How do we know which nutrition advice to follow?

I've been interested in nutrition and dietetics since I was 14.  I knew that I wanted to major in Dietetics in college, and I chose James Madison Univ. for that reason.  It's interesting though to think back about what I was taught from 1995-1999 about nutrition.  Some of the things I learned have remained the same, but some of them haven't.  For example, in the late 90s dietetians were encouraging everyone to use  margarine instead of butter.  Margarine had less saturated fat, and no one taught about trans fats yet.  Today however, most people know that butter is the better choice because trans fats are so highly linked with cardiovascular disease.  

Another example is the diabetic diet.  At that time dietetians were still teaching the diabetic exchange program, where you could eat so many exchanges from each of the food groups, and diabetics were not supposed to eat too many carbohydrates.  It didn't differentiate a whole lot between refined carbs and whole, complex carbs.  And there was no mention of fat making a difference in diabetics' blood sugar levels, the focus was completely on carbs.  Today, a lot of studies have shown how fat can be the main culprit behind insulin resistance and diabetes; the fat can make the cells "sticky" and insulin cannot get into the cell with glucose.  Some people still teach diabetics to restrict all carbohydrate intake, whether it's refined or complex, but newer information shows that complex carbohydrates combined with a low-fat, vegan diet can reverse diabetes.  It worked for me.

So how do we know what to believe?  Is the nutritional advice 15 years from now going to be completely different from what it is today?  For some things it will be (my prediction is that high protein/low carb diets will be history, and hopefully by then the whole omega 3 debate will be settled).  But, I have a read a lot of books and articles about nutrition and there are 2 recommendations that have not changed over time:

1)  Eat a lot of fruits and vegetables

2)  Choose whole grains and complex carbohydrates over refined ones.

No one that I have read has argued with these 2 points.  Of course, most people suggest organic fruits/vegetables over conventional ones, but if you can't afford organic, it's probably still better for you to eat conventional fruits and vegetables than none at all.  And who would possibly claim that refined, sugary foods are good for you (beside the makers of Froot Loops who claim that they are "whole grain")?  If refined carbs and sugar were good for you, we would all be so healthy!  

So if you are confused by the information out there and aren't sure which diet is healthiest, start with the 2 recommendations above.  And maybe things will be a little clearer in the future.