Blood chemistry homeostasis during prolonged fasting in the northern elephant seal

Serum electrolytes, 3 enzymes, and 11 metabolites were monitored for 32-68 days in weaned, naturally fasting elephant seal pups. Serum glucose, urea nitrogen and creatinine levels declined as the fast progressed, whereas total protein, albumin, and globulin levels remained nearly constant. By contrast, triglycerides, cholesterol, uric acid, and bilirubin were quite variable and no definite trends were apparent. Alkaline phosphatase activity appeared to increase during fasting, while serum glutamic oxalacetic transaminase and lactate dehydrogenase remained fairly uniform. Comparisons of averaged blood chemistry values from singly and multiply sampled fasting pups to that of four nursing pups showed significant differences in the levels of blood urea nitrogen, creatinine, cholesterol, bilirubin, and albumin, but sampling uncertainty limited physiological interpretation. Electrolyte levels in all animals were maintained within narrow limits under all conditions with little interindividual variation. These results further document the remarkable homeostasis achieved during prolonged fasting in elephant seals and support the hypothesis that fat is the primary energy substrate during the protracted natural fasts characteristic of this species.