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

Rationale Ecologists increasingly determine the delta N-15 values of amino acids (AA) in animal tissue; source AA typically exhibit minor variation between diet and consumer, while trophic AA have increased delta N-15 values in consumers. Thus, trophic-source delta N-15 offsets (i.e., Delta N-15(T-S)) reflect trophic position in a food web. However, even minor variations in delta N-15(source AA) values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDFT-S). Diet digestibility and protein content can influence the delta N-15 values of bulk animal tissue, but the effects of these factors on AA Delta N-15(T-S) and TDFT-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 delta N-15 using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), and were also hydrolyzed into free AA that were analyzed for delta N-15 using gas chromatography-combustion-IRMS. Results As dietary protein increased, Delta N-15(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 Delta N-15(T-S) and on TDFT-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 Delta N-15(Consumer-Diet) values. However, the similar responses of Delta N-15(T-S) and of TDFT-S to diet variation suggest that if diet samples are available, Delta N-15(T-S) accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Delta N-15(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).

Automated summary

Ecologists determine the delta N-15 values of amino acids in animal tissue to reflect trophic position in a food web. The effects of dietary protein on Delta N-15(T-S) and on TDFT-S varied by amino acid but were consistent between variables. High dietary protein may result in greater Delta N-15(Consumer-Diet) values in bulk muscle tissue.