how come a lot of people are lactose intolerant if everyone started off by drinking milk as a baby?


how come a lot of people are lactose intolerant if everyone started off by drinking milk as a baby?

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Most of the world is lactose intolerant. The ones that aren’t have lactose persistence. Everyone can digest lactose as a baby, but those with persistence never lose the ability as they grow up. It is a trait that evolved in regions where people drink a lot of dairy, like Scandinavia. In places without milk drinking, producing the enzyme to digest milk as an adult is wasted metabolic effort.

Lactose intolerant has nothing to do with breast milk, it’s all about cow milk. Many children develop intolerance starting at age 5 when lactase enzyme activity decreases.

Most animals that drink milk are lactose intolerant as adults.

Mothers only produce milk for a short time after having a baby, which means that being able to drink milk is kind of useless a year or two after being born (since your mom isn’t feeding you milk anymore).

It takes energy to generate lactase, the enzyme that digests lactose. But, if you’re not drinking milk, there’s no reason to generate lactase, so doing that just decreases your chances of surviving because you’re spending energy on something useless.

As a consequence, most mammal children grow out of producing lactase as they get older.

Humans did something weird though. Rather than relying on their mothers for milk, people decided to start stealing milk from mother cows, and drinking that.

Now, there’s a way for humans to drink milk at any age, not just as children, so it’s advantageous to produce lactase as an adult, which allows natural selection to produce humans who can drink milk.

There hasn’t been enough time for this adaptation to spread to the entire human population though, so only some people with origins in specific areas of the world where drinking milk was common produce lactase as adults. People who haven’t developed this adaptation, just like every other mammal, are called lactose-intolerant.

Lactose is about 4-5% of cow milk and about 7% of human milk. It is possible for infants to be lactose intolerant and it causes all sorts of gastric issues for the infant.

As we age, the amount of the enzyme lactase (which breaks down lactose,) we produce tends to decrease with age and if stop consuming milk it happens more rapidly.

People who are lactose intolerant can consume it but suffer from gastric symptoms. So people who are intolerant can consume small quantities without ill effects but at larger quantities get horrible symptoms.

What is lactose intolerance:

Lactose intolerance is caused by the lack of an enzyme called lactase present in the small intestine. Lactose cannot be absorbed by the body. Lactase breaks down lactose into glucose and galactose which can be absorbed by the body. The symptoms of lactose intolerance are a result of intestinal bacteria feasting on the intact lactose.

How can lactose intolerant people exist if they all drank milk as babies:

Babies *can* be lactose intolerant but most aren’t. You’re asking why most people can tolerate lactose as babies but some can’t as adults.

There is a gene called the LTC gene that allows mammals to generate lactase. There is another gene called the MCM6 gene that turns the LTC gene on and off. In most mammals the MCM6 gene turns off the LTC gene after a baby is weaned. The reason humans can tolerate milk past weaning is that a mutation of the MCM6 occurred several thousand years ago that stopped it from turning off the LTC gene.

Generally speaking, adult lactose intolerance in humans can be the result of a few factors:

1. The absence of the MCM6 mutation. If an individual did not inherit this mutation their LTC gene gets turned off and they stop producing lactase after weaning.

2. The body is less able to produce lactase as an individual ages.

3. Injury to the colon causes the body to be less able to produce lactase.

One thing to note is that most mammals are lactose intolerant once they are weaned. Feeding cats and dogs milk or ice cream can cause them intestinal discomfort.

Funny thing is my baby was lactose intolerant and the doctor said they grow out of it. I thought BS. After 1 years old he tolerated it and years later drinks it just fine.

Milk from your own species, i.e. breastmilk, can be easier to break down than milk from other animals.


Milk is for babies.

Cow’s milk is for baby cows.

Adult humans drinking cow’s milk is an aberration- some can tolerate it, most can not, bc their bodies *don’t need it*.

The milk industry doesn’t want us to know that it is *completely unnecessary for adults to drink cow milk.*

You get just as much calcium and vitamin D from orange juice, or cauliflower, or red peppers.

(Not judging anyone who likes drinking milk- if you like it you like it and that’s fine- but the very idea that adult humans *need* cow milk in their diet is UDDERLY ridiculous! Lactose intolerance is proof.)

I did read not long ago that it has something to do about your DEEP ancestors, something about having both neanderthal and another kind which i fail to remember.

Genetics factors into who can tolerate milk as an adult.

If your ancestry is to people’s who lived in temperate-to-cold climates, then you likely have the gene to digest milk.

This is because cows were an excellent way of converting vast barren lands (which cows feed off of) into meat and milk. People who lived closer to the equator could reap the land year round. In colder climates, cows make good use of otherwise less nourishing grasslands and steppe.

I’m milk protein intolerant (different), and reacted badly to breast milk as a child. Took them a while to figure it out. So some children may be lactose intolerant at birth also.

Not sure if anyone mentioned beta casein here? Cows produce a different beta casein then humans and its harder for many to digest. Animals like goats, donkeys and camels produce the same beta casein (A2) as humans and it’s easier for us to digest their dairy. There are now many small cow farms producing cows with A2 beta casein through genetic processes so that customers will be able to buy their cow milk products still.

Lactose intolerant people lack the enzyme lactase that breaks down the milk molecules and allow you to digest it adequately.
They lack it because the part of the DNA that tells the body to produce lactase gets “stored away” aka condensed over time.
The natural/normal thing is for animals (including humans) to become lactose intolerant when growing up, and to be able to easily digest lactose is the mutation.

Babies can also be lactose intolerant, and many babies drink their mothers milk, which is made for them and less likely to cause issues instead of another animal’s milk, which can be more likely to cause issues. However, some mothers and babies can’t do breastfeeding, so they have to use something else.

Adults are lactose intolerant. We develop an intolerance as we grow up. Some races deal with it better than others. Be wary of giving your adult cat some milk even though she’ll enjoy it.

We are not supposed to drink milk as adults, but we do because it’s delicious 🙂