When scientists speak of stable isotopes in the context of diet reconstruction, they are referring to the specific ratios of naturally occurring isotope variants of carbon and nitrogen in the organic material of bone.

These ratios are not set at birth and maintained regardless of diet — they are continuously updated through the metabolic processes that build and rebuild bone tissue.

Bone collagen, which turns over gradually over the course of years, reflects the diet of the period preceding death rather than simply the diet at birth or at any single moment.

The carbon isotope ratio in bone collagen reflects the primary sources of dietary carbon.

Marine organic matter has a different carbon isotope signature than terrestrial organic matter, because the processes by which carbon moves through marine food webs are different from those in terrestrial food webs.