system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people who are lactose intolerant become gassy or sick instead of getting rid of lactose like the body does with fiber?

I know the body lacks a certain enzyme to digest lactose in these cases, but the body can't digest fiber either. I guess another way to word this question is "Why don't people get sick when they eat fiber but lactose intolerant people get sick when they consume lactose?"
gpt
with lactose, if *you* don't digest it, the bacteria in your colon will happily digest it. When they do that, they make gasses and ferment sugars. The gasses and sugars have a variety of effects ranging from the sensation of bloating, to causing water to enter the colon and give you diarrhea. Fiber does not have the same chemical composition, so you don't necessarily get the same by-products when it ferments inside you. Lactose also results in pretty significant production of those by products, which can overwhelm your system. Symptoms typically worsen with more consumption.