IBS is often misunderstood. It is a lack of ability to digest one or more of nature's basic sugars, up to seven... (lactose, fructose, fructans, sorbitol, polyals [?], GOS, mannitol, and a unique sugar in avocado). When the wrong sugars are digested, you feel weak, nauseated, sick, and now it is believed there may be a connection to MS for some(which I have). Digesting caffeine makes it considerably worse.
I have the inability to digest all seven sugars, and finding foods I can eat, between following a regiment for MS and IBS is nearly impossible.