Seasonal reductions in food availability may cause animals to catabolize endogenous tissue and the resulting loss of lean mass can hinder their ability to forage and reproduce. While several studies have considered nitrogen isotopes (δ15N) as an indicator of catabolism, relationships between protein intake, body condition, and tissue δ15N have not been assessed simultaneously in controlled conditions. We conducted a feeding experiment on laboratory mice (Mus musculus) to test the effects of low (5%) versus high (30-40%) dietary protein content on lean mass, fat mass, and tissue δ15N. This approach enables the distinction between use of exogenous and endogenous nitrogen, illuminating a framework of protein metabolism and tissue synthesis. As expected, lean mass and body fat were lower in mice fed low-protein diets. Nitrogen isotope discrimination (Δ15N) between blood plasma-diet and liver-diet did not differ between diet treatments. In contrast, Δ15N for hair decreased while Δ15N for muscle and RBC increased in the low-protein treatment. These patterns suggest that animals in negative nitrogen balance catabolize labile endogenous protein (e.g., muscle) to maintain vital tissues (e.g., liver) required to survive. Consequently, muscle and RBC δ15N values appear to be the most useful in assessing the nutritional state of animals. Our combination of direct measurements of body condition with δ15N analysis suggest how nitrogen isotopes can be better used as tracers of catabolic and anabolic activity by demonstrating connections between tissue-specific metabolic processes and Δ15N, thus refining the application of δ15N as a tool for assessing nitrogen balance in wild animals.
Keywords: Catabolism; Isotopes; Metabolism; Nitrogen; Nutrient-stress.