Relationships between plasma ketones and fasting duration in neonatal elephant seals.

Long-duration fasting in mammals can ultimately lead to stage three terminal starvation, which is characterized by depleted fat stores, a metabolic shift away from fat metabolism toward lean tissue catabolism, and a sharp decline in circulating levels of plasma fatty acids and ketone bodies. However, this biochemical shift has never been observed outside of the laboratory in a naturally fasting, nonhibernating mammal. In the current study, plasma levels of the ketone body D-beta-hydroxybutyrate (beta-HBA) were assayed in 10 Northern elephant seal pups during suckling and the postweaning fast and in 12 fasting adult seals. Plasma beta-HBA concentration in the pups was minimal during suckling (0.09 +/- 0.06 mM; n = 10) and began to increase immediately after weaning. The concentration rose until about 55 days into the fast (1.34 +/- 0.36 mM; n = 10) and then declined sharply. Within 10 days of this deflection point, the seal pups left for sea. By contrast, adult elephant seals showed consistently low levels of beta-HBA after several months of fasting (0.06 +/- 0.07 mM; n = 12). The data suggest that the duration of fasting in elephant seal pups may be determined, in part, by biochemical shifts that occur near the end of the fast and that the regulation of ketone concentration is different in fasting neonatal and adult elephant seals.