I generally avoid MFS. I had been under the assumption that it's corn, and okay to have. Until I ran into the following problem:
I was trying Dannon Light and Fit yogurts. It has MFS, so I took a gamble. I started to get sick after eating it, so I looked at the label closer. It has MFS, and Modified Corn Starch. I was confused, and contacted the company to ask for clarification. They said "We have to list wheat as a major food allergen", but when I pressed them for what was their MFS source, they said "It's a blend, but I don't know what's all in it." I ended the conversation with "So, there could be wheat in it if you don't even know for sure," and they admitted that could be true.
Needless to say, I'm avoiding MFS for a while again.