Perlmutter's stuff is way out of the left field. If you read the actual study, it does not support his conclusions. Interestingly, there are studies that show that a vegan grain-rich diet lowers hemoglobin A1C. So much for his made-up "grain brain" condition. Of course most diets studied lower it, because they are much healthier than the standard american diet of overeating.

And it's also perfectly possible to be a moderate consumer of sugar and have low hemoglobin A1C depending on other lifestyle factors.