Isotope techniques in the measurement of human body composition.

Most of the papers in this symposium are concerned with dynamic studies: isotopic tracers may provide information about the metabolism of labelled molecules within the living body which cannot be obtained by any other means. However, in the measurement of body composition, isotope techniques are in general less powerful. If you wish to know how much water, protein, fat or mineral a whole body or a particular tissue contains the most accurate way of obtaining the answer is to subject the body or tissue to the appropriate chemical analysis. With small animals this is a perfectly practicable procedure: for example one can determine the total energy content of a rat by direct bomb calorimetry (Miller & Stock, 1969). In the measurement of human body composition, however, such an approach is obviously not acceptable, so the total energy content of a living man must be determined by less direct, and consequently less accurate, methods. Before going on to consider the place of isotope techniques in the estimation of human body composition, it is useful to review briefly the information which has been derived from the chemical analysis of a small number of human adult cadavers. Table I summarizes the results obtained by Mitchell, Hamilton, Steggerda & Bean (1945), Widdowson, McCancc & Spray (195 I), Forbes, Cooper & Mitchell (1953) and Forbes & Lewis (1956) from the analysis of six adult bodies. Table z shows the composition of some human tissues, determined by direct chemical analysis (Dickerson & Widdowson, 1960; Widdowson & Dickerson, 1960). The composition of the whole bodies shown in Table I is given in relation to fat-free weight, and on this basis there is quite good agreement between the analyses of bodies