Have you ever drunk a cup of milk and immediately felt your stomach gurgle? Probably not, given that we students at MGCNLCA have gotten used to drinking milk tea every day. However, don’t be embarrassed if the answer is a nervous “yes.” It is actually likely to be so since 80-100% of East and Southeast Asians share the trait that causes this: lactose intolerance. Not only that, but in the wider tropical and hot regions of the world, lactose intolerance rates are similarly high. But why is it that something so inconvenient even exists in such a wide portion of the population? What causes this inconvenience in the first place? Let’s start by answering the latter.
Lactose intolerance, also known as lactase deficiency, is the inability to digest lactose, which is a sugar found in milk. This is caused by the lack of lactase secreted by the small intestine during digestion. Lactase is the enzyme that breaks down lactose, and if the body cannot do this function, you can experience a general feeling of sickness, diarrhea, flatulence, stomach bloating, stomach cramps and pains, and stomach rumbling. However, the degree of lactose intolerance can vary amongst different people, with some being able to consume more milk than others before feeling the effects, and with reactions ranging from negligible to severe. It can even be asymptomatic due to bacteria in the small intestine breaking lactose down instead of lactase.
Primary lactose intolerance is what we will be talking about since it is what occurs in most lactose-intolerant people around the world. In this type, the gene that tells cells how to create lactase expresses itself less over time after the age of two. In other words, it becomes less active. This type of lactase deficiency is not abnormal in humans, and in fact, it is the norm. The ability to digest lactose is simply not needed once a baby is weaned off of its mother’s milk. As a result, this now instead becomes a question of why Europeans and those living in less tropical areas have mutations leading to lactose persistence (the ability to digest lactose throughout life).
When migrating out of the Fertile Crescent, Europeans found themselves in a cold land with fewer land animals for meat and with better conditions to preserve milk and herd cattle. As such, they satiated their protein needs with milk. Those who developed lactose persistence reproduced and passed on their genes more often than those who did not because the former had an inherent advantage in having an easier and healthier life (not as many stomach cramps and diarrhea). As such, Europeans would have much higher rates of lactose persistence. On the other hand, upon migration out of Mesopotamia, Asians could not preserve milk as well in their hotter environment and could just easily derive protein from soy.
Not only that, but the calcium assimilation hypothesis is another piece of evidence. Vitamin D allows the body to absorb calcium, which is important for bone growth and strength and is mostly obtained by the body from the Sun’s ultraviolet rays. In tropical countries, this is not a problem, but in temperate regions like Europe, it is. The hypothesis states that the Europeans instead relied on calcium and small amounts of Vitamin D in milk to keep their bones strong. This further magnifies the need for lactose persistence in northern latitudes.
The reasons above make up the bulk of the evidence relating to lactose intolerance and the environment. There were some groups that did consume dairy regularly though, like the Mongols, Hindus, and the Dinka, Nuer, and Herero tribes found in Africa. However, they ate fermented cheeses and yogurts, which had less lactose than fresh milk. Because of this, there was less of a need to develop lactose persistence. Overall, Europeans lived in a cold and barren environment which catered to and required a diet of dairy, and so adapted to this need over time.
In this world, there are many things that are influenced simply by where you were born. Whether it be your beliefs, language, behaviors, or even the way your body ingests milk. Seemingly arbitrary and random facets of our biology, like lactase production, are actually tailor-fit to our environment. And, if that environment entails a lifestyle of consuming unhealthy amounts of milk tea, the body will adapt to it as well through the generations (you’ll probably get a heart attack first though).
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Photo:
A lactose intolerance map of the world clearly indicates a trend amongst tropical and temperate regions
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