Measurements of delta15N of consumers are usually higher than those of their diet. This general pattern is widely used to make inferences about trophic relationships in ecological studies, although the underlying mechanisms causing the pattern are poorly understood. However, there can be substantial variation in consumer-diet delta15N enrichment within this general pattern. We conducted an extensive literature review, which yielded 134 estimates from controlled studies of consumer-diet delta15N enrichment, to test the significance of several potential sources of variation by means of meta-analyses. We found patterns related to processes of nitrogen assimilation and excretion. There was a significant effect of the main biochemical form of nitrogenous waste: ammonotelic organisms show lower delta15N enrichment than ureotelic or uricotelic organisms. There were no significant differences between animals feeding on plant food, animal food, or manufactured mixtures, but detritivores yielded significantly lower estimates of enrichment. delta15N enrichment was found to increase significantly with the C:N ratio of the diet, suggesting that a nitrogen-poor diet can have an effect similar to that already documented for fasting organisms. There were also differences among taxonomic classes: molluscs and crustaceans generally yielded lower delta15N enrichment. The lower delta 15N enrichment might be related to the fact that molluscs and crustaceans excrete mainly ammonia, or to the fact that many were detritivores. Organisms inhabiting marine environments yielded significantly lower estimates of delta15N enrichment than organisms inhabiting terrestrial or freshwater environments, a pattern that was influenced by the number of marine, ammonotelic, crustaceans and molluscs. Overall, our analyses point to several important sources of variation in delta15N enrichment and suggest that the most important of them are the main biochemical form of nitrogen excretion and nutritional status. The variance of estimates of delta15N enrichment, as well as the fact that enrichment may be different in certain groups of organisms should be taken into account in statistical approaches for studying diet and trophic relationships.