Simulating nitrogen metabolism and urinary urea nitrogen: Creatinine ratios in ruminants

Northern cervids have adapted to a seasonal reduction in food quality, quantity, and availability in winter. A negative energy balance forces these animals to catabolize fat and protein reserves to survive. Chemical analysis of urine collected in snow from free-ranging animals and controlled experiments with captive animals show that the urinary urea nitrogen:creatinine (UN:C) ratio is an indicator of nutritional restriction. We added a simulation model of nitrogen metabolism to a previously developed simulation model of energy metabolism and used this model to predict urinary UN:C ratios. Predicted urinary UN:C ratios, nitrogen partitioning, and body mass changes were similar to measured values from wild and domestic ruminants. Predicted urinary UN:C ratios were not sensitive to small changes in parameter values related to nitrogen metabolism, but were sensitive to predicted creatinine production and to the predicted fraction of urine nitrogen composed of urea nitrogen. As body condition declined, the predicted urinary UN:C ratio increased because catabolism of body protein increased the concentration of urea nitrogen in the blood. The increased concentration of urea in blood decreased the recycling rate of urea nitrogen, and consequently increased excretion of nitrogen in the urine. About half of the urea nitrogen excreted in urine is derived from nitrogen that has been recycled until body fat declines to about 3% of ingesta-free weight, when nitrogen from catabolism of protein increases in importance. Periodic severe nutritional restriction, analogous to severe winter storms, increased the predicted urinary UN:C ratio. The model improves our understanding of the physiological basis for changes in urinary UN:C ratios as body condition declines and during periods of severe food restriction.