The adrenal hormone cortisol impacts nearly all the body’s systems. Cortisol levels fluctuate throughout the day, normally peaking in the morning hours and bottoming out at night. In studies, deviations from this pattern are associated with signs and symptoms of adrenal dysfunction. One such dysfunctional pattern is a flat cortisol curve in which the amount of cortisol secreted (high or low) shifts very little throughout the day. In both healthy and clinical populations, a flattened cortisol curve has been robustly associated with a shorter lifespan and negative health indicators.
Results of one such study indicated that a flattened cortisol curve statistically predicted poor survival time in patients with metastatic breast cancer.1 Natural killer cell numbers and activity were also decreased. In the patients studied, 70% had flattened cortisol curves, presumably due to the physical and emotional stress of their diagnosis and treatment.1 Flattening of the cortisol curve has also been shown to predict early death from lung cancer and has been associated with low T-cell lymphocyte counts.2
In a study of healthy individuals, diurnal cortisol patterns were measured for two years, and participants were then followed for six to eight more years.3 Participants were mostly middle-aged (average of 61 years). Flattened cortisol curves correlated with mortality from all causes, including cardiovascular deaths.3
A 2018 study of police officers in the United States shows a higher prevalence of cardiovascular disease compared to that of the general public, thought to be related to high levels of stress.4 An atypical or flatter waking cortisol pattern predicted a significant worsening of brachial flow-mediated dilation (a biomarker of subclinical cardiovascular disease) during a seven-year follow-up in men, but not women. Similar associations of cortisol with disease have been found in populations other than police officers.4
Diabetes and depression
A blunted cortisol curve may also be correlated with diabetes and higher HbA1c in individuals with diabetes.5 Some researchers consider HPA axis dysregulation to be a critical link in the high prevalence of co-existent depression and diabetes, based on the results of two studies.5 In the first study, diabetic persons reported greater depressive symptoms and higher stress than did healthy controls.6
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