Sequential measurements of body density or total body water yield inaccurate estimates of body composition change unless the proportions of protein, water and mineral in lean tissue remain constant. Since rates of change of bone mass in adults are likely to be small compared with those of fat or protein mass, volume and total body water measurements can be combined in a three-compartment model to estimate fat and protein changes assuming constant mineral mass. This new model minimizes the bias inherent in estimates of fat or protein changes made from separate density or total body water measurements.
