1997; Seeman and Robins, 1994; McEwen, 1998; Dhabhar and McEwen, 1996; McEwen et al., 1997).
Emphasis on allostatic load is not meant to overshadow the importance of diverse avenues of ongoing research on single systems of physiological risk (e.g., syndrome X and cardiovascular disease, dysregulation of the HPA axis and cognitive impairment). However, to date, there has been insufficient attention given to co-occurring risk across multiple physiological systems as well as to more temporally distant assessments of such risk. Because of its dual emphasis on cumulative risk across multiple physiological systems and cumulation across time, the concept of allostatic load is a particularly promising contributor to integrative health research, the theme of this report. Growing evidence of life course trajectories of comorbidity (i.e., individuals are ever more likely to suffer from multiple chronic conditions as they age) further underscores the importance of attending to co-occurring risk factors and their cumulation through time. Specifically, 45 percent of women and 35 percent of men ages 60-69 report two or more chronic conditions; figures rise to 61 percent of women and 47 percent of men ages 70-79 and 70 percent of women and 53 percent of men ages 80-89 (Jaur and Stoddard, 1999). Understanding the etiology of these co-morbid profiles requires attending to multiple co-occurring risk factors.
A provisional operationalization of allostatic load has been provided by Seeman et al. (1997, unpublished manuscript), using measures of the HPA axis (cortisol, dihydroepiandrosterone sulfate), sympathetic nervous system (epinephrine, norepinephrine), cardiovascular activity (systolic and diastolic blood pressure), metabolism and adipose tissue deposition (waist-hip ratio), glucose metabolism (glycosylated hemoglobin), and atherosclerotic risk (serum HDL and total cholesterol). Underscoring cross-system linkages, high allostatic load was found in the MacArthur Studies of Successful Aging to predict later life mortality, incident cardiovascular disease, and decline in cognitive and physical functioning. Looking to socioeconomic and psychosocial precursors, the same operationalization has been linked to cumulative economic and social relational adversity from childhood through age 59 in a 40-year longitudinal study (Wisconsin Longitudinal Study; Singer and Ryff, 1999). A slightly modified version of this operationalization of allostatic load has been associated with lower levels of education as well as greater hostility in the Normative Aging Study (Kubzansky et al., 1999). We underscore the provisional nature of the operationalization of allostatic load, and in Chapter 10 we discuss in detail the future research program needed to refine assessment of cumulative physiological risk. In Chapter 3 we also discuss the optimal functioning of these multiple physiological systems via the concept of allostasis.