The touchstone that I've been using personally is when I read a nutrition slanted question, I ask myself "Would knowing this information impact their workout/fitness goals, or allow them to get more out of the exercise they are doing?"
If I can answer yes to that question, then it's generally worthy of an answer. If I answer no, then it's off topic for the site. If it's "maybe", then I read through it a few times to see if I can fit it into yes or no.
What I am a little concerned about is the response of "Well, it's off topic unless you relate it to exercise", and then they toss off a one line addition to make it fit. (My prime example for this would be the back and forth in the comments from this question: Acetyl L CarnitineAcetyl L Carnitine), that have evidently now been deleted.
That technically fits the guidelines, but if it's persistent, then imho we open the gates to nutrition questions with the tag line "Cause I wanna lose weight" or other quasi exercise validation.