Moving Beyond Static Body Composition Paradigms to Assessments of Change, Plasticity, and Function

Body weight is one of the most commonly and easily performed measurements in a physician’s office. Rapid weight loss provides clear grounds for concern, yet the interpretation of stable or gradually changing weight and its relevance to older individuals’ future health status represents a far more complex clinical issue. As for other important clinical questions in geriatric medicine, research findings relating body weight to clinical outcomes in younger populations cannot be simply transferred to older adults. For example, aging-related losses in lean muscle mass, especially when accompanied by modest gains in fat mass, may take place even in the absence of changes in total body weight. Furthermore, changes in body composition in older persons not necessarily associated with weight loss may predict poor outcomes, including disability and death. Thus, it is not surprising that lower and upper extremes in body weight have emerged as risk factors for frailty. As a result, many unresolved questions remain, and no formulas or tables exist that would permit the clinician to confidently recommend ‘‘optimum’’ target weights to individual older patients. Technologies designed to measure body composition offer the ability to identify older adults whose lean muscle, fat, or bone mass lies outside of population-based norms, but with the exception of dual energy X-ray absorptiometry (DXA)-derived measurements of bone mineral density (BMD), assessments of body composition have failed to affect everyday clinical geriatric practice. DXA was first developed to measure bone mineral content (BMC), yet even though it has also been validated as a reliable research tool for measuring fat and lean mass in older adults, clinicians do not use it for these purposes, and even in the case of BMD, growing evidence indicates that, with aging, the importance of low bone mass as a predictor of a hip fracture diminishes, and other risk factors such as a history of a previous fracture, difficulty rising from a chair without use of arms, poor depth perception, use of longacting benzodiazepines and recent weight loss begin to outweigh it. Efforts at expanding older adults’ overall ‘‘healthspan’’ and functional independence through clinically realistic strategies designed to prevent or postpone disability lie at the core of modern geriatric practice. The article by Koster and colleagues addresses many of these issues from the perspective of a 7-year prospective observational cohort study involving nearly 3,000 older adults enrolled in the Health, Aging and Body Composition (Health ABC) Study. Koster and colleagues evaluated the ability of physical fitness to predict subsequent changes in body composition and muscle strength. The observation that physical fitness (as measured according to ability and time needed to complete a 400-m walk) is related to differences in body composition and muscle strength is interesting but not particularly surprising. The major novel contribution of this study lies in the investigators’ capacity to provide longitudinal data that, when stratified according to physical fitness, provides a dramatic illustration of the tremendous physiological variability and plasticity that different subsets of older adults experience as they age. Not only is physical fitness associated with current and future differences in body composition, but analysis of the differences in the longitudinal changes involving specific body composition parameters suggests the presence of significant variability in the nature of the specific physiological factors that may promote or hinder homeostatic responses, thus affecting the maintenance of function in different subsets of older adults. Moreover, because a long-distance walk test could be incorporated into a typical outpatient clinical encounter, such an approach might ultimately be used to offer older adults more-individualized interventions. These could be selected in a manner designed to target specific populations based on knowledge gained regarding the nature of the underlying physiological mechanisms that place those particular individuals at a greater risk of future disability. Aging is associated with a large number of now welldefined changes in body composition. These involve alterations in relative or absolute amounts of tissue, as well as qualitative changes that affect tissue quality and performance. Some changes, such as mild age-related accumulations of abdominal fat or modest declines in femoral bone mass, become so common in late life that they are typically viewed as representing normal or usual aging, leading in some cases to the development of age-adjusted nomograms. Other categories of altered body composition have been viewed as being more pathological in nature. Examples include quantitative abnormalities involving excessive fatty deposits in regions (e.g., abdomen) that may occur at any age and qualitative abnormalities with de novo fatty infiltration (e.g., skeletal muscle) that are seen in aging-related sarcopenia and specific categories of muscle diseases. With aging, there is a fat redistribution in which fat progressively infiltrates tissues such as pancreas, muscle, DOI: 10.1111/j.1532-5415.2009. 02687.x