Metabolic response to fasting in 6-week-old Steller sea lion pups (Eumetopias jubatus)

Four Steller sea lions (Eumetopias jubatus) aged 6 weeks were fasted for 2. 5 d to determine how young pups mobilize energy reserves during short perio ds of fasting similar to those experienced in the wild. At 6 weeks of age, the pups lost 5.1 +/- 0.3% of their body mass during 2 d of fasting, with a n average daily mass loss of 0.7 +/- 0.1 kg.d(-1). Plasma blood urea nitrog en (BUN) concentration increased significantly from 3.0 +/- 0.1 mM, after a n overnight fast, to 4.8 +/- 0.5 mM, after 2.5 d of fasting. It is apparent that BUN levels are quickly depressed, since after only an overnight fast, these pups showed BUN levels 2- to 4-fold lower than those measured after the same pups, when 9 months of age, had recently been fed fish. Plasma ket one body (beta-HBA) concentrations of the 6-week-old pups increased signifi cantly from 0.32 +/- 0.08 to 0.42 +/- 0.08 mM between 0.5 and 1.5 d of fast ing. There was no significant change in mean plasma concentration beyond 1. 5 d, owing to variable individual responses to extended fasting. Plasma bet a-HBA levels at 9 months of age ranged from 0.07 to 0.18 mM. Six-week-old S teller sea lion pups showed blood chemistry consistent with metabolic adapt ation to fasting within 16 h but were unable to sustain a protein-sparing m etabolism for a prolonged period. The pups appeared to revert to protein ca tabolism after only 2.5 d of fasting. This infers a decrease in lipid catab olism that might be due to the depletion of available lipid resources.