> He argues that it doesn't exist based on the fact
  > that people who claim to be allergic don't suffer
  > ill effects from other MSG-containing products.

Some people who claim to be allergic don't suffer ill effects, and some do. This effect is very well known for other fad allergies, such as gluten intolerance. The existence of "gluten intolerant" people who eat cookies does not preclude the existence of celiacs.