how does lactose intolerance work and why do only certain people have it
In: Biology
You lack the enzymes needed to break down/digest lactose and you end up with the sugars being digested late in the digestion process which leads to gas pressure and bloating in your intestinal track.
It can be wildly painful and not at all comfortable.
Most mammals naturally stop producing the enzymes that allow them to digest lactose (the sugar in milk) after they leave the infancy stage of their development. Humans however have developed a mutation that allows them to digest it into adulthood at least 3 times. Once in Northern Europe, once in the Central Asian Steppes, and once in Sub Saharan Africa. But that said something like 75% of the global human population is lactose intolerant since most people are from phenotype groups that do not have the mutation.
If you do not have the mutation then the lactose passes through your digestive tract without being broken down and it is intact when it gets to your lower intestines and the bacteria that lives there can consume it. Their waste from doing this will cause you to have a lot of gas, and can cause you to have diarrhea.
Humans are mammals. That means that when we are babies, our mothers produce milk that we drink. Lactose is a type of sugar found in milk.
In most mammals (and most humans, actually), only babies can digest lactose. As they get older and are weaned off milk, they stop producing the enzyme lactase, which is responsible for digesting lactose. Without lactase, the lactose remains intact until it reaches the intestine where it is literally fermented by bacteria, producing gas; this causes the symptoms of lactose intolerance in humans.
As I said, this is the norm for all mammals, including humans. However, some populations of humans, particularly in Europe and parts of Africa and the Middle East, developed genetic mutations that produce lactase even into adulthood. This is a particularly clever evolutionary trick, as it allows those of us with the mutation to drink milk from domestic animals like cows and goats. However, only around 45% of humans worldwide actually have the mutation, and in some regions like East Asia it is as low as 10%. Even if you belong to a population where lactose persistence is common, it’s not guaranteed you’ll inherit the trait.
Humans are not meant to drink milk. That simple really. Even those who can handle it, have other negative effects from it.
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