Great post by JP over at Primal Journal on why nutrition science sucks:
The pillars of nutrition science are weak because of the approach used. Population studies are irrevelant in most cases and, yet, they are very popular. …
Most studies are short term studies. Also, using questionnaire as a way to gauge food intake and choices has been done extensively. We all know how unreliable these questionnaires are : People lie in these questionnaires. …
The biggest problem in my opinion is the lack possible comparison between diets. … Another problem we face is the lack of multidisciplinary knowledge amongst nutrition experts.
Lots of good points — be sure to check out the whole post. I particularly liked his bottom line (emphasis mine):
Modern nutrition science sucks because too many variables are into play with no mean of knowing what’s really harmful or not. People are getting fat and most of their focus is nutrition since it’s so popular in the media. However, making people healthier require people with multidisciplinary knowledge and unbiased information. Both are not too present and this is why -not carbs, not fruits, not tubers, not fat, etc.- people are not getting healthier. As a side note, question the claims of all medias. If you want to eat an healthy diet, eat foods you can pick in the nature.
I’m convinced that nearly all diets — low-carb, low-fat, and everything in between — work because they largely remove industrial food. The rest (like the Twinkie diet) work by reducing calories in the short term.
When in doubt, go with real food!
Weight Maven is written by Beth Mazur. Beth believes that obesity is more symptom than cause and that the real problem is our Western diet -- especially sugar, refined grains, and industrial oils. Beth writes about nutrition, ancestral health & food policy. And cats! 
The reason our current nutritional approach doesn’t work is because it operates on an incomplete model of what it physiologically means to be human We are both a matter as well as an energy field but to sustain the matter field model currrently used is to be incomplete in our physiologic approach to healing