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).

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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.

Recommended textbook: Stipanuk or Gropper.

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calories proper

For more on renal gluconeogenesis, see HERE.