Mohammed T. Abou-Saleh

Principles and Practice of Geriatric Psychiatry.Editors: Professor John R. M. Copeland, Dr Mohammed T. Abou-Saleh and Professor Dan G. BlazerCopyright & 2002 John Wiley & Sons LtdPrint ISBN 0-471-98197-4 Online ISBN 0-470-84641-013Chronological and Functional AgeingJack M. Guralnik and David Melzer*National Institute on Aging, National Institutes of Health, Bethesda, MD, USA,*Department of Community Medicine, Institute of Public Health, University of Cambridge, UKChronological age is the major descriptor by which aging isdefined. In an effort to characterize the dynamics and variabilityof the aging process, recent approaches have used measures offunctional aging to reflect the observation that individuals mayfunction at a level above or below that expected for theirchronological age. The relationship between chronological andfunctional age has implications for major questions of agingresearch and public policy. Do humans have a fixed lifespan? Howcan we maximize functioning for any given age? How is functionalaging associated with vulnerability to disease? Should retirementbe mandatory at a certain chronological age or should functionalage play a role in this decision?CHRONOLOGICAL VS. FUNCTIONAL AGINGChronological age is defined as time since birth; its effect isanalyzed in virtually all health studies and indeed in nearly allareas of human research. Although chronological aging isstrongly associated with mortality and nearly all diseases, itshould not be viewed as an etiologic factor. It is rather a proxy fornumerous factors that change or accumulate over time, such ascumulative exposure to toxins and trauma, changes in hormonelevels, immunological defenses and genetic repair mechanisms. Agreat deal of work has gone into identifying factors that explainthe relationship between chronological age and susceptibility todisease and dysfunction, but much of this relationship remainsunexplained. Identifying chronological age as a risk factor may beof little value, not because age is not strongly related to diseaseincidence but because an intervention to change one’s age is notavailable. For developing prevention strategies, however, understandingan age association may be important, such as the findingthat bone loss accelerates for 10–15 years after menopause, slowsfor a number of years, and again accelerates after age 75.A number of different concepts, including functional age,biological age and biomarkers of aging, have been developedbased on the observation that physiological measures andfunctional performance show a range of values in a populationof a given chronological age. Functional age is a concept that restson the premise that a measure other than chronological age couldbetter reflect one’s position in the aging process. Althoughbiological and functional age have been defined differently, theyare frequently used interchangeably. In contrast to chronologicalaging, which occurs at a universal fixed rate, functional aging hasbeen termed ‘‘non-chronological’’ because its rate may accelerateor decelerate and, in fact, functional age may be greater or lessthan one’s chronological age.The concept of biomarkers of aging is particularly appealingbecause it implies that there are biological measures that reflectthe rate of aging and that successful interventions on the agingprocess would have an effect on these markers. It is unclear at thistime whether there is an underlying biological state of aging thatcan be summed up as a single number that indicates how far alongin the aging process the individual has progressed. It has beenargued that aging is a complex, uncoordinated phenomenon thatcan not be summed up in such a way 1 . Others believe that,although a well-validated set of biomarkers does not currentlyexist, there may be techniques developed that can work well asgeneral indicators of biological age 2 . The requirements of abiomarker are not that it simply be different in persons or animalsof different ages but that, in a group of subjects of the same age, ithas a distribution of values that relates to other age-sensitivetraits, such as longevity 2 .Much of the work done on functional aging has focused onphysiological changes that are part of normal human aging.Another aspect of functional aging, which may be termed‘‘functional health status’’, assesses functioning at the level ofthe whole older person, describing how that person functions indaily life. Functional health status has been found to be related tochronological age, disease and a variety of other modifyingfactors. Measures of functional health status have also provedvaluable for clinical and health services research 3 . The USA’snational goal to increase the span of disability-free life exemplifiesthe high level of interest in the measurement of functional healthin recent years 4 .FUNCTION VS. DISEASE IN CHARACTERIZINGOLDER PERSONS AND OLDER POPULATIONSUnderstanding the functional aspects of aging has been animportant part of geriatric medicine for several decades. In themedical model of disease, the clinician gathers symptoms andsigns, makes a diagnosis, and bases the therapeutic approach onthis diagnosis. Complementing this disease-orientated approach,functional assessment provides an understanding of the impactand consequences of the older person’s disease or diseases, givinginformation on level of independence and prognosis, as well ashealth care, rehabilitation and social needs. As aging research hasincreased in recent years, the functional approach has played animportant role in its agenda.Although normal physiological changes with aging may havean impact on the older person, the far greater functional impactcomes from the effects of disease. A framework that represents thePrinciples and Practice of Geriatric Psychiatry, 2nd edn. Edited by J. R. M. Copeland, M. T. Abou-Saleh and D. G. Blazer&2002 John Wiley & Sons, Ltd