system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If enzymes are catalysts and catalysts don't get used up, how do people become lactose intolerant?
gpt
Lactose intolerance people don't produce enough of the enzyme they need to digest the lactose. 

Think of the lactose as an invading army, and the enzyme as an adamantium spear. The spear never gets used up, but if there's one spear, and 100,000 people in the invading army, that doesn't really matter.  Most of the lactose still gets past the solitary, spear-wielding defender, ravaging the countryside (of your digestive tract).