Tarheel Health Portal/Lactose Intolerance

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Lactose intolerance is a common disorder that has shown interesting patterns of concentration in certain areas of the world. While milk has become a common source of nutrients to humans, humans are the only mammals to possess the ability to consume milk after infancy. In fact, some populations of humans express the trait of lactase tolerance (the ability to break down lactose in adulthood) far more than others. For example, 96% of people in Asian countries, including China, are affected with lactose intolerance.[1]. However, some Northern European countries (such as Sweden), have low levels of lactose intolerance (near 1-2%). The difference in allelic frequencies of lactose intolerance in Asia versus Northern Europe may be due to the difference between the environments of the two regions drove evolution, causing natural selection to select for lactase tolerance in Scandinavia. This affected the gene pool, as well as the culture, of the regions.


Evolutionary Significance[edit]

The evolutionary histories of Europe and Asia are important when considering the discrepancy in allelic frequencies between Sweden and China. This depends on random genetic mutations that are introduced into the gene pool. The mutations are then inherited (through natural selection). However, it is important to consider the specific environmental conditions that conserved these random mutations in gene pools. Eastern Asia harbors more land mass and less water than Northern Europe, which is surrounded by the sea. Many portions of Eastern Asia are covered by dry grasslands and highlands.[2] Asia's arid and large landscape is vulnerable to large temperature swings. Northern Europe has a much more moderate climate.[2] The proximity of the ocean to Scandinavian countries leads to much more moisture in the atmosphere.

The dry and harsh climate in Asia may have made it rather impractical to raise large groups of livestock. Fertile grazing land was more scarce. Therefore, the herding of cattle was more prominent in Northern Europe. Additionally, cattle were simply more widely available in Europe than in Asia. This prevalence of dairy-producing mammals eventually allowed humans to develop a dependence on their product, which included not only skin (to make leather) and meat, but also milk.

Milk contains lactose lactose, a disaccharide. Humans utilize glucose, a monosaccharide, as their primary source of energy. Lactose is usually not the body’s first choice as an energy source, as the breakdown of the disaccharide into its monosaccharide components requires extra energy. However, lactose may have been consumed in addition to glucose for extra nutrition in Scandinavia. Extra nutrients may have been necessary due to the harsh climate; the presence of cattle may have provided an evolutionary advantage. Natural selection may have selected for lactase tolerance as a method for coping with the severely weather in Scandinavia.[1] Dairy products proved to be essential for the survival of populations in Northern Europe.[3]

Genetic Significance[edit]

The gene pools of the two different populations selected for different traits based on the environment. Different gene markers were identified when examining and comparing DNA from individuals of Northern European descent with individuals of Eastern Asian descent.[3] Gene markers are variable sequences found in DNA that can be used to differentiate between individuals, or even species. As such, these markers can be used to identify variations in the gene sequences between two different populations of humans. The gene markers suggested that groups probably had a different evolutionary history.

The difference in environmental and geographical location was most probably a factor that spurred the rise of the variant gene markers. The genetic differences between Europeans and Asians were initially few and random, as all humans share a common ancestor. However, as time progressed and humans further spread throughout Europe and Asia, these gene markers became more prominent and widespread. These markers were then inherited by offspring, due to natural selection by the respective environments.[4] These allelic shifts eventually began to stabilize.

Modern gene frequencies of the lactose intolerance trait were reached about 6000-9000 years ago.[5] This corresponds to the beginning of the Agricultural Revolution, which is when humans began to domesticate livestock. This correlates with the cultural differences between Northern Europeans (who placed a large emphasis on farming and livestock) and Eastern Asians (where the arid landscape tended to encourage a more nomadic lifestyle). One concern when examining the genetic basis of lactose intolerance was if lactase persistence existed prior to milk use.[1] This probably occurred as a random mutation that was then preserved in an environment that favored the trait- the environment was that of Scandinavia. Genetic traits that were selected by the environment were vital to forming the two different gene frequencies that today exists between Northern Europe and Eastern Asia. The trait of lactase intolerance would have provided no benefit to individuals in Asia, where cattle were not as prevalent, and where milk products were not a diet staple. Therefore, the environment did not favor this trait.

Benefits of Decoding the Disparity[edit]

Discovering the reason behind the disparity in lactose intolerance allelic frequencies in different populations could be a major milestone in understanding the path of human evolution throughout the centuries. This information would be valuable to evolutionary biologists, but also to the scientific community as a whole, because it may help us understand more about the theory of evolution. It would further assist geneticists in the task of sequencing genomes and understanding the purpose of specific genes related to lactose intolerance. This information may also be relevant to the public because it may aid them in understanding their own health and genetic background, especially in the near future, as using gene sequences to plan ahead for the future (for example, creating pedigrees to determine health risks) is becoming more common. Knowing one’s own genetic history may help one manage his or her own health, which may lead to breakthroughs in fields such as personalized medicine.

Lactose intolerance is a fairly relevant topic to UNC students. Even if a student doesn’t actually have this condition, it is highly likely that they know someone else who does suffer from it. Not much research about the genetic background of lactose intolerance specifically is actually conducted at UNC, although there is a fair amount of gastrointestinal research that occurs. However, this research could prove beneficial in that knowledge may help design more lactose-free food options, which could be introduced for public consumption. Carolina Dining Services offers several lactose-free options around campus. Students can also visit Campus Health, or even UNC Hospitals to determine whether they are lactose intolerant or not.

This map displays the frequency of lactose intolerance in the world.

Further Readings[edit]

WebMD Lactose Intolerance

Mayo Clinic- Lactose Intolerance

NPR

UNC Dining Services

UNC Campus Health

References[edit]