So looking it up, all I can find is that the brain imposes relatively high caloric needs with respect to other organs, but not that those calories need to come

So looking it up, all I can find is that the brain imposes relatively high caloric needs with respect to other organs, but not that those calories need to come from carbohydrates. Do you have any sources on the latter claim? The last time I got exposed to this claim was when a doctor (not a nutrition expert) told me that we must eat fruit for breakfast because our brains need the sugar. To me, both anthropology and personal experience seem to contradict this idea. Maybe I can change my mind.