The δ15N isotopic difference between methionine and phenylalanine consistently distinguishes nitrogen derived from terrestrial sources from nitrogen derived from aquatic sources in animals consuming both, with an offset of approximately 12‰.

When animals eat food, the nitrogen in methionine and phenylalanine stays unchanged no matter how many times the food moves up the food chain, but other amino acids change their nitrogen fingerprint during digestion. This happens because methionine and phenylalanine are not broken down in a way that removes nitrogen, so they keep the original nitrogen signature from the plants or algae at the start of the food chain. If the food came from land, the nitrogen has one fingerprint; if it came from water, it has a different one. The difference between these two amino acids is always about 12 parts per thousand, so scientists can tell if an animal ate mostly land-based or water-based food just by measuring these two molecules.