Constitutional, Somatic Genetic and Environmental Aspects of the Phenotypic Diversity of Aging in Human Subjects

Abstract

For the case of a large number of parameters, gerontologists typically report an increase in variance as a function of age. This is particularly true when the parameter of interest is subject to physiological stress, thus testing the degree to which the organism can maintain homeostasis. Such observations are so common that the phenomenon could be a candidate for one of the few “laws” of gerontology so far uncovered. Like all laws, there are exceptions, the best example of which is the loss of accommodation of the ocular lens as a function of age in human subjects. As one approaches age 50, the lens has already lost most of its mobility, with very little interindividual variation, much less than what is observed during the early phases of life (Friedenwald, 1952; Ponten, 1977). Most such studies of individual variations are cross-sectional in design. The question can therefore be raised that the observed differences may derive, at least in part, from a cohort effect—some differential environmental experience of the contrasting age groups. Therefore, longitudinal studies of individual variations during aging should be pursued whenever feasible. Unfortunately, such studies are rare, although we can expect to learn a great deal from ongoing investigations such as those of the Baltimore Longitudinal Study of the National Institute on Aging (Shock et al., 1984).