The relative proportions of certain isotopes in a sample of tissue, water, air, or other material. In biology the proportions of stable isotopes of common elements in organic matter, especially carbon, nitrogen, and oxygen, can provide useful clues about growing conditions, lifestyle, and metabolism of the organism from which the material was derived. Differences in the relative tissue abundance of the natural stable isotopes of carbon (i.e. 12C and 13C), oxygen (16O and 18O), and nitrogen (14N and 15N) reflect their differing availability in the environment or in the diet or some form of selective metabolism (see isotopic discrimination) by the organism. So, for example, human hair samples with different isotopic signatures can indicate the dietary preferences of the individuals. A relatively high 12C/13C ratio indicates that a person has consumed chiefly food derived from C3 plants (e.g. wheat, rice, and potatoes), which concentrate the lighter 12C isotope (derived from carbon dioxide in air), whereas a 12C/13C ratio closer to that of air indicates a diet dominated by C4 plants (e.g. maize) or food derived from them (e.g. corn-fed beef). Moreover, the ratio of 14N/15N in body tissue changes at successive steps in food chains, with the proportion of 15N increasing at each step. Therefore vegetarians have a greater 14N/15N ratio than meat eaters.
Subjects: Biological Sciences.