Dilution techniques for body composition analysis are based on the principle that water occupies a relatively fixed fraction of the fat-free mass. This technique involves the introduction (e.g., orally, intravenous injection) of a known amount of a tracer that will then equilibrate throughout a given compartment (e.g., total body water) in the body. The concentration of the (nontoxic) tracer is measured in a sample of the compartment, assuming that the tracer has the same distribution volume as the compartment, and in the case of water, is exchanged by the body in a similar manner. This approach can be applied to animals of wide-ranging body size. To extrapolate from total body water to other compartments of body composition such as fat or lean mass, a validation of the relationship to reference values for the particular species and physiological state is required. The primary reference method for the indirect techniques in (farm) animals is chemical analysis or, in some cases, total body dissection.
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