Aging, health behaviors, and the diurnal rhythm and awakening response of salivary cortisol

Background/Study Context: The cortisol diurnal rhythm has previously been examined in relation to age and health behaviors. However, less is known about the relationship between multiple health behaviors and diurnal cortisol in the context of aging, where it is possible that the impact of health behaviors on cortisol varies as a function of age. This study compared the awakening response and diurnal rhythm of cortisol in young versus older adults in relation to health behaviors. Methods: Twenty-four young students (aged 18-22) and 48 community-dwelling older adults (aged 65-88) completed an assessment of health behaviors (exercise, smoking, sleep, diet, alcohol) over the past year. Salivary cortisol was measured over the course of 1 day: immediately upon awakening, 30min later, and then 3, 6, 9, and 12h post awakening. Repeated measures/univariate analysis of variance (ANOVA) was used to test main effects of age and health behaviors, and any interaction effects in relation to diurnal cortisol. Results: Older adults displayed significantly reduced cortisol upon awakening, a lower cortisol awakening response, and a flatter diurnal profile represented by a reduced area under the curve and cortisol slope. There was also a significant interaction of age, cortisol, and diet; younger adults with a higher fat and lower fruit and vegetable intake exhibited the flattened diurnal cortisol phenotype of the older adults. Conclusion: These findings suggest that the diurnal rhythm and awakening response of salivary cortisol is significantly reduced in older adults and that variations in the cortisol diurnal rhythm of younger adults are associated with dietary factors. Younger adults with a poor quality of food intake may be vulnerable to a reduction in the amplitude of the cortisol diurnal profile and this may have implications for other aspects of health.