http://news.mongabay.com/2008/1110-corn.html

Here is a very scientific chart (with explanation below) for those of you that can understand this sort of thing.  Carbon and nitrogen stable isotope values of beef and chicken products sampled, positioned according to geographic region. Chains are designated by symbol (circle = McDonald's, square = Burger King, diamond = Wendy's), stable isotope value is designated by color. The 3 restaurants sampled differ by horizontal position, whereas the 3 servings differ by vertical position, creating a grid of 9 values for each chain within each city. Three missing points (C–E) reflect 2 lost substrates from within the entire study (1 servings of chicken and 1 serving of fries). Image and caption text courtesy of PNAS. Click image to enlarge.

Tangentially related: Candy corn is made almost entirely of corn. It's mostly corn syrup and corn starch. It's probably the only candy mock-food that's actually made out of the food it resembles.