Lactase persistence
Encyclopedia
Lactase persistence is the continued activity of the enzyme
lactase
in adulthood. Since lactase's only function is the digestion of lactose
in milk
, in most mammalian species the activity of the enzyme is dramatically reduced after weaning
. However in some human populations lactase persistence has recently evolved as an adaptation to the consumption of non-human milk and dairy products beyond infancy. The majority of the global population remain lactase non-persistent, and subsequently exhibits varying degrees of lactose intolerance
as adults – though not all genetically lactase non-persistent individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase non-persistence allele
.
s “lactase persistent” and “lactase non-persistent (hypolactasia)” is genetically programmed, and that lactase persistence is not necessarily conditioned by the consumption of dairy products after the sulking period.
The lactase persistent phenotype involves high mRNA expression, high lactase activity and thus the ability to digest lactose. On the other hand, the lactase non-persistent phenotype involves low mRNA expression and low lactase activity. The enzyme lactase is encoded by the gene LCT.
Hypolactasia is known to be recessively and autosomally inherited, which means that an individual with the non-persistent phenotype is homozygous and received the two copies of the lactase gene from heterozygous parents. Also, only one active lactase gene is required to be lactase persistent, because lactase persistence is dominant to hypolactasia. Lactase persistence behaves as a dominant trait because half levels of lactase activity are sufficient to show significant digestion of lactose. Cis-acting transcriptional silence of the lactase gene is responsible for the hypolactasia phenotype. Furthermore, studies show that only 8 cases were found where the parents of a child with lactase persistence were both hypolactasic. While a variety of genetic, as well as nutritional factors determine lactase expression, there is no evidence for adaptive alteration in lactase expression. There are two distinct phenotypes of hypolactasia. Phenotype I is characterized by reduced synthesis of precursor LPH while the mechanism of decreased lactase activity in phenotype II is associated with ample precursor synthesis but reduced conversion of the protein to its mature molecular form.
The lactase enzyme has two active sites which break down lactose. The first is at Glu1273 and the second is at Glu1749, which separately break down lactose into two separate kinds of molecules.
Two mutations or SNP (single-nucleotide polymorphism) have been associated to lactase expression. It was found that C−13910 (C at position -13910 upstream of the gene LCT) and G−22018 (G at position -22018) are related to lactase non-persistence while the T−13910 and A−22018 are related to lactase persistence.
In addition, studies have demonstrated that the lactase gene has a higher expression when T−13910 and A−22018 are present and a lower expression when C−13910 and G−22018 are present. It was also proven that the position -13910 has an enhancer function on the lactase promoter (the promoter facilitates the transcription of the LCT gene). T−13910 is a greater enhancer than C−13910, so it is thought that this mutation is responsible for the differences in lactase expression although there is not enough evidence to prove that lactase persistence is only caused by C−13910→T−13910.
In one study involving a Finnish population, a CT SNP at –14 kb was found in all lactase persistent individuals and absent in all hypolactasia individuals. A second SNP (G-22 kbA) was concordant with phenotype in all but a few rare individuals. Since both SNPs are located in the same gene, this has led to the a genetic means of testing lactase expression in individuals.Outside of the Finnish study, a separate study also confirmed that the CT SNP at -14kb is an indicator of lactase persistence, with the exception of two individuals.
Joel Hirschhorn of Harvard Medical School discovered that lactase persistence was due to the presence of a haplotype composed of more than 1 million nucleotide base pairs, including the lactase gene. The presence of this gene is the cause of lactase persistence. Today, this haplotype can be found in 80% of Europeans and Americans of European ancestry. On the other hand, the percentage of the population who are lactase persistent in sub-Saharan Africa and Southeast Asia is very low. It is absent in the Bantu of South Africa and most Chinese populations. These geographical distributions strongly correlate with the spread of domesticated cattle. About 5,000 to 10,000 years ago, this haplotype came under very strong selective pressure. This period matches the rise of dairy farming. As dairy farming originated in Europe, they were exposed to increased lactose nutrition provided by dairy products, resulting in positive natural selection. This additional nutrition provided by the dairy was very important for survival in the recent history of Europe because the supply of fresh milk leads to the favoring of the lactase persistent trait. As dairy farming spread across the globe, after the separation of Europe-derived populations from Asian- and African-derived populations, and after the colonization of Europe, the strong positive selection occurred in a large region, leading to the global spread of lactase persistence.
is a component of evolution
by which a trait affects the chances of the survival of organisms, and consequently, the trait becomes more prevalent in the population over time. Especially in Europe
, the genetic variant -13,910*T has been strongly associated with lactase persistence and has been favored by natural selection in the past 10,000 years. Indeed, the consumption of lactose
has been proven to benefit humans through adulthood. For example, the 2009 British Women’s Heart and Health Study investigated the effects on women’s health of the alleles that coded for lactase persistence. Where the C allele
indicated lactase non-persistence and the T allele indicated lactase persistence, the study found that women that were homozygous for the C allele exhibited worse health than women with a C and a T allele and women with two T alleles. Women who were CC reported more hip and wrist fractures, more osteoporosis
, and more cataracts than the other groups. They also were on average 4–6 mm shorter than the other women, as well as slightly lighter in weight. In addition, factors such as metabolic traits, socioeconomic status, lifestyle, and fertility were found to be unrelated to the findings, and thus it can be concluded that the lactase persistence benefited the health of these women.
Evidence that lactase persistence has been favored by natural selection was found in a 2006 study published in Nature. The analysis process consisted of plotting extensive linkage disequilibrium
of ancestral and current alleles. The researchers were able to conclude that the score did in fact reflect positive selection of lactase persistence. It has also been reported that lactase persistence presents stronger selection pressure of any other known human gene.
is typical for adult mammals, and lactase persistence is a phenomenon likely linked to human interactions in the form of dairying.
Most mammals lose the ability to digest lactose once they are old enough to find their own source of nourishment away from their mother. After weaning
, or the transition from being breast-fed to consuming other types of food, their ability to produce lactase naturally diminishes as it is no longer needed. For example, in a study performed at Ohio State University
, it was found that in the time a piglet aged from five to eighteen days, it lost 67% of its lactose absorption ability. While nearly all humans can normally digest lactose for the first 5 to 7 years of their life, most mammals stop producing lactase much earlier. Cows can be weaned from their mothers milk anywhere from 6 months to a year of age. Lambs are regularly weaned at an age of about 16 weeks. Clearly, lactase persistence is a uniquely human phenomenon.
The statistical significance
of these figures vary 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:
Chinese
and Japanese
populations typically lose between 20 and 30 percent of their ability to digest lactose within three to four years of weaning. Some studies have found that most Japanese can consume 200 ml (8 fl oz) of milk without severe symptoms (Swagerty et al., 2002).
Ashkenazi Jews can keep 20–30 percent of their ability to digest lactose for many years. 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.
and the domestication of animal species that could provide a consistent source of milk. Hunter-gatherer
populations before the Neolithic revolution
were overwhelmingly lactose intolerant, as are modern hunter-gatherers. Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last ten thousand years. Therefore lactase persistence is often cited as an example of both recent human evolution and, as lactase persistence is a genetic trait but animal husbandry a cultural trait, gene-culture coevolution in the mutual human-animal symbiosis initiated with the advent of agriculture
.
Several genetic marker
s for lactase persistence have been identified, and these show that the allele has multiple origins in different parts of the world (i.e. it is an example of convergent evolution
). The version of the allele most common amongst Europeans is estimated to have risen to significant frequencies about 7,500 years ago in the central Balkans
and Central Europe
, a place and time approximately corresponding to the archaeological Linearbandkeramik
and Starčevo cultures. From there, it most probably spread eastwards as far as India. Likewise, one of the four alleles associated with lactase persistence in African population, is also probably of European origin. Since North Africans also possess this version of the allele it is probable that it actually originated earlier, in the Near East
, but that the earliest farmers did not have high levels of lactase persistence and, subsequently, did not consume significant amounts of unprocessed milk. Lactase persistence in Sub-Saharan Africa
almost certainly had a separate origin, probably more than one, and it is also likely that there was a separate origin associated with the domestication of the Arabian camel
. None of the mutations so far identified have been shown to be causal for the lactase persistence allele, and it is thought that there are several more yet to be discovered.
The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. In some East African ethnic groups lactase persistence has gone from negligible to near-ubiquitous frequencies in just three thousand years, suggesting a very strong selective pressure. But some models for the spread of lactase persistence in Europe attribute it primarily to a form of genetic drift
. Competing theories on why the ability to digest lactose might be selected for include nutritional benefits, milk as a water source in times of drought, and increased calcium absorption helping to prevent rickets
and osteomalacia
in low-light regions.
Roman authors recorded that the people of northern Europe, particularly Britain
and Germany
, drank unprocessed milk. This corresponds very closely with modern European distributions of lactose intolerance, where the people of Britain
, Germany
, and Scandinavia
have a high tolerance, and those of southern Europe, especially Italy
, have a lower tolerance.
In east Asia, historical sources also attest that the Chinese
did not consume milk, whereas the nomads who lived on the borders did. Again, this reflects modern distributions of intolerance. China is particularly notable as a place of poor tolerance, whereas in Mongolia
and the Asian steppes
horse milk is drunk regularly. This tolerance is thought to be advantageous, as the nomads do not settle down long enough to process mature cheese. Given that their prime source of income is generated through horses, to ignore their milk as a source of nourishment would be foolish. The nomads also make an alcoholic beverage, called Kumis
, from horse milk, although the fermentation
process reduces the amount of lactose present.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
lactase
Lactase
Lactase , a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers...
in adulthood. Since lactase's only function is the digestion of lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
in milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
, in most mammalian species the activity of the enzyme is dramatically reduced after weaning
Weaning
Weaning is the process of gradually introducing a mammal infant, either human or animal, to what will be its adult diet and withdrawing the supply of its mother's milk.The process takes place only in mammals, as only mammals produce milk...
. However in some human populations lactase persistence has recently evolved as an adaptation to the consumption of non-human milk and dairy products beyond infancy. The majority of the global population remain lactase non-persistent, and subsequently exhibits varying degrees of lactose intolerance
Lactose intolerance
Lactose intolerance, also called lactase deficiency or hypolactasia, is the inability to digest and metabolize lactose, a sugar found in milk...
as adults – though not all genetically lactase non-persistent individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase non-persistence allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
.
Genetics
Multiple studies indicate that the presence of the two phenotypePhenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
s “lactase persistent” and “lactase non-persistent (hypolactasia)” is genetically programmed, and that lactase persistence is not necessarily conditioned by the consumption of dairy products after the sulking period.
The lactase persistent phenotype involves high mRNA expression, high lactase activity and thus the ability to digest lactose. On the other hand, the lactase non-persistent phenotype involves low mRNA expression and low lactase activity. The enzyme lactase is encoded by the gene LCT.
Hypolactasia is known to be recessively and autosomally inherited, which means that an individual with the non-persistent phenotype is homozygous and received the two copies of the lactase gene from heterozygous parents. Also, only one active lactase gene is required to be lactase persistent, because lactase persistence is dominant to hypolactasia. Lactase persistence behaves as a dominant trait because half levels of lactase activity are sufficient to show significant digestion of lactose. Cis-acting transcriptional silence of the lactase gene is responsible for the hypolactasia phenotype. Furthermore, studies show that only 8 cases were found where the parents of a child with lactase persistence were both hypolactasic. While a variety of genetic, as well as nutritional factors determine lactase expression, there is no evidence for adaptive alteration in lactase expression. There are two distinct phenotypes of hypolactasia. Phenotype I is characterized by reduced synthesis of precursor LPH while the mechanism of decreased lactase activity in phenotype II is associated with ample precursor synthesis but reduced conversion of the protein to its mature molecular form.
The lactase enzyme has two active sites which break down lactose. The first is at Glu1273 and the second is at Glu1749, which separately break down lactose into two separate kinds of molecules.
Two mutations or SNP (single-nucleotide polymorphism) have been associated to lactase expression. It was found that C−13910 (C at position -13910 upstream of the gene LCT) and G−22018 (G at position -22018) are related to lactase non-persistence while the T−13910 and A−22018 are related to lactase persistence.
In addition, studies have demonstrated that the lactase gene has a higher expression when T−13910 and A−22018 are present and a lower expression when C−13910 and G−22018 are present. It was also proven that the position -13910 has an enhancer function on the lactase promoter (the promoter facilitates the transcription of the LCT gene). T−13910 is a greater enhancer than C−13910, so it is thought that this mutation is responsible for the differences in lactase expression although there is not enough evidence to prove that lactase persistence is only caused by C−13910→T−13910.
In one study involving a Finnish population, a CT SNP at –14 kb was found in all lactase persistent individuals and absent in all hypolactasia individuals. A second SNP (G-22 kbA) was concordant with phenotype in all but a few rare individuals. Since both SNPs are located in the same gene, this has led to the a genetic means of testing lactase expression in individuals.Outside of the Finnish study, a separate study also confirmed that the CT SNP at -14kb is an indicator of lactase persistence, with the exception of two individuals.
Global spread
Modern-day Lactose Intolerance in Humans by RegionJoel Hirschhorn of Harvard Medical School discovered that lactase persistence was due to the presence of a haplotype composed of more than 1 million nucleotide base pairs, including the lactase gene. The presence of this gene is the cause of lactase persistence. Today, this haplotype can be found in 80% of Europeans and Americans of European ancestry. On the other hand, the percentage of the population who are lactase persistent in sub-Saharan Africa and Southeast Asia is very low. It is absent in the Bantu of South Africa and most Chinese populations. These geographical distributions strongly correlate with the spread of domesticated cattle. About 5,000 to 10,000 years ago, this haplotype came under very strong selective pressure. This period matches the rise of dairy farming. As dairy farming originated in Europe, they were exposed to increased lactose nutrition provided by dairy products, resulting in positive natural selection. This additional nutrition provided by the dairy was very important for survival in the recent history of Europe because the supply of fresh milk leads to the favoring of the lactase persistent trait. As dairy farming spread across the globe, after the separation of Europe-derived populations from Asian- and African-derived populations, and after the colonization of Europe, the strong positive selection occurred in a large region, leading to the global spread of lactase persistence.
Evolutionary advantages
The question then arises: if lactase expression is not necessary after infancy, why has it persisted? Lactase expression persistence is largely due to natural selection. Natural selectionNatural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
is a component of evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
by which a trait affects the chances of the survival of organisms, and consequently, the trait becomes more prevalent in the population over time. Especially in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, the genetic variant -13,910*T has been strongly associated with lactase persistence and has been favored by natural selection in the past 10,000 years. Indeed, the consumption of lactose
Lactose
Lactose is a disaccharide sugar that is found most notably in milk and is formed from galactose and glucose. Lactose makes up around 2~8% of milk , although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from or , the Latin word for milk,...
has been proven to benefit humans through adulthood. For example, the 2009 British Women’s Heart and Health Study investigated the effects on women’s health of the alleles that coded for lactase persistence. Where the C allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
indicated lactase non-persistence and the T allele indicated lactase persistence, the study found that women that were homozygous for the C allele exhibited worse health than women with a C and a T allele and women with two T alleles. Women who were CC reported more hip and wrist fractures, more osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
, and more cataracts than the other groups. They also were on average 4–6 mm shorter than the other women, as well as slightly lighter in weight. In addition, factors such as metabolic traits, socioeconomic status, lifestyle, and fertility were found to be unrelated to the findings, and thus it can be concluded that the lactase persistence benefited the health of these women.
Evidence that lactase persistence has been favored by natural selection was found in a 2006 study published in Nature. The analysis process consisted of plotting extensive linkage disequilibrium
Disequilibrium
Disequilibrium is a term used to describe the lack of or opposite of an equilibrium.* in medicine:** Disequilibrium in cerebral palsy - a syndrome described by Hagberg & all** lack of equilibrioception...
of ancestral and current alleles. The researchers were able to conclude that the score did in fact reflect positive selection of lactase persistence. It has also been reported that lactase persistence presents stronger selection pressure of any other known human gene.
Lactase persistence in nonhumans
Lactose malabsorptionMalabsorption
Malabsorption is a state arising from abnormality in absorption of food nutrients across the gastrointestinal tract.Impairment can be of single or multiple nutrients depending on the abnormality...
is typical for adult mammals, and lactase persistence is a phenomenon likely linked to human interactions in the form of dairying.
Most mammals lose the ability to digest lactose once they are old enough to find their own source of nourishment away from their mother. After weaning
Weaning
Weaning is the process of gradually introducing a mammal infant, either human or animal, to what will be its adult diet and withdrawing the supply of its mother's milk.The process takes place only in mammals, as only mammals produce milk...
, or the transition from being breast-fed to consuming other types of food, their ability to produce lactase naturally diminishes as it is no longer needed. For example, in a study performed at Ohio State University
Ohio State University
The Ohio State University, commonly referred to as Ohio State, is a public research university located in Columbus, Ohio. It was originally founded in 1870 as a land-grant university and is currently the third largest university campus in the United States...
, it was found that in the time a piglet aged from five to eighteen days, it lost 67% of its lactose absorption ability. While nearly all humans can normally digest lactose for the first 5 to 7 years of their life, most mammals stop producing lactase much earlier. Cows can be weaned from their mothers milk anywhere from 6 months to a year of age. Lambs are regularly weaned at an age of about 16 weeks. Clearly, lactase persistence is a uniquely human phenomenon.
Distribution
| Human group | Individuals examined | Intolerance (%) | Reference | Allele frequency Allele frequency Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for... |
|---|---|---|---|---|
| Basques Basque people The Basques as an ethnic group, primarily inhabit an area traditionally known as the Basque Country , a region that is located around the western end of the Pyrenees on the coast of the Bay of Biscay and straddles parts of north-central Spain and south-western France.The Basques are known in the... |
85 | 0.3 | N/A | |
| Dutch Dutch people The Dutch people are an ethnic group native to the Netherlands. They share a common culture and speak the Dutch language. Dutch people and their descendants are found in migrant communities worldwide, notably in Suriname, Chile, Brazil, Canada, Australia, South Africa, New Zealand, and the United... |
N/A | 1 | N/A | |
| Danes | N/A | 4 | N/A | |
| Europeans in Australia Anglo-Celtic Australian Anglo-Celtic Australian are citizens of Australia with British and/or Irish ancestral origins.-Demography:From the beginning of the colonial era until the mid-20th century, the vast majority of settlers were British or Irish... |
160 | 4 | 0.20 | |
| Swedes | N/A | 5-7 | N/A | |
| British Demographics of the United Kingdom This article is about the demographic features of the population of the United Kingdom, including population density, ethnicity, education level, health of the populace, economic status, religious affiliations and other aspects of the population.... |
N/A | 5–15 | 0.184-0.302 | |
| Germans | 1805 | 6-23 | N/A | |
| Swiss | N/A | 10 | 0.316 | |
| European Americans | 245 | 12 | 0.346 | |
| Tuareg | N/A | 13 | N/A | |
| Finns | N/A | 14-23 | N/A | |
| Belorusians | N/A | 15 | N/A | |
| Russians Russians The Russian people are an East Slavic ethnic group native to Russia, speaking the Russian language and primarily living in Russia and neighboring countries.... |
N/A | 16 | N/A | |
| Ukrainians Ukrainians Ukrainians are an East Slavic ethnic group native to Ukraine, which is the sixth-largest nation in Europe. The Constitution of Ukraine applies the term 'Ukrainians' to all its citizens... |
N/A | 13 | N/A | |
| Austrians Austrians Austrians are a nation and ethnic group, consisting of the population of the Republic of Austria and its historical predecessor states who share a common Austrian culture and Austrian descent.... |
N/A | 15–20 | N/A | |
| Spaniards (non-Basque) Spanish people The Spanish are citizens of the Kingdom of Spain. Within Spain, there are also a number of vigorous nationalisms and regionalisms, reflecting the country's complex history.... |
N/A | 15 | N/A | |
| Northern French French people The French are a nation that share a common French culture and speak the French language as a mother tongue. Historically, the French population are descended from peoples of Celtic, Latin and Germanic origin, and are today a mixture of several ethnic groups... |
N/A | 17 | N/A | |
| Central Italian Italian people The Italian people are an ethnic group that share a common Italian culture, ancestry and speak the Italian language as a mother tongue. Within Italy, Italians are defined by citizenship, regardless of ancestry or country of residence , and are distinguished from people... s |
65 | 19 | N/A | |
| Indians Demographics of India The demographics of India are inclusive of the second most populous country in the world, with over 1.21 billion people , more than a sixth of the world's population. Already containing 17.5% of the world's population, India is projected to be the world's most populous country by 2025, surpassing... |
N/A | 20 | N/A | |
| African Tutsi Tutsi The Tutsi , or Abatutsi, are an ethnic group in Central Africa. Historically they were often referred to as the Watussi or Watusi. They are the second largest caste in Rwanda and Burundi, the other two being the Hutu and the Twa .... |
N/A | 20 | 0.447 | |
| African Fulani | N/A | 23 | 0.48 | |
| Bedouin Bedouin The Bedouin are a part of a predominantly desert-dwelling Arab ethnic group traditionally divided into tribes or clans, known in Arabic as ..-Etymology:... s |
N/A | 25 | N/A | |
| Portuguese Portuguese people The Portuguese are a nation and ethnic group native to the country of Portugal, in the west of the Iberian peninsula of south-west Europe. Their language is Portuguese, and Roman Catholicism is the predominant religion.... adults |
102 | 35 | N/A | |
| Southern Italian Italian people The Italian people are an ethnic group that share a common Italian culture, ancestry and speak the Italian language as a mother tongue. Within Italy, Italians are defined by citizenship, regardless of ancestry or country of residence , and are distinguished from people... s |
51 | 41 | N/A | |
| African American African American African Americans are citizens or residents of the United States who have at least partial ancestry from any of the native populations of Sub-Saharan Africa and are the direct descendants of enslaved Africans within the boundaries of the present United States... Children |
N/A | 45 | N/A | |
| Saami Sami people The Sami people, also spelled Sámi, or Saami, are the arctic indigenous people inhabiting Sápmi, which today encompasses parts of far northern Sweden, Norway, Finland, the Kola Peninsula of Russia, and the border area between south and middle Sweden and Norway. The Sámi are Europe’s northernmost... (in Russia Russia Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects... and Finland Finland Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside... ) |
N/A | 25–60 | N/A | |
| Northern Italian Italian people The Italian people are an ethnic group that share a common Italian culture, ancestry and speak the Italian language as a mother tongue. Within Italy, Italians are defined by citizenship, regardless of ancestry or country of residence , and are distinguished from people... s |
89 | 52 | N/A | |
| North American Hispanic Hispanic Hispanic is a term that originally denoted a relationship to Hispania, which is to say the Iberian Peninsula: Andorra, Gibraltar, Portugal and Spain. During the Modern Era, Hispanic sometimes takes on a more limited meaning, particularly in the United States, where the term means a person of ... s |
N/A | 53 | N/A | |
| Balkans Balkans The Balkans is a geopolitical and cultural region of southeastern Europe... |
N/A | 55 | N/A | |
| Mexican American Mexican American Mexican Americans are Americans of Mexican descent. As of July 2009, Mexican Americans make up 10.3% of the United States' population with over 31,689,000 Americans listed as of Mexican ancestry. Mexican Americans comprise 66% of all Hispanics and Latinos in the United States... Males |
N/A | 55 | N/A | |
| Cretans | N/A | 56 | N/A | |
| African Maasai | 21 | 62 | N/A | |
| Southern French French people The French are a nation that share a common French culture and speak the French language as a mother tongue. Historically, the French population are descended from peoples of Celtic, Latin and Germanic origin, and are today a mixture of several ethnic groups... |
N/A | 65 | N/A | |
| Greek Cypriots Greek Cypriots Greek Cypriots are the ethnic Greek population of Cyprus, forming the island's largest ethnolinguistic community at 77% of the population. Greek Cypriots are mostly members of the Church of Cyprus, an autocephalous Greek Orthodox Church within the wider communion of Orthodox Christianity... |
N/A | 66 | N/A | |
| Jews, Mizrahi (Iraq, Iran, etc.) Mizrahi Jews Mizrahi Jews or Mizrahiyim, , also referred to as Adot HaMizrach are Jews descended from the Jewish communities of the Middle East, North Africa and the Caucasus... |
N/A | 85 | N/A | |
| Jews, Ashkenazi Ashkenazi Jews Ashkenazi Jews, also known as Ashkenazic Jews or Ashkenazim , are the Jews descended from the medieval Jewish communities along the Rhine in Germany from Alsace in the south to the Rhineland in the north. Ashkenaz is the medieval Hebrew name for this region and thus for Germany... |
N/A | 68.8 | N/A | |
| Jews, Sephardic Sephardi Jews Sephardi Jews is a general term referring to the descendants of the Jews who lived in the Iberian Peninsula before their expulsion in the Spanish Inquisition. It can also refer to those who use a Sephardic style of liturgy or would otherwise define themselves in terms of the Jewish customs and... |
N/A | 62 | N/A | |
| Jews, Yemenite Yemenite Jews Yemenite Jews are those Jews who live, or whose recent ancestors lived, in Yemen . Between June 1949 and September 1950, the overwhelming majority of Yemen's Jewish population was transported to Israel in Operation Magic Carpet... |
N/A | 44 | N/A | |
| Sicilians | 100 | 71 | N/A | |
| South Americans | N/A | 65–75 | N/A | |
| Rural Mexicans | N/A | 73.8 | N/A | |
| African American African American African Americans are citizens or residents of the United States who have at least partial ancestry from any of the native populations of Sub-Saharan Africa and are the direct descendants of enslaved Africans within the boundaries of the present United States... s |
20 | 75 | 0.87 | |
| Kazakhs Kazakhs The Kazakhs are a Turkic people of the northern parts of Central Asia .... from northwest Xinjiang Xinjiang Xinjiang is an autonomous region of the People's Republic of China. It is the largest Chinese administrative division and spans over 1.6 million km2... |
195 | 76.4 | ||
| Lebanese Demographics of Lebanon This article is about the demographic features of the population of Lebanon, including population density, ethnicity, education level, health of the populace, economic status, religious affiliations and other aspects of the population.... |
75 | 78 | N/A | |
| Central Asia Central Asia Central Asia is a core region of the Asian continent from the Caspian Sea in the west, China in the east, Afghanistan in the south, and Russia in the north... ns |
N/A | 80 | N/A | |
| Alaskan Eskimo Alaska Natives Alaska Natives are the indigenous peoples of Alaska. They include: Aleut, Inuit, Tlingit, Haida, Tsimshian, Eyak, and a number of Northern Athabaskan cultures.-History:In 1912 the Alaska Native Brotherhood was founded... |
N/A | 80 | N/A | |
| Australian Aborigines | 44 | 85 | 0.922 | |
| Inner Mongolia Inner Mongolia Inner Mongolia is an autonomous region of the People's Republic of China, located in the northern region of the country. Inner Mongolia shares an international border with the countries of Mongolia and the Russian Federation... ns |
198 | 87.9 | ||
| African Bantu | 59 | 89 | 0.943 | |
| Asian American Asian American Asian Americans are Americans of Asian descent. The U.S. Census Bureau definition of Asians as "Asian” refers to a person having origins in any of the original peoples of the Far East, Southeast Asia, or the Indian subcontinent, including, for example, Cambodia, China, India, Indonesia, Japan,... s |
N/A | 90 | N/A | |
| Northeastern Han Chinese Han Chinese Han Chinese are an ethnic group native to China and are the largest single ethnic group in the world.Han Chinese constitute about 92% of the population of the People's Republic of China , 98% of the population of the Republic of China , 78% of the population of Singapore, and about 20% of the... |
248 | 92.3 | ||
| Chinese Chinese people The term Chinese people may refer to any of the following:*People with Han Chinese ethnicity .... |
71 | 95 | 0.964 | |
| Southeast Asians | N/A | 98 | N/A | |
| Thai Thai people The Thai people, or Siamese, are the main ethnic group of Thailand and are part of the larger Tai ethnolinguistic peoples found in Thailand and adjacent countries in Southeast Asia as well as southern China. Their language is the Thai language, which is classified as part of the Kradai family of... s |
134 | 98 | 0.99 | |
| Native American Native Americans in the United States Native Americans in the United States are the indigenous peoples in North America within the boundaries of the present-day continental United States, parts of Alaska, and the island state of Hawaii. They are composed of numerous, distinct tribes, states, and ethnic groups, many of which survive as... s |
24 | 100 | 1.00 |
The statistical significance
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
of these figures vary 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 AmericanMexican AmericanMexican Americans are Americans of Mexican descent. As of July 2009, Mexican Americans make up 10.3% of the United States' population with over 31,689,000 Americans listed as of Mexican ancestry. Mexican Americans comprise 66% of all Hispanics and Latinos in the United States...
s - 25%, 45%, and 60% in black South Africans
- approximately 10%, 20%, and 25% in Chinese and Japanese
- 30–55%, 90%, and >90% in MestizoMestizoMestizo is a term traditionally used in Latin America, Philippines and Spain for people of mixed European and Native American heritage or descent...
s of PeruPeruPeru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
Chinese
Chinese people
The term Chinese people may refer to any of the following:*People with Han Chinese ethnicity ....
and Japanese
Japanese people
The are an ethnic group originating in the Japanese archipelago and are the predominant ethnic group of Japan. Worldwide, approximately 130 million people are of Japanese descent; of these, approximately 127 million are residents of Japan. People of Japanese ancestry who live in other countries...
populations typically lose between 20 and 30 percent of their ability to digest lactose within three to four years of weaning. Some studies have found that most Japanese can consume 200 ml (8 fl oz) of milk without severe symptoms (Swagerty et al., 2002).
Ashkenazi Jews can keep 20–30 percent of their ability to digest lactose for many years. 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.
Evolutionary history
The ability to digest lactose into adulthood (lactase persistence) would have only been useful to humans after the invention of animal husbandryAnimal husbandry
Animal husbandry is the agricultural practice of breeding and raising livestock.- History :Animal husbandry has been practiced for thousands of years, since the first domestication of animals....
and the domestication of animal species that could provide a consistent source of milk. Hunter-gatherer
Hunter-gatherer
A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...
populations before the Neolithic revolution
Neolithic Revolution
The Neolithic Revolution was the first agricultural revolution. It was the transition from hunting and gathering to agriculture and settlement. Archaeological data indicates that various forms of plants and animal domestication evolved independently in 6 separate locations worldwide circa...
were overwhelmingly lactose intolerant, as are modern hunter-gatherers. Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last ten thousand years. Therefore lactase persistence is often cited as an example of both recent human evolution and, as lactase persistence is a genetic trait but animal husbandry a cultural trait, gene-culture coevolution in the mutual human-animal symbiosis initiated with the advent of agriculture
History of agriculture
Agriculture was developed at least 10,000 years ago, and it has undergone significant developments since the time of the earliest cultivation. The Fertile Crescent of Western Asia, Egypt, and India were sites of the earliest planned sowing and harvesting of plants that had previously been gathered...
.
Several genetic marker
Genetic marker
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify cells, individuals or species. It can be described as a variation that can be observed...
s for lactase persistence have been identified, and these show that the allele has multiple origins in different parts of the world (i.e. it is an example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
). The version of the allele most common amongst Europeans is estimated to have risen to significant frequencies about 7,500 years ago in the central Balkans
Balkans
The Balkans is a geopolitical and cultural region of southeastern Europe...
and Central Europe
Central Europe
Central Europe or alternatively Middle Europe is a region of the European continent lying between the variously defined areas of Eastern and Western Europe...
, a place and time approximately corresponding to the archaeological Linearbandkeramik
Linear Pottery culture
The Linear Pottery culture is a major archaeological horizon of the European Neolithic, flourishing ca. 5500–4500 BC.It is abbreviated as LBK , is also known as the Linear Band Ware, Linear Ware, Linear Ceramics or Incised Ware culture, and falls within the Danubian I culture of V...
and Starčevo cultures. From there, it most probably spread eastwards as far as India. Likewise, one of the four alleles associated with lactase persistence in African population, is also probably of European origin. Since North Africans also possess this version of the allele it is probable that it actually originated earlier, in the Near East
Near East
The Near East is a geographical term that covers different countries for geographers, archeologists, and historians, on the one hand, and for political scientists, economists, and journalists, on the other...
, but that the earliest farmers did not have high levels of lactase persistence and, subsequently, did not consume significant amounts of unprocessed milk. Lactase persistence in Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
almost certainly had a separate origin, probably more than one, and it is also likely that there was a separate origin associated with the domestication of the Arabian camel
Dromedary
The dromedary or Arabian camel is a large, even-toed ungulate with one hump on its back. Its native range is unclear, but it was probably the Arabian Peninsula. The domesticated form occurs widely in North Africa and the Middle East...
. None of the mutations so far identified have been shown to be causal for the lactase persistence allele, and it is thought that there are several more yet to be discovered.
The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. In some East African ethnic groups lactase persistence has gone from negligible to near-ubiquitous frequencies in just three thousand years, suggesting a very strong selective pressure. But some models for the spread of lactase persistence in Europe attribute it primarily to a form of genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
. Competing theories on why the ability to digest lactose might be selected for include nutritional benefits, milk as a water source in times of drought, and increased calcium absorption helping to prevent rickets
Rickets
Rickets is a softening of bones in children due to deficiency or impaired metabolism of vitamin D, magnesium , phosphorus or calcium, potentially leading to fractures and deformity. Rickets is among the most frequent childhood diseases in many developing countries...
and osteomalacia
Osteomalacia
Osteomalacia is the softening of the bones caused by defective bone mineralization secondary to inadequate amounts of available phosphorus and calcium, or because of overactive resorption of calcium from the bone as a result of hyperparathyroidism...
in low-light regions.
Roman authors recorded that the people of northern Europe, particularly Britain
Roman Britain
Roman Britain was the part of the island of Great Britain controlled by the Roman Empire from AD 43 until ca. AD 410.The Romans referred to the imperial province as Britannia, which eventually comprised all of the island of Great Britain south of the fluid frontier with Caledonia...
and Germany
Germanic peoples
The Germanic peoples are an Indo-European ethno-linguistic group of Northern European origin, identified by their use of the Indo-European Germanic languages which diversified out of Proto-Germanic during the Pre-Roman Iron Age.Originating about 1800 BCE from the Corded Ware Culture on the North...
, drank unprocessed milk. This corresponds very closely with modern European distributions of lactose intolerance, where the people of Britain
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, and Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
have a high tolerance, and those of southern Europe, especially Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, have a lower tolerance.
In east Asia, historical sources also attest that the Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
did not consume milk, whereas the nomads who lived on the borders did. Again, this reflects modern distributions of intolerance. China is particularly notable as a place of poor tolerance, whereas in Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
and the Asian steppes
Steppe
In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...
horse milk is drunk regularly. This tolerance is thought to be advantageous, as the nomads do not settle down long enough to process mature cheese. Given that their prime source of income is generated through horses, to ignore their milk as a source of nourishment would be foolish. The nomads also make an alcoholic beverage, called Kumis
Kumis
Kumis, also spelled kumiss or koumiss in English is a fermented dairy product traditionally made from mare's milk. The drink remains important to the peoples of the Central Asian steppes, of Turkic and Mongol origin: Bashkirs, Kazakhs, Kyrgyz, Yakuts, Mongols and Kalmyks...
, from horse milk, although the fermentation
Fermentation (food)
Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...
process reduces the amount of lactose present.