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Males of king penguins (Aptenodytes patagonicus) naturally fast during one month at the beginning of their breeding cycle in the sub-Antarctic islands. Previous qualitative data have shown that this species adapts to prolonged fasting by mobilizing fat stores and minimizing protein loss and that this strategy ends with a progressive increase in protein utilization. In the present study, the quantification of nutrient utilization from body composition of captive birds indicates that, during the phase of protein conservation, 93% of the energy produced derives from the oxidation of fat stores, body protein accounting for the remainder (7%). Tissue composition analysis shows that integument (feathers, skin and subdermal fat) is the main lipid source (65% of the fat loss) during this period, and that pectoral muscles provide the majority of body protein (57% of the total loss). If the fast is prolonged until a body mass below 10 kg is reached, there is a progressive four-fold increase (from 1 68 to 6.50 gN/24h) in nitrogen excretion, together with a progressive exhaustion of fat stores. This shift in fuel metabolism is not a direct consequence of total lipid depletion, because 22% of the initial fat content still remains when proteins are no longer spared. During this later metabolic phase, protein is not only provided by pectoral muscles (71% of the loss), but also by hindlimb muscles (13%), and there remains only 2% of the initial amount of lipid in the integument at the end of the fast. Total energy expenditure is close to the fasting basal metabolic rate during the phase of protein conservation (2.52 W/kg), but it increases by 33% (3.36 W/kg) during the phase of protein wasting. This difference is probably due to a rise in locomotor activity, that is interpreted as reflecting a stimulation of food foraging behaviour before the lethal depletion of nutrient reserves.