The strict dependence of the (human) brain on glucose has always been puzzling to me. While ketones can substitute for a portion of the brain's energy needs, it cannot substitute completely: blood glucose levels below 2--3mM somewhere causes serious neurological problems and can lead to unconsciousness.
Other body tissues are not strictly dependent on glucose, but can oxidize amino acids as well, which is a good backup solution since there is always lots of protein around. But not the brain.
In terms of evolution, this strict glucose dependence must be a major drawback --- falling unconscious just because you don't get enough sugar is probably a bad thing out in the wilderness, when the lions are after you ... So there must be a very important reason.
So why is the brain so dependent on glucose?
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