Figure 77-8 Use of metabolic fuels during fasting. The energy fuel substrate flux shown in terms of calories per day has been calculated from experimental data. Protein loss is equal to approximately 16.6 g of nitrogen. Aa, amino acid; ADP, adenosine diphosphate; ATP adenosine triphosphate; FFA, free fatty acid; Glc, glucose; KB, ketone body; RQ, respiratory quotient; Tg, triglyceride. (Adapted from Blackburn GL, Phinney SD: Lipid metabolism in injury. In Burke JF [ed]: Surgical Physiology. Philadelphia, WB Saunders, 1983, p 113.)