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 negative energy balance, we investigated the relationship between the time-course of marrow fat mobilization and the metabolic states associated with body fuel utilization during a prolonged fast. In order to mimic the winter fast of the barn owl (Tyto alba), captive birds were subjected to fasts of various durations at 5 °C ambient temperature. Body mass and plasma metabolites were used to determine the metabolic state at the end of fasting. Skeleton composition remained unchanged 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 bone water content. This marrow fat mobilization occurred in all parts of the skeleton. This observation supports the hypothesis that bone marrow fat is not only involved in local nutrition, but can also be used as a lipid reserve for total energy requirements. However, in contrast to other fat deposits, marrow fat is mobilized only during phase III of the fast, when the last shift from lipid to protein fuel metabolism 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 used as an accurate index of the nutritional status (i.e. phase II or phase III) in barn owls. Accepted: 4 July 1996