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In evolution, genes survived that could code for metabolic pathways, promoting long term survival during famines or fasting when suffering from trauma, disease or during physiological growth. This requires utilization of substrates, already present in some form in the body. Carbohydrate stores are limited and to survive long, their utilization is restricted to survival pathways, by inhibiting glucose oxidation and glycogen synthesis. This leads to insulin resistance and spares muscle protein, because being the main supplier of carbon for new glucose production. In these survival pathways, part of the glucose is degraded in glycolysis in peripheral (muscle) tissues to pyruvate and lactate (Warburg effect), which are partly reutilized for glucose formation in liver and kidney, completing the Cori-cycle. Another part of the glucose taken up by muscle contributes, together with muscle derived amino acids, to the production of substrates consisting of a complete amino acid mix but extra non-essential amino acids like glutamine, alanine, glycine and proline. These support cell proliferation, matrix deposition and redox regulation in tissues, specifically active in host response and during growth. In these tissues, also glucose is taken up delivering glycolytic intermediates, that branch off and act as building blocks and produce reducing equivalents. Lactate is also produced and released in the circulation, adding to the lactate released by muscle in the Cori-cycle and completing secondary glucose cycles. Increased fluxes through these cycles lead to modest hyperglycemia and hyperlactatemia in states of healthy growth and disease and are often misinterpreted as induced by hypoxia.
Soeters et al. (Wed,) studied this question.