My dad developed diabetes when he was in his twenties, and struggled with it for the rest of his life. Throughout my childhood, I had a bunch of conversations with him about what diabetes was and how he was dealing with it. As I got older, I watched his health decline faster and faster. He died suddenly of diabetes related heart failure while I was in grad school.
Since then, I've read a lot more about nutrition and health. The more I learn, the more I believe that my dad was treated using the best advice politicians could give. GCBC goes into some detail about how politics played a huge part in how heart disease and diabetes were treated historically, and how a lot of evidence about the origins of those diseases was ignored.
Large portions of GCBC are spent explaining the history of nutrition science in excruciating detail. Study after study is held up and compared to everything else. After slogging through all of that, the takeaway seems to be that one guy was sure he was right, and everyone else fell in line and stopped studying (or believing studies about) other hypotheses.
Ansel Keys first popularized the idea that cholesterol was the main cause of heart disease after WWII. He then found a link between eating high fat diets and having lots of cholesterol. Nevermind that no causal studies were performed, he was convinced. The next fourty years saw the production of a number of not very convincing studies that were used to inspire huge changes in the American diet.
Once Keys had convinced a critical mass of scientists and politicians, everyone who questioned the idea that cholesterol was to blame was treated with derision in an epic example of groupthink. A government panel released a report called "Dietary Goals for the United States", which recommended a diet for all Americans based on Keys' cholesterol ideas.
The motivation for all of this seemed to be the mass prevention philosophy of Geoffrey Rose. The idea was that, even though all the studies on dietary fat hadn't shown much impact for a given person, the impact on society was huge. For example, it was estimated that you could live an extra 4 months if you completely cut saturated fat out of your diet. That may not be much for you, but if everyone did that the effect would be huge.
Since 4 months of additional lifetime might not be enough to convince someone to completely cut saturated fat out of their diet, Rose explicitly advocated creating social pressure to be healthy. Social pressure, unfortunately, is often highly resistant to scientific evidence. This effectively meant that future scientific discoveries would have a very difficult time changing anything.
The science became political goals. The political goals led to social manipulations. After that, the early scientific ideas became dogma. Subsequent research on nutrition was interpreted in light of that dogma and very little advancement happened.
Or that's the idea that GCBC espouses. I take less of a cynical attitude. It's easy to make the right decisions with the benefit of hindsight. I've seen some talks that take a more conspiratorial view of things. I don't think that these nutrition decisions were made optimally, but why ascribe to malice what can be explained by human nature?
Whatever the reason, the increasing incidence of heart disease and diabetes since "Dietary Goals for the United States" was released is only tragic.