Why are so many people in the world unable to digest lactose?
In: Biology
Evolutionary pressure. Once you’re weaned off of your mother’s milk, there’s really not a lot of need for dairy products. These products are very recent in human history, as we didn’t really have access to milk, cheese, etc. before the domestication of cows. From a biological perspective, it wouldn’t have made sense to keep producing lactase (the enzyme that digests lactose) if you weren’t eating anything WITH lactose.
You can see this influence based on lactose intolerance by region. Europe has significantly lower rates of lactose intolerance, most likely due to earlier attempts at widespread dairy farming. Evolution would dictate that having the ability to digest milk into adulthood would have an evolutionary benefit in that situation.
Okay so like, humans back in the day really only drank milk as babies. We evolved to be able to digest lactose as babies and then as we grow up, we stop drinking mom’s milk and we stop needing to spend precious resources making lactose digesting enzymes that we didn’t use. Things were real rough before agriculture really took off.
In cultures where humans *raised dairy livestock* (like European cultures), you see really low levels of lactose intolerance. This is because children, teenagers, and adults who could eat milk had a new food source to them! Cows (goats, etc) can turn water and grass into nutritious milk! So people who could eat dairy products lived and those who couldn’t died from a mixture of having less food at their disposal and wasting all their good nutrients when they experience vomiting and diarrhea from their force fed dairy products.
In cultures where humans *didn’t raise dairy livestock*, we see much higher levels of lactose intolerance. This is because their ancestors were not reliant on dairy as a nutritional source and didn’t need to keep the lactase genes running after childhood, so they continued as is. After all, if it ain’t broke, nature ain’t gonna fix it.
Most mammals cant once they reach adulthood, because the only milk they tend to drink comes from their mothers. Once they don’t need to breastfeed anymore, they don’t need to digest lactose anymore. It’s actually super weird that so many adults _can_ digest lactose.
So gran and poppa came from the old country. In the old country they used cows for many things, yes yes uncle explitive existed, but thats VERY not ok. One of those things was milk, you like mac and cheese right? Milk comes from cows and we use that to make dairy products. Because we have been eating cow juice for a long time our bodies became friends and thats why we arent lactose intolerant. No your friends who can’t eat dairy are not goblins from middle earth. Their gran and poppa just didn’t cheese when they were kids. What? Sure I think the popsicle tree should exist too. Remember to brush your back teeth. Where did your brother go!!?
Adult version: Cattle were domesticated draft animals in certain areas. Their milk was abundant in fat and protein, which are vital nutrients. The areas that had cattle used the cattle juice, it made them poo. After many years they got a fun happy friend in their tummy and didn’t poo anymore. The areas that didn’t have cattle did not consume cow juice and didn’t get the happy friend.
Why is ELI5 a cliff notes sub now?
Lactose *tolerance* in adulthood is a beneficial mutation that some humans have. Outside of humans, other animals would never have the opportunity to drink milk beyond their infancy; it’s not like dogs can keep nursing cows around to get some milk from.
We are the only animal that will drink milk from other animals willingly post maturity.
Biologically it makes no sense, Because milk is ONLY from female animal’s mammary gland, and specifically female animals that are pregnant. Its not a stable widespread food source. Pre civilization, once you wean off milk, the most common food you’ll run into is meat, fish, vegetable, and fruits, there was no point in keeping the ability to process milk.
Everyone can actually. At least at first. The gene that produces lactase (the enzyme that breaks down lactose, the sugar in milk) gets turned of after puberty normally. If the gene doesn’t get turned off you can continue to drink milk just fine into old age. This mutation is believed to have first come out of Europe (which explains why it is so much more common in white people) and spread out of Europe into other ethnic groups.
I love recommending Kurzgesagt, they did a video on milk including its digestion and how the ability to do so varies by region. Here: https://youtu.be/oakWgLqCwUc
they were able to survive on substitutes long enough to pass problem to children. more to come.
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