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1. Social Network Characteristics and Salivary Cortisol in Healthy Older People
The last two decades have witnessed a proliferation of research on the impact of psychosocial factors on cortisol secretion in aging populations, which has been motivated by the assumption that the capacity to adapt to stressful forces in life declines with age and this is marked by a progressive dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis [1, 2]. Being the endproduct of the HPA axis, cortisol has been extensively studied in relation to the aging process [3–5]. There is evidence showing that basal cortisol levels (e.g., [6, 7]) and reactivity to psychosocial and pharmacological challenges increase with age (e.g., [8, 9]). Despite the significance of these findings, their interpretability is moderated by mixed findings (e.g., [10, 11]) and a gender effect reported in a number of studies (e.g., [8]). This may partially reflect the remarkably heterogeneous responses of the aged population to stressful challenges, which have not yet been adequately characterized. It is possible that some old people may age more successfully than others [12] and the two sexes cope with daily stressors and distress differentially [13].
One of the most challenging tasks in studying the impact of aging on cortisol is the development of reliable markers of cortisol secretory activity because the HPA axis is highly adaptive and is characterized by marked inter- and intra-individual variability. Cortisol exhibits a robust diurnal rhythm, peaking within 60 minutes after waking and declines thereafter until reaching the nadir around midnight [14]. Recent evidence suggests that the cortisol awakening response (CAR), which is marked by an increase from 50% to 150% within the first 30 minutes after waking up in the morning [15], is a reliable marker of HPA axis functioning [16].
This distinct component of the cortisol diurnal rhythm has a salient genetic component [17]. An absence or accentuation of the CAR is associated with various psychopathological conditions or adverse outcomes (e.g., [18–21]). On the other hand, attenuation in postawakening cortisol levels has been observed in people who are high in optimism [22] or experience more positive affect [23].
Another reliable marker of the integrity of the HPA axis is the diurnal variation or decline which has been operationalized as a descent of cortisol levels measured at...