During fasting, animals rely on a mixture of fats, carbohydrates, and proteins that are derived solely from endogenous sources. The relative contributions of these metabolic fuels chiefly depend on the duration of the fast, but other factors including previous diet, environmental temperature, and activity level can modulate the fuel mixture. It has long been held that endogenous proteins are spared from catabolism until the final stages of prolonged fasting and contribute a significant proportion of energy once the other metabolic fuels have been depleted. However, evidence is mounting that protein is catabolized supplemental to fat metabolism under some circumstances. This has been shown in migratory birds that exhibit dramatic reductions in lean mass during flights. One hypothesis to explain this seemingly maladaptive metabolic strategy is that the catabolism and oxidation of protein in situ yields five times more metabolic water than that generated through fat oxidation alone. Recent support for this hypothesis is that birds benefit from such a strategy due to their uricotelic nature. However, it remains unclear whether ureotelic mammals would also employ this strategy. Here we test the protein-for-water hypothesis in resting laboratory mice subjected to water deprivation during fasting while we tracked rates of protein and lipid catabolism using endogenously incorporated C-13-leucine and C-13-palmitic acid. We found no differences in instantaneous leucine oxidation; however, cumulative differences in instantaneous leucine oxidation ultimately resulted in a higher total leucine oxidation after 72 h of fasting in water-deprived animals. We also found that lipid oxidation was 8% higher in the hydrated mice, but the difference was not significant presumably because of a concomitant reduction in metabolic rates of the water-deprived mice. Our results indicate that mammals do increase rates of protein catabolism during dehydration but to a lesser degree than birds. The ability of mammals to produce highly concentrated urine and their lower inherent rates of protein turnover apparently preclude mammals from taking full advantage of the protein-for-water strategy during fasting under dehydrating conditions.