Metabolism of normothermic woodchucks during prolonged fasting

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The energy metabolism of hibernators has not been characterized for normothermic fasting, and our goal was to quantify oxidative fuel selection of non-hibernating woodchucks Marmota monax during prolonged food deprivation. Indirect calorimetry and nitrogen excretion measurements were used to assess changes in metabolic rate (VO2), fuel selection and composition of nitrogen wastes, as well as seasonal differences. For reference, matching experiments were also performed on rabbits. The results show that woodchucks have a higher metabolic rate in summer (271 micromol O2 kg(-1) min(-1)) than in spring (200 micromol O2 kg(-1) min(-1)) and that fasting-induced metabolic depression is only possible in summer (-25% in 14 days). The metabolic rate of rabbits is high at all times (383 micromol O2 kg(-1) min(-1)), but they show a more rapid depression in response to fasting (-32% in 7 days). Woodchucks have a naturally low reliance on proteins in the fed state (accounting for 8% VO2) in spring; 17% VO2 in summer; vs 28% VO2 in rabbits) and are able to decrease it even further during fasting (spring, 5% VO2); summer, 6% VO2; vs 20% VO2 in rabbits). This study shows that, apart from their notorious capacity for hibernation, woodchucks are particularly well adapted for normothermic fasting. Their ability to cope with prolonged food deprivation is based on a series of integrated responses eliciting deep metabolic depression and a rapid change in fuel selection to spare limited protein reserves. Information presently available on prolonged fasting suggests that such an ability for metabolic depression, possibly down to minimal levels still compatible with normothermic life, may be common among mammals. In contrast, the extreme protein sparing demonstrated in woodchucks is a unique metabolic feature of fasting champions.