Department of Biological Sciences, Old Dominion University, Norfolk, VA, 23529, USA.
Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA.
Rapid Commun Mass Spectrom. 2021 Jun 15;35(11):e9073. doi: 10.1002/rcm.9073.
Ecologists increasingly determine the δ 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 N values in consumers. Thus, trophic-source δ N offsets (i.e., Δ N ) reflect trophic position in a food web. However, even minor variations in δ N values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF ). Diet digestibility and protein content can influence the δ N values of bulk animal tissue, but the effects of these factors on AA Δ N and TDF 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 bulk tissue δ N using elemental analyzer-isotope ratio mass spectrometry (EA-IRMS), and were also hydrolyzed into free AA that were analyzed for δ N using gas chromatography-combustion-IRMS.
As dietary protein increased, Δ N 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 N and on TDF varied by AA but were consistent between variables.
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 N values. However, the similar responses of Δ N and of TDF to diet variation suggest that if diet samples are available, Δ N accurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ N 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).
生态学家越来越多地确定动物组织中氨基酸（AA）的δ N 值；“来源”AA 在饮食和消费者之间通常变化较小，而“营养”AA 在消费者中具有增加的δ N 值。因此，营养源δ N 偏移量（即Δ N ）反映了食物网中的营养位置。然而，δ N 值的微小变化可能会影响代表营养步骤的偏移量的大小，这称为营养辨别因子（即 TDF ）。饮食消化率和蛋白质含量会影响动物组织的总δ N 值，但这些因素对哺乳动物 AA Δ N 和 TDF 的影响尚不清楚。
我们用（A）低脂肪、高纤维、低、中或高蛋白饮食或（B）高脂肪、低纤维、低或中蛋白饮食喂养圈养小鼠（Mus musculus）。用元素分析仪-同位素比质谱（EA-IRMS）分析小鼠肌肉和饮食蛋白质的总组织δ N，并将其水解成游离 AA，用气相色谱-燃烧-IRMS 分析其δ N。
随着饮食蛋白质的增加，两个实验中肌肉组织的总δ N 值略有下降；在低脂肪、高纤维饮食（A）中 AA 的Δ N 值增加；而高脂肪、低纤维饮食（B）中 AA 的Δ N 值保持不变或下降。饮食蛋白质对 AA 的Δ N 和 TDF 的影响因 AA 而异，但在变量之间是一致的。
与实验 B 相比，实验 A 中的饮食更难消化且包含更多的蛋白质。因此，实验 A 中的小鼠可能氧化了更多的 AA，导致更大的Δ N 值。然而，Δ N 和 TDF 对饮食变化的相似反应表明，如果有饮食样本，Δ N 准确地跟踪营养位置。如果没有饮食样本，这里呈现的模式为解释Δ N 值提供了依据。Pro-Lys 的营养源偏移量在不同饮食中没有变化，因此对于杂食动物来说可能比其他偏移量（例如 Glu-Phe）更可靠。
