Brain's Caloric Intake

Your brain burns 20% of your daily calories, despite only being 2% of your body mass. This is partly because of the blood-brain barrier. It's basically a filter that keeps the big nasty molecules like viruses out, while only letting the tiny ones pass through the cracks. Unfortunately, this also applies to helpful molecules. So while other cells might be able to metabolize carbohydrates or lipids, glucose is pretty much the only energy source that’s small enough to get through the blood-brain barrier, so the brain gobbles it up.