as far as i have seen, people do tend to follow the diet even b4 they discovered this. I have avoided milk for years b4 i found out about the Blood Type Diet , i have a friend who hates chicken so much and recently i found out that she's of blood type B which has chicken on their no no list.
as for myself, i love vegetables and tofu from the start, so it was not much of a problem. but i'm uncomfortable with omitting meat altogether, so i always have at least one meat dish for dinner every evening.
i found that as soon as i omitted dairy food and deep-fried meat (chicken especially), i feel so much better.. so i do recommend all type A to avoid those two items. as far as the rest go, you don't need to follow the guidebook thoroughly... design your own diet and lead a healthy life.