Cortisol, stress responsivity and cognitive function in older adults : relationship to waist hip ratio and metabolic parameters
Basal cortisol activity is postulated to be elevated in individuals with central obesity yet this elevation is not mirrored in the basal diurnal cortisol profile. Individuals with central obesity may demonstrate an enhanced clearance capability evidenced by elevated urinary cortisol metabolites, hence basal salivary cortisol levels appear normal or reduced. Those with central obesity have been found to demonstrate an elevated cortisol response to psychological stress due to dysregulation of the HPA axis. Cognitive decrement has been observed in individuals with central obesity who show insulin resistance, high blood pressure and other features of the metabolic syndrome, however, research to date has failed to address the collective influence of elevated cortisol and metabolic risk factors on cognitive performance. A study was conducted (n=83) to explore the basal diurnal cortisol profile in those with central obesity (high waist-hip ratio) compared to those without (low waist-hip ratio). The findings demonstrated that profiles exhibited by high waist-hip ratio individuals mirrored those exhibited by low waist-hip ratio individuals, although mean cortisol was reduced. A subsequent study (n=70) explored cortisol responses to a psychosocial stressor (The Trier Social Stress Test-TSST). High waist-hip ratio individuals exhibited greater cortisol during stress induction (TSST) than low waist-hip ratio individuals. Both the stress induction procedure and actual stress responses (cortisol and blood pressure) reduced performance on the Auditory Verbal Learning Task (AVLT). Performance on a paired associates learning task was impaired in high waist-hip ratio males who demonstrated a cortisol and/or blood pressure (BP) response to the stressor/no stressor. Impairment was further evident in high waist-hip ratio males who demonstrated a cortisol and/or BP response to the stress induction. The findings presented in this thesis suggest that individuals with central obesity exhibit altered basal and stress induced cortisol which may contribute along with metabolic factors, to cognitive impairment. Finally, it was observed that differences in the shape of the diurnal cortisol profiles were attributable to various psychological and metabolic characteristics. Flattened profiles (non-classic), which have been associated with non-compliance, were associated with greater subjective reporting of stress, intensity of daily hassles, poorer sleep quality and more severe metabolic syndrome symptomology. This highlights the usefulness of the basal profile in determining individual vulnerability to stress and poorer health. In conclusion, diurnal cortisol, central obesity and markers of metabolic syndrome may interact to influence hippocampal memory function.