Abstract
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.
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Acknowledgements
We thank Mauriel Curras Rodriquez, Jen Noble, Deborah Boro, Allyson Richins, Christopher Hughes, and Shannon Archuleta for their help mixing diets, feeding mice, analyzing mice in the QMR, and collecting tissues. We also thank Jeremy Roth and Jacob Fields for assistance with animal husbandry, and Dr. Kevin O’Hair for helping with dissections. Thanks to the UNM Center for Stable Isotopes, especially Drs. Nicu-Viorel Atudorei and Laura Burkemper for analytical assistance.
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KLH and SDN conceived the ideas and designed methodology, KLH performed analytical measurements, all authors analyzed the data, and KLH led the writing of the manuscript.
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Funding for this project was provided by a UNM Resource Allocation Committee grant to SDN.
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All applicable institutional and/or national guidelines for the care and use of animals were followed. Animal handling and husbandry was conducted with the approval of the UNM Institutional Animal Care and Use Committee (16-200492-MC).
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All body condition and tissue δ15N values are present in the manuscript.
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Communicated by Blair Wolf.
This study is the first to simultaneously assess protein intake, direct measurements of body condition, and tissue δ15N. Results reveal differences in tissue-specific catabolic and anabolic activities during protein stress that are not only informative on physiological level, but also provide insight into the interpretation of δ15N values in free-ranging animals.
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Hughes, K.L., Whiteman, J.P. & Newsome, S.D. The relationship between dietary protein content, body condition, and Δ15N in a mammalian omnivore. Oecologia 186, 357–367 (2018). https://doi.org/10.1007/s00442-017-4010-5
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DOI: https://doi.org/10.1007/s00442-017-4010-5