DETERMINATION OF TOTAL BODY FAT BY ABSORPTION OF AN INERT GAS; MEASUREMENTS AND RESULTS IN NORMAL HUMAN SUBJECTS *

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Abstract
A method is presented for the measurement of total body fat in humans by absorption of an inert, highly fat-soluble gas (cyclopropane) from a closed respiratory system. The amount of gas absorbed at equilibrium is primarily a function of the body fat content. In a group of normal to moderately obese subjects, body fat averaged 15.4 kg, about 12% lower than fat values for the same subjects calculated indirectly from total body water. Fat-free lean mass differed by 5.1% when calculated by the two methods. The various uncertainties in the method are discussed and estimated. Since equilibrium of body tissues with cyclopropane in the closed respiratory system is prolonged for human subjects, observed absorption data were extrapolated to obtain the necessary equilibrium values. For extrapolation, a mathematical formulation was used which involves a sum of exponential terms suggested by generally accepted theories of inert gas absorption and transport. Various aspects of these theories are discussed particularly those relating to intertissue diffusion and to the perfusion of blood to body fat. The gas absorption technique enables measurement of total body fat directly and essentially independently of the size or condition of the non-fat portions of the body. It may be utilized, therefore, in conjunction with the measurement of total body water, to study changes in hydration of the fat-free lean mass.