For about 5,000 out of the 10,000 years we’ve been drinking milk, we’ve mostly been lactose intolerant.
Wait, what?
Today, around 68% of the population is lactose intolerant. This means we can still get fats and protein from milk, but can’t unlock it’s biggest nutritional bounty - the vitamins, calcium, and carbs stored in lactose. This is a fun fact because milk is the base of one the biggest healthy food industries, and consumed by adults all around the world.
All healthy babies come with a package of lactase enzymes, which allows us to break down milk sugars into all the necessary calcium, vitamins, proteins, and carbs in milk. Production of lactase ends after nursing, around when we’re five. After that, we slowly lose the ability to digest lactose as the amount of this enzyme continues to decrease into adulthood. When this happens, drinking milk often results in gas, cramping, and diarrhea.
The lucky ones
Lactase persistence is an exception that occurred in select places around the globe, most likely because of how the environments shaped our survival conditions. This is virtually non-existent in East Asia, while in Scandinavian countries the rate of lactase persistence is over 90%.
To see why this happens, let's go back 12,000 years or so, when we started domesticating cows, sheep, and goats that could provide milk for adults to drink in public settings. Farmers around the world independently evolved genes to keep their lactase enzymes - in different parts of the world, a different gene is responsible for keeping lactose intolerance at bay.
It's relatively straightforward to see why farmers in cold climates gained an advantage from being able to digest milk sugars: they get more nutrients and calories, with the added benefit that fresh milk can last longer in cold climates without having to turn it into cheese.
However, the deserts of Africa and the Middle East seen like the exact opposite of this, and yet have some of the highest lactase persistence percentages in the world. Here, we can speculate that selection pressure could have worked because herders could survive droughts by drinking camel milk - and if you get diarrhea in the desert from lactose intolerance, you're probably going to die without good water sources.
Lactose tolerance today
For the vast majority of us that haven't managed to evolve this adaption, today's world thankfully offers more friendly survival options.
Cheese and yoghurt making are partly cultural adaptations for lactose intolerant people, which allows everyone to access the nutrition stored in milk they otherwise wouldn’t be able to digest - Milk is around 5% lactose by weight, while cheddar cheese is 0.1%, and tzatziki is 0.3%.
Modern technology has also allowed us to start pre-processing dairy products: lactose-free milk is milk that has been treated with lactase, which breaks down the milk sugars for us (that's why it's sometimes sweeter).
Whey Protein Isolate is another processing method for lactose-intolerant people. This time, we filter out the lactose along with other fats to get a much purer product, which is digested far more easily.