Stable carbon isotopes in human tissues

Abstract

ON studying the distribution of ¹⁴C from nuclear weapon tests in a wide range of human tissues, it was observed that significant modification of ¹⁴C/¹²C isotopic ratio had occurred, largely as a function of variable carbon renewal rates^1,2. As a parallel exercise, ¹³C/¹²C isotope ratios have been assayed, ¹³C and ¹²C being the nonradioactive isotopes of carbon which are assumed to be unaffected by nuclear testing. In principle, however, it is feasible that the differing biochemistries and carbon renewal rates of various body tissues could induce measurable stable isotope ratio changes (fractionation) as a result of the discrepancy in nuclear mass between ¹²C and ¹³C. This inequality leads to a difference in basic thermodynamic properties manifested as zero point energy, diffusion coefficients and kinetics of reaction for ¹²C- and ¹³C-substituted molecules. Thus, in reactions at equilibrium or those kinetically controlled, preferential selection of either the light or heavy isotope, and hence ¹³C/¹²C ratio change, may occur. We report here the results of over 80 measurements on human tissue samples which have increased the data available on natural baseline isotope ratios. This is particularly relevant because of the increasing use of stable, rather than radioactive, tracers in metabolic studies.