It can cause your body to not be able to absorb the nutrients you need. Your other choices
for treatment are good choices
for people with IBS, especially if your IBS tends toward constipation more than diarrhea.
Yes, fast food does tend to irritate IBS, regardless of whether it tends toward diarrhea or constipation. I get nauseous if I eat something that I know irritates my IBS, then I get constipated.
Many with IBS have different kinds of food intolerances.