This is a good question, and one for which nobody seems to know the answer. Many people with IBD, like my dad, don't find that foods do cause them problems while others, like me, find that certain foods can aggravate the gut. I also know other IBDers who can eat what they want unless they are in a flare-up, then they have to eat an easier-to-digest diet.
It's a frustrating part of IBD, that no two of us seem to react exactly the same to way to any type of protocol, whether prescription medications or foods or holistic approaches.
IBD is caused by inflammation in the gut and there are theories that certain foods can cause, or worsen, inflammation in the body. You can read Expert Nutritionist Jennifer Rackley's Sharpost on the topic here: http://www.healthcentral.com/ibd/c/91113/28136/inflammatory-craze
I think the most important thing is for each person living with IBD to figure out if any foods do make their symptoms worse and to avoid them. It can be difficult at first, but when you start feeling better it makes the alterations to your diet worth the benefit.
I hope this helps you a little.
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