Body composition analysis must divide the whole into component parts (Fig. 1). Depending on the scientific question, those parts are different. They might be the chemical elements, e.g., carbon, hydrogen, oxygen, sodium, etc.; they might be anatomical entities, e.g., intestinal tract, liver, muscles, skeleton, etc.; or they might be defined by cellular location, e.g., cell mass, extracellular space, extracellular water, etc. Of pragmatic interest to animal scientists is the division of the eviscerated body or carcass into skeletal muscle, fat, bone, and sometimes skin. Dissection of the carcass into these components is a viable but laborious approach for large animals. The water, protein, fat, and mineral content of the body or carcass is an approximation of the muscle, fat, and bone mass. Carbohydrate is a very small percentage of the total and usually is not analyzed. The simplest compartmen-talization of the body or carcass is into fat mass plus fat-free mass. This two-compartment model can be obtained by underwater weighing using estimates of the density of the two compartments.
Extensive reviews of body composition are available although they usually are slanted toward a single species, e.g., humans or pigs. Techniques have been developed to approximate body composition without the laborious procedures necessary for chemical composition. The fat-free mass (muscle plus bone) may be approximated using 40K determination of intact animals, assuming that ions, including potassium, are primarily present in tissues with a high water concentration, e.g., muscle, bone, internal organs, but are present at very low concentrations in fat. Some methods determine the water or ionic compartment to approximate muscle mass using techniques based on the water space or electrical conductance (bioelectric impedance). Other techniques employ imaging by ultrasound, X-ray based computerassisted tomography, or magnetic resonance imaging. The latter two techniques can evaluate internal organs in an intact animal, as well as the muscle, fat, and bone mass. Most of these techniques can be used with a live animal, although the imaging techniques require anesthesia to eliminate movement.
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