Why are most adults lactose intolerant?
In: Biology
Most humans are lactose intolerant however Middle East and Europe developed a gene LCT which allowed extended production to an enzyme that digests their milk after infancy so we can continue to drink milk.
When you stop drinking milk your body stops producing this enzyme
*All* adult mammals are lactose intolerant except for some humans.
Lactose sugar is only found naturally in one place – mammalian milk. Milk is produced exclusively by mothers for young. It’s very beneficial for mammalian young, because it’s full of calories and nutrients. But that also makes it hugely problematic for mammalian mothers. Those calories and nutrients have to come from somewhere, and that somewhere is extra food that the mother has to eat.
She does not benefit from that extra food, because she has to use it all for milk. For that reason, mammals only produce milk for young, and only when they are young. Once the offspring are old enough and strong enough to fend for themselves, mothers stop producing milk. The young don’t need it and continuing to make has a cost. Mom *could* keep trying to take care of the young, but that becomes increasingly dangerous for her. At some point she has to worry about possible future generations and let the current generation figure out its own survival, whether it wants to or not.
Lactose requires a special enzyme to break it down – lactase. All (normal) mammal babies make lactase. That enzyme is *only* useful for breaking down lactose. And building lactase also has a cost. If an animal isn’t ingesting any lactose, there’s no reason to waste energy making lactase. So once mom stops giving it lactose, it stops making lactase. No other animal gets milk as an adult, so there’s no reason to keep making it.
That has been the pattern for *hundreds of millions of years*, since milk and lactose evolved. Humans began domestication of animals only 11,000 years ago. And it was only about 5,000 years ago that a mutation entered the human genome that delays when we stop producing lactase. That’s only 5,000 years for that mutation to pass down and get spread around the human population. That mutation appeared in Europe, so it’s only people with European ancestry in their history that have it. Granted, Europeans have gotten around to most of the world so people all over the world have European ancestry, even if it’s only a fraction of it. Despite that, most of Asia and Africa do not have the gene and become lactose intolerant as adults. And, it’s not *everyone* with European ancestry. If you have the gene you are guaranteed to have European ancestry, but having European ancestry does not guarantee that you will have the gene.
Lactose intolerance is considered to be the default state for humans. Humans did not evolve the ability to tolerate and digest cow milk products until very recently.
When you take a step back and think about it, Cow milk is a strange thing for us to be eating. It is not part of a humans natural diet.
European populations were the first to develop the enzymes and gut bacteria needed to digest milk products properly. One theory postulates that humans living with herds of Aurochs (Wild Cows, now extinct) bred the animals for their meat, but were unable to consume the milk because it made them sick.
Attempts to preserve the milk resulted in the first cheeses. The cheese making process at a basic level is pretty straight forward, all it requires is milk + time. The bacteria that turns milk into cheese processes the lactose and converts it into something we can more easily digest.
Eating cheese as part of their diet over centuries resulted in cow gut bacteria becoming adapted to our own biome and a permanent part of our digestive tract. This resulted in Europeans developing the ability to make the enzymes needed to break down lactose and consume cow milk directly.
While Asian and African populations never developed this because they didn’t eat Aurochs cheese as part of their traditional diet.
Most people are lactose intolerant. The level of severety is often low enough that people are able to ignore it. But as you age allergies can change for the worse or for the better.
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