During Prolonged Periods of Fasting or Undernutrition, Glucose and Amino Acids Are Conserved by Extensive Utilization of Fats and Ketone Bodies for Energy Production
From the previous discussion of postabsorptive metabolism, you can appreciate that amino acids form an important depot for glucose precursors and energy-produci ng substrate. During prolonged fasting or undernutrition, however, it would not be advantageous for animals to rely heavily on their skeletal muscle for energy and glucose production; doing so would soon lead to severe weakness as the skeletal muscle protein was consumed. Thus, protective mechanisms have developed by which skeletal muscle is preserved during periods of insufficient energy intake. In utilization of stored fuels, shifts away from glucose and toward adipose fat stores are necessary for protein sparing. Figure 32-15 illustrates the general scheme of metabolism during prolonged catabolic periods.