Lactase persistence in humans is very simply the continued production of lactase, the enzyme that allows us to metabolize the lactose that we get from eating dairy, in our intestines. The allele for it however is rare amongst certain populations of the world. China for instance has approximately only 1% of its population continues to create lactase throughout their lives versus the 90% of the Finnish, Danish, and Tutsi populations that all continues to create lactase. This discrepancy between the different populations of the world makes lactase persistence an interesting problem. How can different populations so far away, Northern Europe versus Eastern Africa, have similar lactase persistence frequencies despite the huge racial, geographic, historic, and cultural differences? Why is it even there, does it provide an advantage to survival? To find this out, the rates of lactase persistence across the globe must be considered along with the cultural and historical backgrounds of the various populations to see if there are any proximate causes for the isolated nature of lactase persistence or is there an ultimate point for humans to be able to digest dairy products after being weaned.
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Human culture has for approximately 10,000 years been domesticating and keeping sheep, goats, and cattle for our own personal usage. We know that mammals of the world in their infancy have the ability to digest lactose; the primary carbohydrate found in milk and associated dairy products. Upon weaning, most animals lose their ability to produce lactase and consequently are unable to further digest lactose. Humans are different. We on average only have 65% of our population lose the ability to produce lactase, the key enzyme responsible for our ability to digest lactose. For them, lactose causes cramps, stomach bloating, gas, and general abdominal discomfort. The other 35% of the population then has lactose as a legitimate source of glucose.
On a genetic level we know that carrying theÂ Tâˆ’13910Â variant 14 kb upstream of lactase (LCT)Â gene is correlative to all populations of the world that have lactase persistence. (Enattah, 2008) This however is not the only genetic component in deciding lactase persistence. It has been found that in a wider scope, inclusive of the C/Tâˆ’13910Â variant allele, the DNA contains enhancer elements with binding sites for several transcription factorsÂ such as Oct-1 and GATA-6 (region from âˆ’13909 toÂ âˆ’13934), HNF4Î± and Fox/HNF3Î± (region âˆ’13857 toÂ âˆ’13817), andÂ Cdx-2(region âˆ’14022 to âˆ’14032). (Enattah, 2008) All this has an effect on the lactase regulation processes of the genes in the intestines.
In order to find out what genetic makeups create individuals with lactase persistence, first populations must be found that are still producing lactase. Several non-invasive measures and examinations can be administered to find this out. One popular method is for a surveyor to simply give a subject a load of lactose and see what happens. If the subject is lactase persistent, then there will be a marked increase in their blood sugar levels while subjects without the allele for lactase persistence will become sick. The lactose will pass all the way to their colon where it will be fermented by gut bacteria into fatty acids, hydrogen, and a variety of other gases. Hydrogen here can pass easily into the blood stream and can be easily detected with a hydrogen analyzer. (Itan, 2010) However this and other ways of testing for lactase persistence are given to error, so any study to find the percentage of a population with lactase persistence from a sample is somewhat inaccurate.
Recent notice in the nucleotide changes associated with lactase persistence has also made it apparent that there are multiple separately found alleles that correspond to lactase persistence. These different alleles each are part of the populations of the world that have a strong inclination towards lactase persistence like populations in Europe and certain Middle Eastern and African populations. So far four strong candidates for alleles that cause lactase persistence in populations, but the number of populations tested has been small so it is very likely that we as of yet do not know all of the alleles responsible for lactase persistence in humans all over the globe.
Yuval Itan and fellow researchers tried to overcome this lack of continuity in information by applying a surface interpolation of the data categories available to them, categories like genetic, phenotypic, etcâ€¦, and then put the aforementioned statistical procedure over a grid covering the worlds land mass. In this way regions of relatively dense or particularly sparse lactase persistence in the populations there can be singled out for future study. The results of their cartographic study gave them an understanding of the limited nature of the so far completed studies of lactase persistence in human populations around the world. The lack of data only means here that we cannot yet sufficiently explain the lactase persistence phenotype frequency in Eastern Africa and other regions of the world. (Itan, 2010)
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There are two major hypotheses as to why lactase persistence persists in human populations of Northern Europe and part of Africa and the Middle East. One is the Gene-Culture Coevolution hypothesis which states that there is a nutritional advantage for pastoral populations to be able to digest lactose and the other is the Calcium Assimilation which posits that people who carry the allele(s), LCT*P, are favored in high-altitude regions of the world where proper Vitamin D synthesis in the skin does not occur properly. (Gerbault, 2009) In a study to see which to see which hypothesis was more efficacious, Gerbault and others analyze data on lactase persistence and attempted to find a correlation between LCT*P variations, geography, and the variation of other independent polymorphisms at a variety of geographic locations in the world to try and see if whether LCT*P patterns were an oddity of genetic drift or whether they were formed by population demographics and histories of an area. Gerbault found in his study that there was a high and significant correlation between pastoralism and the LCT*P frequencies in the African populations included in the analysis and the European populations the gradient LCT*P frequencies could be explained by positive selection on the allele, by population history, or both. This finding fits into the Gene-Culture Coevolution hypothesis and is also in agreement to another finding that the frequencies of LCT*P in the Northwestern part of Europe were too high to be explained by genetic drift, hence a novel environment combined with a culture centered around surviving in a pastoral setting makes lactase persistence a useful allele to have as dominant in that population. (Gerbault, 2009)
However, the Genetic-Culture Coevolution hypothesis works best with explaining the frequencies of lactase persistence in Africa. Northern Europe is best explained through a combination of the Calcium Assimilation hypothesis and demographic effects. The study points out that in Northern Europe, that selective pressure for lactase persistence was higher than it was in the Southern populations, but the full reason as to why that is has yet to surface. (Gerbault, 2009) Essentially here, both the coevolution of the allele frequency and the cultural growth of pastoral origins reinforced one another and selected for individuals who had lactase persistence and the in the North, people evolved a way to make up for the interrupted Vitamin D synthesis in their skin and a as of yet explained reason that the demographics of the population selected for a higher allele frequency of lactase persistence.
Historically Northern to Central Europeans started to have increased lactase persistence within approximately the last 20,000 years. The trait is thought to have given an advantage to those individuals who had consumed unfermented dairy on a regular basis. Though, this theory opens up discussion for two theories the first is the Cultural-Historical hypothesis, wherein some believe that lactase persistence alleles were rare until pastoralism was invented and people started keeping sheep, cattle, and other milk producing livestock that added to their diets and natural selection for individuals with the correct alleles took over from there. The other hypothesis is Reverse Cause hypothesis, where subscribers to this train of thought stipulate that it was pastoralism and dairy collection that was adapted to a population who was already pre-adapted to having the lactase persistence allele. (Burger, 2007)
In the Burger article, DNA was obtained from ancient skeletons of individuals from the Mesolithic and the Neolithic. From there they sequenced the mitochondrial DNA and found through short tandem repeat genotypes across the eight Neolithic remains and the one Mesolithic remains, that upon searching for the allele most common to lactase persistence, it was absent. This means that the Cultural-Historical hypothesis is correct and it was only at a later date that a relatively strong and recent selection process amongst that population brought forth the dominance of the lactase persistence allele. (Burger, 2007) This new information along with evidence based off of the decay of long-range haplotypes and the variations in the microsatellites, the rough age of the âˆ’13.910*T allele in Europe is approximately 2,188 and 20,650 years ago and 7,450 and 12,300 years ago respectively these dates fit into the historical time scale where there is evidence of the domestication of cattle in Europe.
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Europeans have been shown through studies to have seven polymorphisms in the lactase gene that are highly associated to only 3 common haplotypes. The frequencies of these three haplotypes in the Harvey article were shown to exist in Caucasians from North and Southern Europe as well as the Indian-sub continent. In the Southern regions, the latter two types of haplotypes, labeled simply B and C, were far more common the former haplotype A. Studies conducted by Harvey show allelic association to either lactase persistence or nonpersistence. Eleven unassociated individuals were tested for their RNA transcripts to determine their haplotype and it was found that haplotype A was almost always associated to the highly expressed allele of lactase persistence. (Harvey, 1998) This evidence points out the high prevalence of haplotype A amongst Northern Europeans along with their predominant lactase persistence allele. The two, haplotype A and the lactase persistence allele most often occur together, but this is not a guarantee. Haplotype B can also result in lactase persistence in a few small instances and even a case of homozygosity with the C haplotype has resulted in lactase persistence. Overall, haplotype A is the best predictor of lactase persistence, but not always. (Harvey, 1998)
African populations offer a different problem than their European counterparts in explaining the prevalence of lactase persistence amongst themselves. The -13910*T allele which is extremely prevalent in the European populations is sparse amongst the African milk drinkers. Ingram and her team conducted a series of gene resequences which revealed three new SNPs close to the original -13910*T allele of which two were within the Oct-1 binding site. The most common of the three, -13915*G, is curiously associated with lactose tolerance rather than lactase persistence. -13915*G breaks down Oct-1 binding and instead of becoming involved with lactase persistence. This opposite approach that garners the same ends as the original allele shows the complexity of the polymorphism and of how it has adapted over time to meet the needs selected for humans in that area. (Ingram, 2006)
The Sudan study conducted by Bayoumi in 2008 found an interesting misnomer in the dairying peoples of Africa. In the Sudan where there is a long history of nomadic pastoralism, the rate of individuals who were not lactase persistent or lactose tolerant was high. It was found that in all subgroups, excluding a group called the Bedja, an allele for hypolactasia was showing up more than the allele for lactase persistence. Among the groups, one common thread connected them together as a reason for their falling lactase persistence frequencies and rising hypolactasia, settlement. The once nomadic peoples were beginning to settle into permanent locations and to start agricultural endeavors. Proof of this lied with already settled peoples like the residential agriculturalist of the Nile valley, who had been settled for a longtime and not surprisingly had some of the highest hypolactasia rates and lowest lactase persistence rates. This cultural gradient of allele frequencies mixed with the racial gradient of allele frequencies to create a highly complex frequency distribution over the population of Sudan whose details are nearly impossible to ascertain due to the size of the initial sample. However, this large coverage of a number of different frequencies presents a prime research spot in the future to observe the evolution of lactase phenotypes over time. (Bayoumi, 2008)
The discrepancy in explaining lactase persistence between African and Middle Eastern populations and those of Northern Europe seems to stem from a difference of regional history despite the nutritional similarities, i.e. - dairy. The dietary needs of different races might leave open a possibility of a bias in nutritional needs as set forth by the federal government. Dietary guidelines as set by the government, best represented with the well-known "Food Pyramid", it is recommended for the average person to consume 2-3 servings of dairy a day. However, lactase nonpersistence tends to occur in disproportionately high rates amongst African-, Asian-, Hispanic-, and Native Americans than it does in Caucasians which makes them less likely to properly digest the lactose in the food that has been suggested to them. Instead, calcium can be obtained from other sources that do not contain lactose and minorities here actually suffer less from osteoporosis than Caucasians do. So, minorities have even less incentive to consume dairy when it has little to no positive effect on their health, and as such an alteration of the food pyramid to make the dairy products optional in view of other calcium sources might be helpful to the public health. (Bertron, 1999)
This article, while not directly telling of the source, proximate or ultimate, of lactase persistence, it does however tell of the application of an understanding of its source can affect policy change in law and in understanding of basic nutritional need. Current understanding of nutrition disregards the ability of different populations to actually digest the food that they are supposed to eat. Dairy here is a matter of contention over lactase persistence and the usefulness of actually ingesting dairy versus the majority of people who are unable to reap the benefits of the carbohydrate lactose.
At a proximate level, lactase persistence is due, as far as we can tell, to the dominance of haplotype A with regards to where the allele for lactase production is located along with geographic and cultural specificities that have allowed for the dominance of lactase persistence to continue.
At an ultimate level the Northern Europe region can be best explained with the Calcium Assimilation hypothesis which explains the prevalence through assuming that the human body adapts to a novel environment in which normal Vitamin D synthesis cannot occur in the skin and is then forced to find and cope with another source of Vitamin D, dairy here, to make up for the lost production of an important vitamin. Along with that trait becoming the most successful trait to survive in the North, it then became the dominant trait passed down to the next generation, furthering itself over time not from genetic drift but from the adaptation to an environment. African and Middle Eastern lactase persistence is best explained with the Gene-Culture Coevolution hypothesis which sees the persistence of lactase as an adaptation to a food source that is readily available to a continuously pastoral people who lived in a region that lacks a variety of other sources of sustenance. The environment in Eastern Africa and into the Middle East are extreme and milk from domesticated cows are one of the few truly reliable sources of calories and nutrients, as such individuals who could ingest and properly digest the dairy would carriers of a haplotype that confers the highly advantageous lactase persistence onto their progeny.
Though in all of this, it is important to note that the amount of research and field work that has been completed in this area of study so far, cannot warrant any definite answers to the exacting proximate or ultimate causes of lactase persistence. The above articles are based off of limited data that has been collected so far, but as of yet we still don't know if we have found all of the haplotypes associated with lactase persistence in the world. There may still be some population in the world that has a completely different genetic approach to lactase persistence than neither the Northern Europeans nor the African/Middle Easterners possess and yet they are lactose tolerant.