Flux of metabolic fuels during starvation in the rat

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Abstract
Rats were starved for 6 days to determine whether the N sparing observed during starvation in humans was also present in rats. The urinary N excretion decreased on the 1st day, probably due to metabolism of remaining dietary protein. From the 2nd day of starvation to the end of the starvation period, the urinary N excretion increased progressively. The hepatic glycogen stores were depleted at the end of the 1st day. The blood glucose concentration remained constant throughout the starvation period except for a 15% decrease on the 1st day. There was increased mobilization of lipid stores, starting on the 1st day, reflected by an increase in the blood free fatty acids, glycerol and ketone body concentrations. These metabolite concentrations began to increase on the 3rd and 4th day which probably reflected depleted fat stores since no visible body fat was observed by the 4th day. Apparently the rat does not spare body protein during starvation, probably because it depletes its glycogen and fat stores rapidly and must then depend on body protein as the major fuel for energy metabolism.