I believe that nutrition science is pretty dubious because it mainly relies on epidemiological studies. The extraordinary claims that are touted as fact are numerous. This is why we get many conflicting studies on cholesterol, fats, carbs, etc. It's very, very difficult to isolate one variable in nutrition epidemiological studies and attribute them to correlations in population outcomes. The one major success story for epidemiological studies was smoking and lung cancer. Of course, difference there is that smoking is binary: one either smokes or they don't. They may smoke more or less...but the variables are pretty low. Whereas dietary intake is diverse and no two people eat the same exact diet.