| Summary: | The principle objective of this investigation was to develop an understanding of the biochemical and physiological response of free-ranging adult polar bears (Ursus maritimus) to prolonged fasting. A body condition index was developed from two measures, total body mass and straight-line body length, and was used as a covariate in the analyses of all other data. Protein and amino acid catabolism and urea synthesis were significantly lower in fasting bears when compared to feeding bears, and in fat bears when compared to lean bears. The inference from these results is that the energy metabolism in both states (fasting and fat) is one in which lipid is the predominant fuel for energy and nitrogen is conserved. Nutritional state (feeding versus fasting) had no significant effect on the plasma concentrations of non-esterified fatty acid, glycerol, and ketone bodies, or on the plasma ratio of acyl-carnitine to free carnitine. Furthermore, acetoacetate concentration was below the level of detection (
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