BONE-MARROW FAT MOBILIZATION IN RELATION TO LIPID AND PROTEIN CATABOLISM DURING PROLONGED FASTING IN BARN OWLS

To assess the role of bone marrow fat in survival during a period of n egative energy balance, we investigated the relationship between the t ime-course of marrow fat mobilization and the metabolic states associa ted with body fuel utilization during a prolonged fast. In order to mi mic the winter fast of the barn owl (Tyto alba), captive birds were su bjected to fasts of various durations at 5 degrees C ambient temperatu re. Body mass and plasma metabolites were used to determine the metabo lic state at the end of fasting. Skeleton composition remained unchang ed throughout phase II of fasting, during which the birds essentially rely on lipid fuels. During the following phase III, characterized by an increase in net body protein breakdown, the lipid mass in skeleton marrow decreased sharply by 78%, concomitant with an increase of the b one water content. This marrow fat mobilization occurred in all parts of the skeleton. This observation supports the hypothesis that bone ma rrow fat is not only involved in local nutrition, but can also be used as a lipid reserve for total energy requirements. However, in contras t to other fat deposits, marrow fat is mobilized only during phase III of the fast, when the last shift from lipid to protein fuel metabolis m occurs. Thus, metabolic and/or hormonal changes associated with this transition could be involved in bone marrow fat mobilization. Lastly, our results suggest that the measurement of bone marrow fat can be us ed as an accurate index of the nutritional status (i.e. phase II or ph ase III) in barn owls.