This meta-analysis examines sources of variation in consumer-diet δ¹⁵N enrichment. δ¹⁵N of consumers is typically higher than that of their diet, a pattern used to infer trophic relationships. However, the underlying mechanisms are poorly understood. The study analyzed 134 estimates from controlled studies to test potential sources of variation. Results showed that ammonotelic organisms had lower δ¹⁵N enrichment than ureotelic or uricotelic organisms. Detritivores had significantly lower enrichment. δ¹⁵N enrichment increased with the C:N ratio of the diet, suggesting that a nitrogen-poor diet can have effects similar to fasting. Taxonomic classes like molluscs and crustaceans generally had lower enrichment. Marine organisms had lower enrichment than terrestrial or freshwater ones. The main biochemical form of nitrogen excretion and nutritional status were identified as key sources of variation. Variability in δ¹⁵N enrichment should be considered in statistical approaches for studying diet and trophic relationships. The study highlights the importance of understanding these variations to improve the accuracy of ecological inferences. The results suggest that δ¹⁵N enrichment varies significantly among organisms, depending on factors such as nitrogen excretion form, diet type, taxonomic class, environment, and food composition. This variation can affect the reliability of δ¹⁵N-based trophic level estimates. The study emphasizes the need for more research into the factors influencing δ¹⁵N enrichment to improve the application of stable isotope ecology in ecological studies.This meta-analysis examines sources of variation in consumer-diet δ¹⁵N enrichment. δ¹⁵N of consumers is typically higher than that of their diet, a pattern used to infer trophic relationships. However, the underlying mechanisms are poorly understood. The study analyzed 134 estimates from controlled studies to test potential sources of variation. Results showed that ammonotelic organisms had lower δ¹⁵N enrichment than ureotelic or uricotelic organisms. Detritivores had significantly lower enrichment. δ¹⁵N enrichment increased with the C:N ratio of the diet, suggesting that a nitrogen-poor diet can have effects similar to fasting. Taxonomic classes like molluscs and crustaceans generally had lower enrichment. Marine organisms had lower enrichment than terrestrial or freshwater ones. The main biochemical form of nitrogen excretion and nutritional status were identified as key sources of variation. Variability in δ¹⁵N enrichment should be considered in statistical approaches for studying diet and trophic relationships. The study highlights the importance of understanding these variations to improve the accuracy of ecological inferences. The results suggest that δ¹⁵N enrichment varies significantly among organisms, depending on factors such as nitrogen excretion form, diet type, taxonomic class, environment, and food composition. This variation can affect the reliability of δ¹⁵N-based trophic level estimates. The study emphasizes the need for more research into the factors influencing δ¹⁵N enrichment to improve the application of stable isotope ecology in ecological studies.