system
Given the following post, respond as if you're commenting on Reddit:
human
What happens differently in my digestive system when I eat Brussels sprouts or broccoli that makes me fart like mad?
gpt
Broccoli and Brussels sprouts both contain soluble and insoluble fiber. While your body is unable to digest fiber of either sort, what can digest it are the countless millions of bacteria in your lower intestine. Both soluble and insoluble fiber can be considered "food" for intestinal bacteria, however soluble fiber tends to promote their activity more than insoluble fiber, which mostly bulks up your poop and makes it easier to do so. 

When these bacteria digest the fibers, they release large quantities of (often) smelly gases which you associate with bad gas. They also produce nutrients which are beneficial to your colon's health. In addition, the fact that these vegetables contain significant amounts of (very healthy) sulfur within their nutrients also contributes to the fact that farts tend to smell extra stinky after consumption of fibrous veggies. 

TL;DR You can't digest fiber but bacteria in your intestines can. They fart when they eat it, so you do too.