Stress can have detrimental health consequences, and the hypothalamic-pituitary adrenal (HPA) axis has been proposed to play a key role in stress-health linkages (McEwen, 1998). Thus, interest in the HPA axis has been strong, and has exploded over the past decade with the availability of low cost cortisol measurement in saliva.
This allowed cortisol assessment in population-based and epidemiological research, providing valuable evidence on links between salivary cortisol with job stress, trauma, depression, socioeconomic and demographic status, negative health outcomes, metabolic disease, and cancer mortality (Adam & Kumari, 2009).
The HPA axis has intrinsic circuits that follow a diurnal pattern, with a sharp rise in cortisol levels following morning awakening (termed the cortisol awakening response, CAR) and a subsequent decline throughout the day. Different aspects of this diurnal curve have been proposed to reflect regulatory influences of different physiological systems (Clow et al., 2010); and the CAR specifically has received much attention as a potentially promising marker of psychosocial health (Clow et al., 2010).
In addition to its circadian activity, the HPA axis is also a critical stress response system. In response to a stressor, the paraventricular cells of the hypothalamus secrete corticotropin-releasing hormone (CRH) and vasopressin (AVP), which stimulate the release of adreno-corticotropin hormone (ACTH) in the pituitary gland. ACTH reaches the adrenal glands through the bloodstream and initiates the release of cortisol into the blood (Tsigos & Chrousos, 2002). Most secreted cortisol is bound to proteins in the blood, but a small fraction is unbound. Unbound cortisol can enter cells by passive diffusion, which allows measurement in bodily fluids, such as saliva. Assessment of salivary cortisol levels thus reflects momentary snapshots of HPA axis activity, capturing acute or short-term cortisol production over the past 15-20 minutes.
Salivary cortisol has numerous advantages. Salivary cortisol collection is easy (without medical personnel), non-invasive (pain/stress-free), low cost, and allows for sample collection in many different contexts. For these reasons, it has become extremely popular in field-based research (Adam & Kumari, 2009), studies with children (Jessop & Turner-Cobb, 2008), and laboratory studies examining psychological stress reactivity (Kirschbaum & Hellhammer, 1994).
However, a word of caution is warranted. Salivary cortisol cannot provide a simple measure of accumulated biological stress, partly because stress exposure alters regulatory control of the system over time in order to keep circulating levels within a fairly narrow range. The impact of stress exposure may be more accurately captured by alterations in those regulatory components, but we do not yet know if those regulatory changes are “visible” in saliva levels. Levels in saliva within a day and across days are also strongly influenced by a host of other factors. For example, measurement of cortisol in blood and saliva is influenced by circadian variation (Kirschbaum & Hellhammer, 1994; Posener, Schildkraut, Samson, & Schatzberg, 1996), situational factors (e.g., novelty; Davis, Gass, & Bassett, 1981), food intake (Gibson et al., 1999), or intra-individual day-to-day variability (Hellhammer et al., 2007). Thus, it is important to repeatedly sample salivary cortisol over time and to assess potentially confounding psychosocial, biological, and methodological factors in laboratory and field studies (e.g., see Adam & Kumari, 2009). There is some evidence that averaging across at least 3 days of collection may be necessary to get trait-related information about a study participant. (Hellhammer et al., 2007).
The utility of saliva cortisol also depends on the scientific question of interest. Saliva cortisol levels reflect the amount of free cortisol in the blood, which is the active component, but this represents only a small fraction of the total amount in the blood because the majority is carried by binding proteins. This perhaps enhances the value of saliva (free) cortisol measures if interests focus on the effects of cortisol on its target receptors, such as MR and GR receptors in the brain. However, if trying to capture stress reactivity, its value may be somewhat diminished since the amount of binding protein (e.g., cortisol binding globulin, or CBG) in the blood can vary widely. As a result, only an unknown fraction of the cortisol secreted in response to a stressor can be “seen” in a salivary measure. One factor that influences CBG levels, for example, is estrogen, so anything that alters estrogen levels (e.g., sex, menstrual cycle phase, birth control pills) can alter the percentage of secreted cortisol that is actually seen in the saliva. This reduces the accuracy of free cortisol in reflecting what is actually happening in the brain in response to stress. The appearance of cortisol in the saliva also takes approximately 20 minutes after activation of the stress response within the brain, and individual variation in the temporal dynamics of the system can thus intervene and create noise in efforts to fully quantify stress reactions to brief stressors (Lopez-Duran, Mayer, & Abelson, 2014).
Cortisol collection in saliva using Salivettes is described on the Biomarker Network website. Dr. Clemens Kirschbaum also answers frequently asked questions on technical issues in the downloadable document available here.
Authors and Reviewer(s):
Prepared by Stefanie Mayer, PhD and reviewed by James L. Abelson, PhD, who made substantive contributions during the review process. Please direct suggestions and feedback to firstname.lastname@example.org.
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