The nitrogen isotope ratio of glutamic acid relative to phenylalanine in animal tissues reliably correlates with trophic position in food webs, with glutamic acid showing consistent 15N enrichment of 7–8‰ per trophic level while phenylalanine remains stable, enabling quantitative estimation of an organism's position in aquatic and terrestrial food chains.

When animals eat other organisms, their bodies break down proteins and reuse the amino acids. One amino acid, glutamic acid, gets altered in a way that removes lighter nitrogen atoms and keeps heavier ones, making it progressively heavier with each step up the food chain. Another amino acid, phenylalanine, stays unchanged because its structure doesn't allow this kind of alteration. This difference lets scientists measure how high up the food chain an animal feeds by comparing the heaviness of these two amino acids in its tissues.