After about 5 to 6 weeks of fasting, splanchnic
glucose production totals about 80 g per day with 10 to 11 g
of glucose per day synthesized from ketones, 35 to 40 g per
day from recycled lactate and pyruvate, 20 g per day from
glycerol, and 15 to 20 g from amino acids (mostly alanine)
released from muscle [1].
With metabolic
stress, urinary 3-methylhistidine excretion increases, reflecting
increased protein catabolism, and overall urinary
nitrogen losses frequently total 30 g or more per day. Each
gram of nitrogen lost can be translated into the breakdown
of approximately 30 g of hydrated lean tissue (whereby 1 g
nitrogen = 6.25 g protein; muscle is about 80% water, so 80%
of 30 g = 24 g, and 30 g muscle = 24 g water + 6.25 g protein).
In metabolic stress, the body systems prioritize wound
repair and host defense at the expense of body tissues, in essence
gambling that convalescence or a return to health will
occur before tissue depletion threatens survival.
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