The relationship between dietary protein content, body condition, and Δ 15 N in a mammalian omnivore

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 (δ N) as an indicator of catabolism, relationships between protein intake, body condition, and tissue δ N 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 δ N. 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 (Δ N) between blood plasma-diet and liver-diet did not differ between diet treatments. In contrast, Δ N for hair decreased while Δ N 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 δ N values appear to be the most useful in assessing the nutritional state of animals. Our combination of direct measurements of body condition with δ N 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 Δ N, thus refining the application of δ N as a tool for assessing nitrogen balance in wild animals.