I'm curious if left untreated or at least undiagnosed, can bipolar disorder become full on depression?

Sure: Yes there are suibstitute foods depending on what needs to be achieved. Most fish has good protein and some fish oils which are good for your health.Chicken can also give good proteins but it does not have the oils that the fish provides. But make sure that you remove the skin from the chicken in order to remove the anima lfats which are not healthy.