Our dogs are both neutered and, although they get sardines, olive oil, etc., their coats aren't very glossy at all.
I have noticed, though, that their coats feel really soft whenever they get to swim in the sea. Don't know if other COLers have noticed this? Ours used to swim in the sea almost every day but now we've moved it's quite infrequent and their coats aren't so good. Don't know if you live near the sea Nelly?