I get asked about this a lot. Gluconeogenesis. The brain uses the equivalent of 120 g glucose/d. During starvation, brain still uses glucose, but less (because ketosis) and a bit differently (less oxidation, more lactate release).
The brain and the obligatory glycolytic tissues always use at least some glucose. During starvation, this glucose comes from gluconeogenesis.
The substrates for gluconeogenesis are primarily lactate, pyruvate & alanine, glycerol, propionate, and some amino acids (not leucine or lysine). The relative proportions change depending on #context, eg, the duration since your last meal, diet composition (keto, LF, etc.), etc.
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For more on renal gluconeogenesis, see HERE.