With all of the confusing misinformation out there, there is plenty that we do know, based on sound science. The Harvard School of Public Health is a great resource that distills what scientific research has shown us about nutrition (as opposed to what lobbyists have influenced the USDA to tell us about nutrition).

More info here: http://www.hsph.harvard.edu/nutritionsource/what-should-you-...

And an overview here: http://www.hsph.harvard.edu/nutritionsource/healthy-eating-p...