THE USE OF TRITIATED-WATER TO DETERMINE PROTEIN AND LIPID UTILIZATION IN FASTING BIRDS - A VALIDATION-STUDY IN INCUBATING GREAT-WINGED PETRELS, PTERODROMA-MACROPTERA

To obtain reference data for the estimation of energy metabolism from body mass (BM) loss in long-term fasting Procellariiformes, we determined the loss of water, lipid, and protein by analysis of body composition in great-winged petrels, Pterodroma macroptera, during the incubation fast. We also examined the validity of the use of total body water (TBW) loss alone to calculate protein and lipid loss. Body composition analysis showed that, during the major part of the 17-d fast, water, lipid, and protein contribute a constant 35.7%, 50.8%, and 13.5%, respectively, of BM loss. The energy content of lipid (adipose triglycerides) was found to be 39.4 kJ/g. The energy equivalent of BM loss was 22.3 kJ/g; 90% and 10% of energy originated from lipid and protein utilization, respectively. The 37.6% contribution of water loss to BM loss, estimated from measurement of TBW by using tritiated water, was similar to that obtained by direct analysis of body composition. The loss of protein and lipid was estimated from TBW loss as determined by using tritiated water, with the assumption that water and protein are lost in direct proportion to their content in lipid-free nonbony tissues (LFM). Calculations were based on the finding from analysis of body composition that the percent water content of LFM from which protein is catabolized remains steady during the fast (72.9%). Furthermore, the dry LFM lost is greater-than-or-equal-to 95% protein. This yielded protein and lipid losses, and an energy equivalent of BM loss, identical to those obtained by direct analysis of body composition. A good estimate of BM loss composition in long-term fasting petrels may, therefore, be obtained from determination of TBW loss using labeled water. This could be extrapolated to other birds.