1. Accurately predicting isotope ratio discrimination is central to using mixing models to estimate assimilated diets of wild animals. This process is complicated when omnivores consume mixed diets because their discrimination is unlikely to be the weighted average of the various dietary constituents as occurs in current models.
2. We sought a basic understanding of how protein quality and quantity determine Δ15N and Δ34S in mammals and birds. Dietary protein is the primary source of both elements in many plants and animals. Low protein quality and high protein content have the potential to increase Δ15N by increasing protein turnover.
3. Protein quality, defined as the relative amount of the most limiting amino acid, accounted for 87–90% of the variation in Δ15N when mammals and birds consumed plant matter and mixed diets of plants and animals with protein of intermediate quality and quantity. However, foods containing relatively large amounts of high quality protein (i.e. vertebrate flesh) and foods with exceptionally low quality protein (e.g. lentils, Lens culinaris) had disparate nitrogen discriminations relative to what would be predicted from protein quality alone.
4. Supplementation of plant and animal diets with nitrogen-free carbohydrates and fats to reduce protein quantity did not reduce Δ15N in three plant-based diets fed to laboratory rats, but reduced Δ15N in two of three meat diets with >50% protein.
5. Δ34S was weakly correlated with Δ15N (R2 = 0·48) but was highly correlated with dietary δ34S (R2 = 0·89). Because methionine, a sulphur amino acid, was the most limiting amino acid in all diets, sulphur should be highly conserved as indicated by the lack of any change in Δ34S when diets were supplemented with carbohydrates and fat to both provide additional energy and reduce protein content.
6. Predictive equations incorporating both protein quality and quantity accounted for 81% of the variation in Δ15N and offer the opportunity to create more realistic mixing models to accurately estimate assimilated diets for omnivores.