Carcass analyses revealed that the amount of water in the FFM is relatively constant at about 73%. Total body water (TBW) can be determined by dilution techniques. Dilution techniques are generally based on the equation:
C1 × V1 = C2 × V2 = Constant
where C is the tracer (deuterium oxide, tritium, or 18O water) concentration and V is the volume.
When a subject is given a known amount of a tracer (C1 × V1), which is known to be diluted in a given body compartment, the volume of that body com-partment can be calculated from the dose given and the concentration of the tracer in that compartment after equilibrium has been reached. Suitable tracers for the determination of TBW are deuterium oxide, tritium oxide, and 18O-labeled water. Other tracers can also be used, such as alcohol and urea, but they are less suitable because they are partly metabolized (alcohol) or because they are actively excreted from the body (urea) during the dilution period. After giving a subject the tracer and allowing around 3–5 hours for equal distribution throughout the body, determination of the concentration of deuterium in blood, saliva, or urine allows the calculation of TBW.
Alternatively, other tracers can be used, such as tritium oxide and 18O-labeled water, and the tracer can be given intravenously, which is advantageous when the subject has gastrointestinal disorders. The reproducibility of the method is 1–3%, depending on the tracer used and the analytical method chosen. From TBW, the FFM, and hence fat mass, can be calculated, assuming that 73% of the FFM is water:
BF% = 100 × (Weight − TBW/0.73)/Weight
The precision for estimations of body fat is about 3–4% of body weight. As with the densitometric method, this error is due to violations of the assump-tion used (i.e., that the relative amount of water in the FFM is constant and equals 73% of the FFM). In sub- jects with a larger than 73% water content in the FFM (pregnant women, morbid obese subjects, and patients with edema), the factor 0.73 will result in an overesti-mation of the FFM. A three-compartment model of the body that contains fat mass, water, and dry FFM has a lower bias than a two-compartment model. An overestimation of body fat by densitometry, for example because of a relatively high amount of water in the FFM, will be counteracted by an underestima-tion using the dilution method.