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Ecologists increasingly determine the δ¹⁵N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ¹⁵N values in consumers. Thus, trophic‐source δ¹⁵N offsets (i.e., Δ¹⁵N_T‐S) reflect trophic position in a food web. However, even minor variations in δ¹⁵N_{source AA} values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF_T‐S). Diet digestibility and protein content can influence the δ¹⁵N values of bulk animal tissue, but the effects of these factors on AA Δ¹⁵N_T‐S and TDF_T‐S in mammals are unknown.

We fed captive mice (Mus musculus) either (A) a low‐fat, high‐fiber diet with low, intermediate, or high protein; or (B) a high‐fat, low‐fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for …