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