The impact of protein quality on stable nitrogen isotope ratio discrimination and assimilated diet estimation

Abstract

Accurately predicting isotopic discrimination is central to estimating assimilated diets of wild animals when using stable isotopes. Current mixing models assume that the stable N isotope ratio (delta(15)N) discrimination (Delta(15)N) for each food in a mixed diet is constant and independent of other foods being consumed. Thus, the discrimination value for the mixed diet is the combined, weighted average for each food when consumed as the sole diet. However, if protein quality is a major determinant of Delta(15)N, discrimination values for mixed diets may be higher or lower than the weighted average and will reflect the protein quality of the entire diet and not that of the individual foods. This potential difference occurs because the protein quality of a mixed diet depends on whether, and to what extent, the profiles and amounts of essential amino acids in the individual foods are complementary or non-complementary to each other in meeting the animal's requirement. We tested these ideas by determining the Delta(15)N of several common foods (corn, wheat, alfalfa, soybean, and fish meal) with known amino acid profiles when fed singly and in combination to laboratory rats. Discrimination values for the mixed diets often differed from the weighted averages for the individual foods and depended on the degree of complementation. Delta(15)N for mixed diets ranged from 1.1 per thousand lower than the weighted average for foods with complementary amino acid profiles to 0.4 per thousand higher for foods with non-complementary amino acid profiles. These differences led to underestimates as high as 44% and overestimates as high as 36% of the relative proportions of fish meal and soybean meal N, respectively, in the assimilated mixed diets. We conclude that using isotopes to estimate assimilated diets is more complex than often appreciated and will require developing more biologically based, time-sensitive models.

Stable carbon and nitrogen incorporation in blood and fin tissue of the catfish Pterygoplichthys disjunctivus (Siluriformes, Loricariidae)

“…S1) were higher than the widely used average of 3.4‰ for this isotope for muscle tissue (Fry and Sherr 1984;Focken 2004;Fry 2007), but were not inconsistent with Δ 15 N tissue-diet (> 4‰) in herbivorous mammals (Robbins et al 2005) or fishes (Mill et al 2007). The major contributing factors to Δ 15 N tissue-diet are growth rate, dietary protein content, protein quality (i.e., the biological value, or how well the amino acid profile of the dietary protein meets the needs of the animal; Robbins et al 2005;Robbins et al 2010), and the dietary δ 15 N (Caut et al 2009;Martínez del Rio et al 2009). Herbivorous animals generally consume protein that is low-quality, and/or has low δ 15 N (Robbins et al 2005), and hence, herbivorous animals have larger Δ 15 N tissue-diet than carnivores.…”

Discrimination Factors for Stable Isotopes of Carbon and Nitrogen in Blood and Feathers from Chicks and Juveniles of the California Condor

“…however, we are not aware of any physical or biochemical processes that could lead to different trophic fractionations based solely on the minuscule absolute differences in isotope concentration between different types of food (Robbins et al 2010).…”

Experimental determination of stable carbon and nitrogen isotope fractionation between mangrove leaves and crabs

“…However, studies have shown high variability for the consumer-diet discrimination factors Δ 13 C and Δ 15 N with regard to specific tissues, taxonomic classes, habitat, diet isotopic ratios, protein quality of the diet, life-history level and other factors (e.g. Caut et al 2009, Robbins et al 2010, Boecklen et al 2011, Florin et al 2011. This indicates the importance of a careful choice of discrimination factors in food web studies.…”