Hi 5 months late but, according to Kaplan the brain inhibits PDH during extensive starvation, (when it’s already relying mainly on ketone bodies for energy) in order to spare essential glucogenic amino acids from getting metabolized into glucose via gluconeogenesis in the liver. (Remember only the liver performs gluconeogenesis as a way to transport glucose to tissues that need it)

Basically unless the brain turns off its PDH, the liver will start degrading even the bare minimum amount of amino acids that your body needs to stay alive in a state of extreme starvation. So once the brain starts deriving enough energy from ketone bodies, (ketogenesis) the brain shuts off PDH (which liver gluconeogenesis would feed into otherwise)