Although mainstream sources still mistake "the brain needs glucose" for "the brain can only run on glucose," regular MDA readers know the truth
: given sufficient adaptation, the brain can derive up to 75% of its fuel from ketone bodies
, which the liver constructs using fatty acids. If we could only use glucose, we wouldn't make it longer than a few days without food. If our brains couldn't utilize fat-derived ketones, we'd drop dead as soon as our liver had exhausted its capacity to churn out glucose. We'd waste away, our lean tissue dissolving into amino acids for hepatic conversion into glucose to feed our rapacious brains. You'd end up a skeletal wraith with little else but your brain and a hypertrophied liver remaining until, eventually, the latter cannibalized itself in a last ditch search for glucose precursors for the tyrant upstairs. It would get ugly.
That's adaptation. But is there an actual cognitive advantage to running on ketones?
Maybe. It depends. It certainly helps people with neurodegeneration.
People whose brains suffer from impaired glucose utilization see cognitive benefits from ketones. In Alzheimer's disease, aging-related cognitive decline, epilepsy, and Parkinson's disease, brain glucose uptake is depressed—even before any actual cognitive decline appears
. Despite high glucose availability, the aging, epileptic, Alzheimer's, or Parkinson's brain can't utilize enough of it to handle cognition. Enter ketones. Ketones act as an alternative energy source for the glucose-starved brains.
It's no coincidence that ketogenic diets can improve symptoms (and in some cases abolish them) and cognitive function in all four conditions.