I'm genuinely curious. Is there any evidence that dieticians are more than just cargo-cultists?

Talking to them usually results in "more of this and less of that and exercise and being thorough and blah blah blah". That sounds a lot like "I have no real idea of what's going on, but something in this broad spectrum of measures is bound to eventually pay off". That's very different from an answer that one would expect from a true professional, which would be more along the lines of "In this long and complex chain of events, this point here is the one that you need to affect in order to experience the effect you're looking for".