Lactase persistence is something that we covered in class briefly, though it is something I found to be interesting. Even though most people are lactose intolerant, they still consume dairy anyways. The reason for that is because the consequences, such as bloating or diarrhea, are not severe enough for most people to turn away delicious ice cream. So the question is this, how exactly did we develop this feature? What caused our bodies to at some point learn to reject lactose? There are a few different explanations for these questions; those included are congenital deficiency, developmental deficiency, then primary and secondary deficiency.
In a population, lactose intolerance will develop at birth when both parents have the recessive gene. Babies that are born with this produce little to no lactase, and will likely struggle with this throughout their entire lifespan. Developmental deficiency sounds like someone develops lactose intolerance as they age, however it is a little different than that. It presents itself when infants are born premature and their digestive systems do not fully develop. In those cases, a person has a chance of improving this condition later in life.
Primary deficiency is the most common explanation, as it is caused by faulty genetics. Primary deficiency is less severe than when it is congenital because symptoms may not come about until adulthood. In most cases in history, adults could no longer digest lactose after childhood. Lastly, secondary deficiency is the result of another disease or condition. Those typically include surgeries or illnesses regarding the digestive system and intestines. The most interesting thing about secondary deficiency is that it is most common in children who live in the UK, or near that region. Secondary deficiency can be temporary or long term.
Something else we didn’t fully cover in class was the history of lactose intolerance. About 65% of the population, mainly those of Asian heritage, cannot digest the milk sugar lactose. The remaining 35% are mainly from European descent. The evolution required to achieve that 35% only took 20 thousand years, which means the feature must have been highly selective and favorable according to genetics and traits. The mutations that existed became more and more frequent around the agriculture communities. Evidence that suggests this dates back to as early as 6000 BC, the Neolithic period, which is right about the time that we know humans started farming.
To expand on what we learned in class, famine in Europe seemed to have played a big role in persistent genes. The reason for that is someone in good health who was lactose intolerant would get diarrhea, but if someone was malnourished, they would pass away. In the slow seasons for crops, it is suspected that milk drinking was more common. With people starting to drink more nutrient rich milk, they didn’t get sick as often. The less fortunate passed away, and therefore didn’t pass genes on. The ones who just suffered from a tummy ache were able to digest it a little better, which is why it became recurrent. Lactose intolerance used to be considered a disease or disorder, but now it is actually very common.
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