Human remains from the Mesolithic site of Yuzhniy Oleniy Ostrov in Karelia, Russia, exhibit mean collagen δ¹⁵N values of 16.1‰ and mean δ¹³C values of -20.0‰, indicating a diet dominated by freshwater aquatic resources and high protein intake with minimal contribution from terrestrial game, as confirmed by multi-proxy isotopic analysis of collagen, enamel apatite, and compound-specific amino acids.

When people eat mostly fish and little fat or plants, their bodies break down large amounts of protein for energy. This produces excess nitrogen, which is flushed out as urine with lighter nitrogen atoms, leaving heavier nitrogen atoms behind in the body. Those heavy atoms get built into collagen in bones and teeth. At the same time, the carbon from the fish protein becomes the main source of carbon in collagen because there isn't enough fat or plants to dilute it. The amino acids in the fish protein keep their original chemical signature, which matches the fish and not land animals, proving the fish came from freshwater sources.