One hundred ninety-one (191) risk factors were analyzed, and 62 negative associations and 30 favorable associations with diagnosed cases of inflammatory bowel disease were found.

The following risk factors had moderate-quality evidence for increasing the risk of ulcerative colitis and/or Crohn’s disease:

Smoking (+61%, 53 studies)
Asthma (+30%–31%, 15 to 16 studies)
Appendix removal (+128%, 6 studies)
Autism spectrum disorder (+57%, 6 studies)
Hidradenitis suppurativa (a skin condition with painful, swollen lumps) (+56%–125%, 3 to 5 studies)

The following risk factors had moderate-quality evidence for decreasing the risk of ulcerative colitis and/or Crohn’s disease:

H. pylori infection (−58%, 19 studies)
Having access to a toilet (−27%, 9 studies)
Higher BMI (−24%, 5 studies)

What risk factors are associated with inflammatory bowel disease? Original paper