But for some of us, food reactions are quite real. And I will +1 the OP. I had a cheat last weekend that led to uncomforatble bloating. There was a day for me when this was't an issue because that was my state 100% of the time. Now I feel better, and notice when I don't. I'm glad that I now understand that grains make me (and a lot of people) sick.
I'd also agree that there is other issues beyond grain just simply being bad. Any dietary switch can cause these types of issues and there is no doubt some placebo affect going on. That said, there is no doubt substantive issues for many, many people, including, but certainly not limited to insulin response.