Why Are Humans Lactose Intolerant?

by Alisha Upadhyay

Why Are Humans Lactose Intolerant?

June 24, 2021

Humans were not born with the ability to digest animal milk, making them lactose intolerant. But why has evolution favored dairy tolerance?

There is rivalry for dairy milk. Alternative “milk” manufactured from soy or almonds is becoming more popular. These substitutes are vegan-friendly and can be used by individuals who are lactose intolerant or allergic to milk. However, the advent of alternative milk is only the latest chapter in humanity’s long association with animal milk. This connection has been through many ups and downs over thousands of years.

It is almost unheard of in many cultures. China started a national drive in 2000 to encourage people to consume more milk and dairy products for health reasons – a campaign that had to overcome many elderly Chinese people’s profound fears. However, many Chinese people are still sickened by cheese, which is simply milk left to spoil.

Drinking milk is a relatively new habit in our species’ 300,000-year existence. Before roughly 10,000 years ago, almost no one drank milk, and if they did, it was only on rare occasions. Early farmers and pastoralists in western Europe were among the first humans to live with domesticated animals, including cows, and were the first to drink milk.  Today, drinking milk is a common practice in many countries.

Lactose, a type of sugar found in milk, differs from the sugars found in fruit and other sweet foods.

Baby Food

Animal milk consumption is unusual for biological reasons.

Lactose, a type of sugar found in milk, differs from the sugars found in fruit and other sweet foods. Our bodies produce a particular enzyme called lactase when we are babies, which allows us to digest the lactose in our mother’s milk. However, for many people, this ends after we are weaned in early childhood. Lactase is required to properly digest lactose in milk. As a result, an adult who consumes excessive amounts of milk may feel flatulence, severe cramps, and even diarrhea, thus making them lactose intolerant. 


But then evolution took over, and some people developed the ability to keep their lactase enzymes active well into adulthood. They were able to drink milk without experiencing any ill effects because of this “lactase persistence.” Mutations in a region of DNA that controls the lactase gene’s function are the cause.

Evolution favored the lactase persistence characteristic, which is now extremely frequent in some groups. Lactase persistence is active in more than 90% of people in northern Europe. The same is true in a few African and Middle Eastern populations. However, lactase persistence is substantially rarer in many populations: many Africans lack the characteristic, which is unusual in Asia and South America.

Lactose intolerant people can still consume a tiny amount of lactose without harm; therefore, drinking a small bit of milk is fine. Alternatively, milk can be processed into butter, yogurt, cream, or cheese, all of which contain less lactose. Lactose content in hard cheeses like cheddar is less than 10% of milk, and butter is equally low.

A Dairy Decline?

The news over the last few years, on the other hand, has given the impression that consumers are abandoning milk. For example, the Guardian published a story titled “How we Fell Out of Love with Milk” in November 2018, outlining the stratospheric rise of companies selling oat and nut milk and implying that regular milk is in jeopardy.

Statistics, on the other hand, reveal a different narrative. Global milk output has increased every year since 1998 in response to rising demand, according to the IFCN Dairy Research Network’s 2018 report. Milk was produced in 864 million tons over the world in 2017.

It’s particularly surprising that Asia, where most people are lactose intolerant, accounts for so much of the increase in milk demand. Whatever benefits people find in milk outweigh any potential digestive troubles or the need to process it.

Indeed, the UN Food and Agriculture Organization has urged people in impoverished nations to raise more non-traditional dairy animals, such as llamas, so that they can benefit from milk even if cow’s milk is unavailable or too expensive.

Furthermore, a key study published in January 2021 described a “planetary health diet” that aims to maximize health while minimizing environmental damage. Even though it involves dramatically reducing red meat and other animal items, it nevertheless includes the equivalent of one glass of milk each day.


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