What is the cause of lactose intolerance?

what is the cause of lactose intolerancearticles

In this post, I will discuss what is the cause of lactose intolerance. First of all, our body knows what it needs exactly. Lactose is a sugar that you can find only in mammalian milk and doesn’t exist in other foods. So, if you are lactose intolerant, your body doesn’t want to drink milk.

This article is mostly based on Dr. Akio Sato’s work and book.


What is lactose?

Lactose is a sugar and a combination of two monosaccharides of glucose and galactose. Lactose exists only in mammalian milk and doesn’t exist anywhere in nature. Human milk contains the highest amount of lactose among all mammalian animals.

What is lactose intolerance?

People who are lactose intolerant have difficulty to digest lactose, a sugar found only in dairy products, causing several symptoms by consuming the dairy product. Lactose intolerance occurs because of the inactivity of the enzyme lactase in the small intestine.

How does the body response to lactose?

I won’t use the word “symptom” because lactose intolerance is not a disorder, it is a natural mechanism. So, I use “body’s response” instead. How does the body response in lactose intolerance? According to Wikipedia, the following conditions appears usually after 30 to 120 minutes of consuming dairy products.

  • abdominal bloating
  • abdominal cramps
  • flatulence: too much gas in the intestine
  • diarrhea
  • nausea
  • borborygmi (making sound by intestine because of gas movement)
  • vomiting (particularly in adolescents).

The more dairy products one consume, the more likely severe conditions will happen. Some people are lactose intolerant, and they don’t feel sick with a little bit of consumption.

abdominal pains by lactose intolerance

How does lactose intolerance begin?

When a baby is breastfed, lactose in milk breaks into two monosaccharides of glucose and galactose by the enzyme lactase, and these two monosaccharides are absorbed into cells.

After the weaning period, lactase activity decreases rapidly. If babies with low lactase activity drink milk, lactose can’t be digested in the small intestine and go to the large intestine where there are many enteric bacteria. These bacteria break down some lactose into organic acids. The organic acids stimulate the Intestinal wall and create gas. These gas and organic acids cause abdominal pain.

The main function of the large intestine is to remove excess water from stools. Since lactose is hyperosmotic, it tends to absorb water.

If lactose remains in the large intestine,  it interferes with large intestine to remove water from stool and pull water from the intestinal wall, which makes stool watery to cause diarrhea.

Why does lactase activity decrease with age?

Lactose intolerance is not a disease.

Why do we lose lactose activity growing up? It is because we don’t need to drink milk anymore and to digest lactose in milk. The body no longer recognizes breast milk as a food source and reacts to lactose in milk.

All mammals begin to get nutrition by eating solid food after weaning, lose in lactase activity and drink milk no more.

Human is the only mammal which eats or drink dairy product after the weaning period. When lactose intolerant people consume dairy products and become sick with it, they think their stomachs are weak and not healthy. No! it is wrong! Lactose intolerance is not a disease. Lactose intolerance is the normal state as a grown-up adult!

Some people can drink milk again by constantly drinking milk. It is probably because the lactase becomes active again to match their habit. But, their lactase activity is not enough as an infant. If they drink milk a lot, they still feel uncomfortable in the large intestine.

Lactose intolerance is a natural mechanism to support the survival of mammalian species

Lactose intolerance does not disorder or disease. it is the mechanism to support the survival of mammalian species.

If a baby keeps drinking mother’s milk, the mother’s body can not have ovulation and not be ready for the next pregnancy. If growing up children drink milk, they may feel pain in the abdomen because of enzyme lactase activity decrease. And then, the children start to eat solid food as same as their parents do.

Lactose intolerance is natural mechanism to support survival of mammalian species

Lactose intolerance incidence is different depending on races.

According to this article, 65–70% of the world’s adult population is lactose intolerant. According to the Physicians Committee for Responsible Medicine, followings are each race’s lactose intolerance rate.

  • 95 percent of Asian Americans,
  • 74 percent of Native Americans,
  • 70 percent of African Americans,
  • 53 percent of Mexican Americans,
  • 15 percent of Caucasians.

Among these races, Caucasians are the only race that is high lactose-tolerant and keeps high enzyme lactase activity even after the weaning period.

What happened to Caucasians 8000 years ago?

According to Dr. Akio Sato, lactose tolerant Caucasians appeared 8000 years ago in Mesopotamia civilization. They moved to northern Europe where the daytime is short, where it is difficult to grow or collect a variety of crops and vegetables. They needed to have cow dairy products that contain a rich amount of protein to survive. Together with dairy products, wheat was also their important food.

Lactose tolerant Caucasians are very new for human history. Yet, lactose tolerant people are the majority of the world population, two-thirds of humans, because it is how human are made originally.

Lactose tolerant Mongoloid in northern Asia.

Ancient Mongoloids who lived in northern Asia get all their food, clothing and shelter from animals such as horses, cows, sheep, goats, and camels. In the region, very short grass can grow and very dry. There was no way to grow crops. These people are ancestors of current Mongolians.

From June to October, they can get milk from animals and make cheese and butter.  From October to May, they couldn’t get milk, so they ate animals. From horse milk which is high in lactose, they make liquor, too.

By trading them, they were getting grains. But, they ate grains very little. Even now, Mongolian people eat grains only 30% of total calories.

Lactose intolerant Mongoloids

Mongoloid in Southeast Asia

A part of Mongoloids moved to southeast Asia. Not like northern Asia. They didn’t need any dairy and animal products to survive. it was very easy to get fruits, nuts, and root vegetables.

They never imagined that they use animal milk as food. Their diet became plant-based. And, they didn’t need to develop lactose intolerant.

Japanese people

Japanese people’s ancestors are Northen Mongoloid. Japan was filled with forest. It was easier to collect plant-based foods instead of hunting animals. Later, southern Mongoloids came to Japan and brought rice cultivation. Later, more advanced rice cultivation technology was introduced from Korea, and Japanese people start to cultivate rice everywhere. Cows are used as a workforce. And, Japanese people never imagined that they drink milk from them until western people came to Japan.

In 1856 Townsend Harris from the U.S., the first Consul General, came to Japan and he asked the Japanese government to find milk for him. How the government answered him was very interesting. The government officer answered to Harris, “Cow milk is for a baby cow to drink, but not for a human.” Japanese people in the 18th century felt very wield about taking milk from a cow that worked for people.

Thus, Japanese people haven’t drunk milk until recently. So, the majority of the Japanese kept the original body system, lactose intolerance.

ethnographic map by Meyers Konversations-Lexikon


Lactose intolerance is not a disease. It is the body mechanism so that more efficiently leave offsprings. Without lactose, a baby wants to drink mother’s milk even after growing up. So, a mother can’t have the next baby.

If you have lactose intolerance, your body is telling you that you don’t need milk nutrition any more because you have already grown up. Let’s listen to the true voice from your body, not from the head thought.

You can get enough nutrition with a plant-based diet.

If you have a comment or questions, please send me a message into a comment box below.

Thank you!


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