Glucose isn't the problem, glucose regulation is. If you eat refined carbs or sugars you get slammed with it all at once, and your body becomes more insulin res

Glucose isn't the problem, glucose regulation is. If you eat refined carbs or sugars you get slammed with it all at once, and your body becomes more insulin resistant and fat-generating as a defense mechanism. Meanwhile, you run into the "blood sugar rollercoaster" effect - a high, followed by a crash causing extreme hunger. The blood sugar shenanigans have the side effect of manipulating blood pressure as well, which can make some activities difficult or dangerous. When you eat fats, proteins and slower-digesting carbs the glucose regulation problem doesn't occur, at least not on the same scale, so your brain and body can perform undistributed for potentially many, many hours. Consequently the only problems come from nutritional content and digestive efficiency. For elite endurance athletes there is still reason to "carb up" to pump as much glyogen into the muscles as possible, but that isn't most of us, and it isn't something you need for brain work.