Seems to be unstudied, but this person[0] reports has both severe mental and physical reactions currently to almost anything other than meat, though she was able to eat some vegetables safely before. Her father has similar reactions to many plant-based foods, and they both have had serious disorders activated seemingly by diet, especially severe depression.

In addition, many people are unable to eat yeast or yeast extracts, which are among the few commonly available non-flesh sources of the crucial vitamin B12.

[0]: http://mikhailapeterson.com/2017/12/18/zero-carb-carnivore-d...