system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Are people born lactose intolerant? And if so, how do lactose intolerant babies drink their mothers milk?
gpt
Lactose intolerance is the default for humans after being weaned. We stop producing lactase, the enzyme needed to digest lactose, which is a sugar. This happens in other mammals and is normal.

The ability to continue to produce the lactase enzyme into adulthood is a mutation that occurred in populations that used animals for milk and dairy products. So those with ancestry from these populations are less likely to be lactose intolerant.  

A person with lactose tolerance can actually become lactose intolerant if they stop consuming dairy because their body may not produce the lactase enzyme after a period of time.