I was an ovo-lacto vegetarian for 7 years. Hardly any sugar either. It was a long time ago and I wasn't dx'd at the time, so I can't say definitively any mood effects it did or didn't have, but I don't think it did. (I did have a reduction in migraines however.)

I do remember what was a hypo time of that era wherein one day while at the laundromat, I suddenly decided we should move a couple thousand miles and build our own house (as in totally hand build(!)) (Move happened, house, not surprisingly, didn't). Even not dx'd at the time, looking back, that spell was a fairly obvious. BUT. That's almost exactly the same thing I did a number of years later when not a vegetarian, so that's just "me", not "me"at, lol.