The energy cost of triglyceride-fatty acid recycling in nonobese subjects after an overnight fast and four days of starvation

The basal blood glycerol concentration was determined and the rate of glycerol turnover was assessed by a nonradioactive infusion technique in six healthy nonobese adults after an overnight fast and again after four days of total starvation. Simultaneously, estimates of total energy expenditure and net fat oxidation were made from measurements of oxygen consumption, carbon dioxide production, and urinary nitrogen excretion. The data were combined to provide quantitative estimates of the activity of the triglyceride/fatty acid cycle. The basal concentration of glycerol in venous blood rose from a mean value of 54 ± 8 μmol/L (SEM) before starvation to 154 ± 5 μmol/L on day 4 of starvation. Glycerol turnover rates correlated well with the basal blood glycerol concentration (r = .95) and increased from a mean value of 115 ± 17 μmol/min before starvation (equivalent to mobilization of about 3.95 kJ triglyceride/min) to 304 ± 20 μmol/min (equivalent to mobilization of about 18.41 kJ/min). The estimated rate of net fat oxidation was 3.00 ± 0.47 kJ/min before starvation and 4.00 ± 0.14 kJ/min on day +4 of starvation. The rate of triglyceride energy recycling or rate of deposition of triglyceride energy into fat stores was calculated from the difference in the rate of fat energy mobilization and the rate of energy released during net fat oxidation. The values were found to be 0.94 ± 0.26 kJ/min before starvation and 6.29 ± 0.54 kJ/min on day +4 of starvation. The mean energy cost associated with cycling of this amount of energy in the triglyceride/fatty acid cycle was calculated to be 0.018 kJ/min (0.36% of resting energy expenditure) before starvation and 0.12 kJ/min (2.49% of resting energy expenditure) after four days of starvation. It is concluded that although the activity of the triglyceride/fatty acid cycle increases considerably during starvation, the energy expended in driving the cycle is only a small proportion of total energy expenditure.