I don't know that a zero carb diet is unhealthy, or even an all protein diet. My understanding is that when one moves away from carbs to protein/fat one's metabolism starts moving into fat burning mode and things that may be considered harmful don't get problems due to the metabolic differences that occur. Believe me, I'm not sure of these claims, but thanks to my mother constantly dieting (I would keep her company sometimes, to egg her on) I have read many "diet books" and tried out a couple. So, I'm not basing that on any studies or anything.
But, I don't know that ANY diet is more healthy than others. I'm kind of a believer in moderation in all things, but I haven't really heard that an all protein diet is deadly, or that protein eaters everywhere are dropping dead, right and left, or anything. Etc.
Anna