Assessment of body composition: methods and controversies

摘要

Body weight can be subdivided into two or more physiologically distinct components. The traditional two compartment model divides body weight into the fat mass (FM) and the fat free mass (FFM). This model forms the basis of the majority of our current knowledge of body composition. The assessment of body composition in the form of FM and FFM provides valuable information about the physical and metabolic status of the individual. The FM can be considered to represent a calorie or energy storage depot. Conversely, the FFM represents the actual health of the animal. It is a heterogenous entity consisting predominantly of intracellular (ICF) and extracellular fluid (ECF), minerals, glycogen and protein. The FFM contains the body cell mass (BCM) whichis the metabolically active part of the body responsible for determining most of the resting energy expenditure. BCM encompasses those lean tissues most likely to be affected by nutrition or disease over relatively short periods. Furthermore, the FFM isgenerally. accepted as an index of protein nutrition and therefore changes in FFM over time are assumed to represent alterations in protein balance. Determination of body composition is essential for understanding how normal body composition changes withphysiological processes such as growth, aging, obesity, and disease. Knowledge of body composition also can be advantageous in the field of exercise physiology and athletic performance where an optimal FM:FFM ratio may maximize performance. Objective quantification of the proportion of FM to FFM can be used to assess nutritional status, monitor the effects of nutritional intervention and other therapeutic regimens. The primary clinical application of body composition analysis is for the evaluation of malnutrition. Obesity is the most prevalent form of malnutrition in veterinary medicine. Surveys suggest that 25 percent to 30 percent of dogs and cats presented to veterinary clinics are overweight. The significance of obesity pertains to its role in thepathogenesis of a variety of diseases and the ability to exacerbate pre-existing disease. The ability to accurately measure body fat will facilitate understanding the causes and effects of obesity and the response to weight reduction programs. Long termsuccessful weight reduction may be related to conservation of the FFM. Therefore it is important to determine if weight changes are accompanied by equal alterations in FM and FFM or if one tissue accounts for most of the overall change in body weight. Nutritional assessment also is crucial in the management of hospitalized patients. Protein-calorie malnutrition with depletion of the FFM and associated BCM can be associated with increased susceptibility to infection, impaired wound healing, decreased strength, immune dysfunction, pulmonary insufficiency, and increased morbidity and mortality. Methods of nutritional assessment including estimation of dietary intake, serum concentrations of albumin and immunological markers have been applied to the critically ill population, however, these markers can be affected by other disease processes and they do not capture the true realm or incidence of malnutrition. Therefore precise methods for the early assessment of nutritional status in critically iii patients are particularly important as medical and dietary interventions may be able to prevent or improve malnutrition in these patients. In addition, reliable serial monitoring of the body composition will allow evaluation of new therapies to enhance recovery. Numerous methods exist for the assessment of body composition. However techniques including densitometry, total body potassium, and neutron activation analysis are not readily available and the remainder of this discussion will focus on clinically relevant methods. The commonly utilized two compartmental model serves as the basis upon which initial body composition methods were developed. This model is dependent upon assumptions regarding the char