I just finished a terrific book called "Grain Brain" by Dr. David Perlmutter, MD. Dr. Perlmutter is a Board-Certified Neurologist and Fellow of the American College of Nutrition - a unique combination that may give him the ability to make connections between diet and brain health that others might miss. The book is heavily sourced and includes the above graph (roughly reproduced by me) from an article published in the journal Neurology, which shows a direct relationship between average blood sugar levels and annual physical brain loss as measured by MRI. Hemoglobin A1C, a standard blood test given to diabetics, provides a 3 to 4 month rolling average of blood glucose levels as indicated by glycated hemoglobin.
He later draws the connection between fasting blood sugar/insulin levels and the size of the hippocampus - the brain's memory center. The function of the hippocampus is dependent on its size. Dr. Perlmutter believes that high carbohydrate consumption is a leading (and preventable) cause of the explosion in Alzheimer's that will cost the United States hundreds of billions in the coming decades. Very scary stuff.
So what do high blood sugar levels have to do with high carbohydrate consumption? Very simply, all carbs are converted to glucose, triggering a release of insulin to signal the muscle and fat cells to transport glucose to the interior of the cell and bring blood sugar levels back into a range that the body considers acceptable. Consumption of a diet rich in carbohydrates may cause consistently high blood sugar levels and an insulin-rich environment that causes insulin resistence, dysfunction, and disease over time.
Although protein triggers an insulin response, it also triggers the release of glucagon, a hormone that signals the liver to create glucose - preventing blood sugar levels from falling too low. Glucagon likely plays a role in the feeling of "fullness," so this may be why protein does not seem to have the same deleterious affect as carbohydrates. Also, low sugar veggies (broccoli, lettuce, etc.) are so bound to fiber that, even though they are carbohydrates, they don't seem to cause the same problems as processed carbs.
Chris Kresser makes the case that carbs from whole-food sources (like potatos) are probably safe. Robb Wolf (The Paleo Solution) , Dr. William Davis (Wheat Belly), and John Kiefer (Carb Nite, Carb Backloading) point the finger at processed carbs as being the culprit behind hyper-insulinemia and inflammation. Gary Taubes (Good Calories, Bad Calories and Why We Get Fat and What to Do About It) thinks that flour and sugar are the likely source of the "cluster" of western diseases including diabetes, heart disease, cancer, Alzheimer's, tooth decay, hemorrhoids, etc.
So what does a non-medical professional (like myself) do? Given the importance of health, I spend most of my leisure-time-reading on this topic. And in the meantime I exercise 5-6 times a week (resistence training, tennis, yoga), try to sleep 8 hours a night, stay connected with the people I love, eat very low carb for breakfast and lunch, and avoid flour and sugar.
Except for ice-cream. As a friend who works at the CIA once told me, the sum of all vices is a constant.