It is pretty confusing. I like the idea of a health nut lifestyle, and I tried to get it on a few sims, but it's confusing, because I have no idea what the Sims considers healthy.
Even before the health nut lifestyle came out, I was confused about how some sims would get a bad moodlet from eating too many quick meals, when they ate healthy things like peas and yogurt. Because of that, I figured that health nuts don't like healthy quick meals, but there was no way to determine which of the other foods count as healthy and which don't.