What exactly do you mean with fine?
Um. Probably lack of noticeable health/fitness problems. But yes, it's a vague word. On the other hand, the general level of uncertainty here is high enough to make a precise definition not worthwhile. We are not running clinical trials here.
By the way, the vagueness of "major ... part of ... diet" is a bigger handwave here :-/
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.