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 mammal 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 people around the world remain lactase non-persistent, and consequently are affected by 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.
Multiple studies indicate that the presence of the two phenotypes "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 suckling 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 their parents, who may be homozygous or at least heterozygous. 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 of lactase expression within an individual in response to changes in lactose consumption levels. 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 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.
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 selection 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. 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 than any other known human gene.
Lactase persistence in nonhumans
Lactose malabsorption 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. Such examples suggest that lactase persistence is a uniquely human phenomenon.
|Human group||Individuals examined||Intolerance (%)||Reference||Allele frequency|
|Europeans in Australia||160||4||||0.20|
|African American Children||N/A||45||||N/A|
|Saami (in Russia and Finland)||N/A||25–60||||N/A|
|North American Hispanics||N/A||53||||N/A|
|Mexican American Males||N/A||55||||N/A|
|Jews, Mizrahi (Iraq, Iran, etc.)||N/A||85||||N/A|
|Kazakhs from northwest Xinjiang||195||76.4|||
|Northeastern Han Chinese||248||92.3|||
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:
- 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 10%, 20%, and 25% in Chinese and Japanese
- 30–55%, 90%, and >90% in Mestizos of Peru
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.
The ability to digest lactose into adulthood (lactase persistence) would have only been useful to humans after the invention of animal husbandry 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 markers 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 mare 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 mare milk, although the fermentation process reduces the amount of lactose present.
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