Measuring fat and fat-free mass: clinical significance and limitations

Measuring body composition is part of clinical nutritional assessment. Simultaneous measurements of fat mass, and fat-free mass, or, preferably, the components of fat-free mass, will yield the most clinical information, but may be limited by laboratory availability. In obese subjects, measurements of fat mass and fat distribution is complemented by a simultaneous assessment of cell mass or total body protein, since management will involve following changes in all of these parameters. Similarly, in osteoporosis, assessment of fat mass and distribution at the same time as bone mass and density, where hormonal replacement therapy has been instituted is a necessary pan of the total management of the patient. Since fat distribution has been recognised as an important indicator of health risk, it has become increasingly important to know how to quantify risk and risk change in individuals with abdominal fat distribution. Little information is available about this, or the risks peculiar to peripheral fat distribution. As compartmental models of body composition become more complex, changes in these compartments in various illnesses are becoming clearer. The use of IVNAA for measuring total body protein changes in haemodialysis, and alcoholic cirrhosis, and the recent development of multifrequency BIA to assess extracellular water provide good examples. Although many hospitals clinics now have DEXA and BIA available, body composition laboratories per se are still a rarity, limiting clinical measurements to anthropometry. The errors involved in measurement using the various techniques mentioned still do not allow assessment of short-term changes in fat mass, or the components of fat-free mass.