I recently came across a piece of investigative journalism in The Guardian that I feel sums up the problems in nutritional guidelines, and how they got so screwed up, better than any other lay article that I have read. It is also a fascinating piece on how science self-corrects, but surprisingly slowly. It is well worth a read.
I would draw attention especially to the paragraph which reads ” This [the social community of scientists] makes scientific inquiry prone to the eternal rules of human social life: deference to the charismatic, herding toward the majority opinion, punishment for deviance, and intense discomfort with admitting error. Of course, such tendencies were exactly what the scientific method was invented to correct for, and in the long run it does a good job of it. In the long run, however, we’re all dead, quite possibly sooner than we would be if we hadn’t been following a diet based on poor advice.”