[scarmig]

Dietitian is the word you're looking for.

As to why the skepticism: the most successful sciences have immediate feedback mechanisms to tell you when you're doing something right. Changing diet and food, however, is slow in effect, hard to monitor, and differs a lot from person to person. Some things are obviously bad--drinking a lot of coke, for instance, and not purposely eating after you're full--but beyond that most things appear to have worked decently for someone at one point or another. So the people in the know have stuck with one set of beliefs for awhile (heavily pushed by the government, which in turn has pretty much acted at the dictates of certain agricultural powers) while alternatives work as often as the party line.