outcomes to establish a new index of allostatic load is a high priority for future research.

Beyond the above classification system and its attendant measurement problems are other methodological issues that warrant attention. First, the measures incorporated in the current operationalization of allostatic load are all based on resting level, or static, assessments. As a collectivity, they are supposed to represent the physiological signature of cumulative psychosocial adversity. This is only one aspect of allostatic load (McEwen and Seeman, 1999). Equally important is abnormal transient response of primary mediators to specific challenges because the repeated overactivity of a physiological system is a source of allostatic load (McEwen, 1998). Impaired transient response to acute challenge should be part of the physiological signature of cumulative adversity. Protocols in animals and humans are needed to document the phenomena and lead to an index of allostatic load incorporating both resting level and transient response measures.

A second missing feature of current operationalization of allostatic load is the specification and implementation of a defensible set of indicators for children. The extant indicators were developed for and calibrated on a longitudinal study of the elderly (Seeman et al., 1997). The same indicators have been used on a midlife sample (ages 59-60; Singer and Ryff, 1999), and a slightly modified operationalization has been utilized in the Normative Aging Study (Kubzansky et al., 1999). A comparison of the distribution of responses across these samples suggests that an age-specific specification of allostatic load would be appropriate. Since a characterization must involve physiological processes that evolve over the life course, the development of a whole-life animal model of allostatic load is likely to yield major payoff in developing the human analog. This would entail setting up experimental protocols that vary the frequency and types of psychosocial challenges to which the animals are exposed over their lives. It would also require age-dependent specification of the various components of the classification system described above.

A third aspect of the further operationalization of allostatic load that warrants further investigation is gender differences in resting levels and in transient response to challenges of particular primary mediators and secondary outcomes. For example, a higher proportion of elderly women show elevated levels of cortisol, epinephrine, and norepinephrine and low levels of DHEA-S. Elderly men tend to have elevated levels of syndrome X markers with greater frequency than women. In response to a 30-minute cognitive challenge, younger men showed greater cortisol reactivity than women. However, in elderly populations the pattern is reversed, with elderly women showing greater transient responses (Seeman et al., in press.)

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