The harsh reality is that there are so many variables when it comes to diet and nutrition information that formal studies are close to worthless. That is why I find information like the studies Weston Price did [1] interesting. His conclusion was that any society on a whole food local diet was generally healthy and that pre-mature physical degeneration only happened when "primitive" diets were replaced with "modern" diets (i.e. highly processed).

Here's a simple rule. If you could find/grow it in your back yard, eat it. If not don't.

[1] http://www.amazon.com/Nutrition-Physical-Degeneration-Weston...