Assessing biological age

As shown in Table 1, methods for assessing biological age in humans can encounter a number of practical difficulties, especially in the selection of individuals regarded as suitable candidates. In a large majority of earlier studies little attempt was made to control for potential variables, and particular confusion has surrounded the issue of whether or not self-recruited individuals should be prescreened for symptoms of disease (reviewed by Bittles and Brightwell (19).

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Table 1 Selection criteria influencing the assessment of human ageing

Table 1 Selection criteria influencing the assessment of human ageing

An extension of this problem is the validity of attempts to differentiate between so-called normal ageing and pathological conditions, such as atherosclerosis, type 2 diabetes mellitus, and degenerative arthritis, which are commonly seen at greater frequency with advancing age. Although a positive correlation clearly exists between the incidence of these disorders and increasing age, few specific causal mechanisms have been identified. With occult disease the task is even more difficult, especially among the elderly who frequently present with a wide spectrum of vague signs and symptoms.

Yet, where possible, for diagnostic and prognostic purposes it is highly desirable that the symptoms of ageing and disease should be separated, which in turn necessitates the establishment of meaningful 'normal' ranges throughout the human lifespan. Direct measurements on human subjects can be conducted as cross-sectional studies, to compare anatomical, physiological, and biochemical parameters in young and old subjects. Alternatively, longitudinal studies may be favoured with serial measurements conducted on given individuals over extended periods.

Both approaches have their limitations. For example, as indicated in Table 1, at what age should the studies begin? In cross-sectional studies can any real knowledge of the ageing process be gained by simply comparing young and old subjects, whose lives have been spent during different time-periods and under variable environmental conditions? From an ethical perspective, to what extent is it permissible to impose the potentially stressful test protocols that may be needed to demonstrate age-dependent changes in elderly subjects? For these reasons, and because of the often large individual variations in the phenotypes of elderly persons, studies directly conducted on human subjects may be of limited applicability in the assessment or definition of biological age.

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