It's hard for me to say how much to change diet based on GCBC. It does a pretty job of motivating a change away from a low-fat diet, but I also didn't fact check any of the studies cited.
I think the best take-away is that making huge, sweeping decisions based on incomplete research is fraut with peril. Human health is a very complicated system, and suggesting that people make huge changes based on a few studies (as the US gov't did), seems like a great way to make things worse instead of better.
After reading GCBC, I'm even more convinced that making big decisions without all the facts is a bad idea. And I don't think we have all the facts now. GCBC makes a pretty good case that current and recent thinking about heart disease is incorrect. It also makes a pretty good case that obesity is a function of the types of things you eat, not just how much. Some of the other claims in the book, especially about cancer and Alzheimer's, seem less well supported. The only thing that I'm really convinced of is that I need to do my own research regarding my health, and I can't just take a doctor's word for anything.
Am I going to make any changes to my diet based on reading this book? I may cut down on sugar intake a bit, though I already have pretty low sugar intake because I'm terrified of getting diabetes. I think I mainly still agree with Michael Pollan's advice:
Eat food. Not too much. Mostly plants.
Although maybe the "mostly plants" part of that advice is mistaken, and my diet will be:
Eat Food. Not too much.