1. Three methods of measuring changes in body fat were compared in seventeen patients undergoing a 2-week course of intravenous nutrition. Patients received all nutrition intraveneously at a steady rate of infusion, calculated to supply energy at a rate equal to 1.5 times the resting metabolic expenditure measured before feeding. Fat change was estimated from measurements of skinfold thickness, by isotopic methods (neutron-activation analysis and dilution of tritiated water) and by daily analysis of expired gases.
2. The mean (with 1 SEM) gain in fat over the 2-week period was 1.14 (0.30) kg for skinfold measurement, 0.53 (0.62) kg for isotopic determination and 1.29 (0.22) kg for expired-gas analysis. There were no significant differences between the mean gains in fat measured by the three methods.
3. The results show that expired-gas analysis is the most sensitive technique, measuring change in fat content with an estimated precision of 0.26 kg. The isotopic method is less sensitive, with a precision of 2.38 kg, but provides a detailed description of body composition. In contrast to these highly-specialized techniques, both of which have limited application, the simple technique of measurement of skinfolds occupies an intermediate position of sensitivity, with a precision for measuring change in fat content of 0.85 kg, and also has the potential to measure total body fat content.