite journal |last1=Itan |first1=Yuval |last2=Powell |first2=Adam |last3=Beaumont |first3=Mark A. |last4=Burger |first4=Joachim |last5=Thomas |first5=Mark G. |date=2009-08-28 |editor-last=Tanaka |editor-first=Mark M. |title=The Origins of Lactase Persistence in Europe |journal=PLOS Computational Biology |language=en |volume=5 |issue=8 |pages=e1000491 |doi=10.1371/journal.pcbi.1000491 |doi-access=free |issn=1553-7358 |pmc=2722739 |pmid=19714206|bibcode=2009PLSCB...5E0491I }}</ref> The genes of adult lactose tolerance have evolved independently in various ethnic groups.<ref>{{citation | first = Nicholas | last = Wade | title= Study Detects Recent Instance of Human Evolution | date = 2006-12-10 | newspaper= New York Times | url= https://www.nytimes.com/2006/12/10/science/10cnd-evolve.html?}}.</ref> By descent, more than 70% of western Europeans can digest lactose as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania.<ref>{{Citation |last= Ridley |first= Matt |title= Genome |publisher= [[HarperCollins]] |year= 1999 |page= 193 |isbn= 978-0-06-089408-5}}.</ref> In people who are lactose intolerant, lactose is not broken down and provides food for gas-producing [[gut flora]], which can lead to diarrhea, bloating, flatulence, and other gastrointestinal symptoms.