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Popcorn

Whole grains are good for everyone—except a person with Crohn's disease who's having a flare-up.

Popcorn is technically a whole grain, and it's probably among the hardest of all those grains to digest. The same goes for corn on the cob.

"Anything that's really rough to digest would possibly be detrimental and cause more symptoms and possibly slow the healing process," says Dalessandro. "That's been my experience with a lot of patients, and myself."

Next: Fried food