In the "flow" phase of severe injury, mobilization of body fat and protein sources are accelerated to meet increased demands. Fat mobilization usually exceeds the need for oxidative substrates and the leftover fatty acids are re-esterified, resulting in a "futile" TG/FFA cycle. This contributes to increased energy expenditure and provision of nutrients may alter the activity of this cycle. We measured in nine adult (age 40 +/- 7 years, weight, 80 +/- 4 kg) severely traumatized (ISS 32 +/- 4) patients, the resting energy expenditure (REE) and net fat oxidation rate (NFO) by indirect calorimetry and whole body lipolysis rate (WBLR) by a two-stage, primed-constant infusion of glycerol. Fasting postinjury kinetic studies were performed within 48-96 hours after admission when the patients were receiving saline without calories or nitrogen. Glucose-based nutritional therapy was then started and continued for 5 to 7 days. The kinetic measurements were repeated after the nutritive solutions were replaced by normal saline for 12 to 15 hours to achieve a postabsorptive state. Trauma elicits an accelerated rate of fat mobilization with increased TG/FFA cycle activity. Adequate nutritional support for 5 to 7 days tends to reduce but cannot normalize the lipid metabolism. In acute trauma 47 +/- 7% of the mobilized fat was recycled to triglyceride and this is increased to 54 +/- 9% after providing nutrition. The energy cost of this cycling was 38 kcal/day, which is five times that seen in normals and corresponds to 1.34% REE.