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.
Gluconeogenesis basically only occurs in the liver and kidney, and maybe the small intestine. For more on renal gluconeogenesis, see HERE. Today, we will focus on liver and gluconeogenic substrates.
THE CORI CYCLE: Obligatory glycolytic tissues and skeletal muscle during exercise release a lot of lactate. This lactate enters the blood and is taken up by the liver where it is made into glucose. This glucose then goes back to peripheral tissues, is converted back to lactate and we have a "cycle!" The Cori cycle. And technically speaking, it doesn't add "new" glucose, just recycles it if you will.
The carbon of liver-derived glucose is carrying energy to the other tissues (eg, skeleta muscle or obligatory glycolytic tissues), in this way the oxidation of fatty acids in the liver can transfer energy from fatty acids to do gluconeogenesis which allows cells without mitochondria to obtain that energy.
Integrated intermediary metabolism.
That's all for now!
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