Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal.

Rationale: Ecologists increasingly determine the δ ¹⁵ N values of amino acids (AA) in animal tissue; "source" AA typically exhibit minor variation between diet and consumer, while "trophic" AA have increased δ ¹⁵ N values in consumers. Thus, trophic-source δ ¹⁵ N offsets (i.e., Δ ¹⁵ N _T-S ) reflect trophic position in a food web. However, even minor variations in δ ¹⁵ N _{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 δ ¹⁵ N values of bulk animal tissue, but the effects of these factors on AA Δ ¹⁵ N _T-S and TDF _T-S in mammals are unknown.
Methods: 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 bulk tissue δ ¹⁵ N using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), and were also hydrolyzed into free AA that were analyzed for δ ¹⁵ N using gas chromatography-combustion-IRMS.
Results: As dietary protein increased, Δ ¹⁵ N _{Consumer-Diet} slightly declined for bulk muscle tissue in both experiments; increased for AA in the low-fat, high-fiber diet (A); and remained the same or decreased for AA in the high-fat, low-fiber diet (B). The effects of dietary protein on Δ ¹⁵ N _T-S and on TDF _T-S varied by AA but were consistent between variables.
Conclusions: Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ ¹⁵ N _{Consumer-Diet} values. However, the similar responses of Δ ¹⁵ N _T-S and of TDF _T-S to diet variation suggest that if diet samples are available, Δ ¹⁵ N _T-S accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ ¹⁵ N _T-S values. The trophic-source offset of Pro-Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu-Phe).
(© 2021 John Wiley & Sons Ltd.)