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.