What is cortisol?
Cortisol is a steroid hormone, a glucocorticoid produced in the zona fasciculata of the adrenal cortex. It is released in response to stress and a low level of blood glucose. Its effects in the body are remarkably wide ranging, including the alteration of immune function, the inhibition of insulin utilization, and stimulating the creation of glucose in the liver.
How and when is cortisol produced?
At times of physically, biologically and psychologically stressful events, the hypothalamus secretes corticotropin-releasing hormones (CRH), which are received by the neighboring anterior pituitary gland. The pituitrary gland then secretes another hormone, adrenocorticotropic hormone (ACTH), into the blood stream. ACTH triggers the synthesis of glucocorticoids (of which cortisol is one) and didehydroepiandrosterone (DHEA).
The stress response is initiated in order to help the body maintain homeostasis under conditions of external and internal stressors. Homeostasis is achieved via a synchronized cascade of activity by the nervous, endocrine and immune systems.
The primary effect of cortisol release in times of stress is the prompt the prioritization of catabolic processes that can provide the body with the energy it needs to survive a threat. These processes are aimed at enhancing survival such as preserving glucose for the brain, converting stored energy for use, and diverting energy from lower-priority activities (such as the immune system) to help organism survive threats.
At the same time, cortisol has inhibitory effects by way of a negative feedback loop through activity in the prefrontal cortex, hippocampus and amygdala.
The exact nature of the stress response and the amount of cortisol that is released depends on several factors including: type of stress, intensity and duration of exposure to the stressor, psychological state, and other factors.
The stress response, like pain, is adaptive. It has been retained under evolutionary pressure because it has conferred an advantage to survival. But also like pain, the failure to regulate it properly can lead to wide ranging deleterious effects.
Effects of cortisol on the body:
Cortisol has numerous effects on the body, including:
- Suppression of inflammation
- Increases blood pressure
- Cortisol stimulates the creation of glucose by the liver
- Inhibits reproductive system
- Lengthens wound healing
- Weakens bones by transporting potassium out of bone cells and reduces calcium absorption in the intestines.
- Inhibits utilization of insulin
- Diuretic
- Works with epinephrine to facilitate the creation of memories of emotional/threatening events.
- Inhibits recall of already stored memories.
Advantages of salivary cortisol measures
Given such wide ranging effects, its measurement in studies of stress is highly desirable. Measuring cortisol used to require blood samples. This necessitated trained venipuncture personnel to collect samples, and prevented its use as a measure of stress in daily life. Plus, the sampling procedure itself could (understandably) trigger the stress response.
Saliva collection, by comparison, is easy and non-invasive. The collection process itself has been shown to not induce stress, which is important (Hellhammer, Wust, & Kudielka, 2009; Kirschbaum & Hellhammer, 2000), and salivary cortisol can be easily obtained real time in situ — as people go about their normal activities, situated in real contexts. Thus, the advantages of salivary cortisol include the following:
- Non-invasive
- Enables real-time in-situ sampling
- The ease and non-invasiveness makes practical more frequent sampling
- No specially trained personnel required. Anyone can do it.
- Saliva cortisol is relatively stable, able to remain at room temperatures for 2-4 weeks without any significant degradation in quality (Kirschbaum & Hellhammer, 2000)
Salivary cortisol is considered to be a reliable measure of activity in a major component of the stress response, the Hypothalamus-Pituitary-Adrenal (HPA) axis.
Measuring salivary cortisol
The majority of cortisol released into the blood stream from the adrenal cortex becomes bound to carriers such as corticosteroid binding globulin (CBG), albumin, and erythrocytes. About 5-15% of cortisol remains “free” or unbound. It is this free cortisol that can be measured in the saliva. In fact, the concentration of cortisol in the saliva accounts for 70% of the non-bound blood cortisol that enters saliva by diffusion through the salivary gland acini (Michael, 2008)
As salivary cortisol is independent of transport mechanisms and the amount and flow of saliva, salivary cortisol correlates highly (r ≥ 0.90) with free blood cortisol (Kaufman & Lamster, 2002; Poll et al, 2007)
Obtaining saliva samples is non-invasive and relatively quick and easy. Minimal special training is required, enabling just about anyone to do it.
Saliva cortisol is fairly stable and samples may be kept at room temperature for up to 4 weeks without sustaining any significant degradation (Bozovic, Racic, Ivkovic, 2013). For long term storage and to prevent mold formation, it is recommended that saliva samples may be frozen at -20 °C (Kirschbaum & Hellhammer, 2007).
In the absence of any threats salivary cortisol follows a circadian rhythm. Levels peak within 30-45 mins of awakening and then decrease throughout the day (Kirschbaum & Hellhammer, 2000). Interestingly, basal levels of cortisol are higher on average during the winter compared to the summer (Malarkey et al, 1995).
Studies that aim to use salivary cortisol as a measure of the stress response should take this circadian rhythm into account. For example, an experimental protocol that aims to induce a stress response (e.g., the Trier Social Stress Test) should not be administered in the morning hours because cortisol are already at high levels and therefore the effects of the stressor may be obscured by a celing effect.
Acute stress episodes result in a substantial increase in cortisol levels secreted (Kirschbaum & Hellhammer, 2000, 2007, 1994; Hellhammer, Wust, & Kudielka, 2009). The amount of cortisol secreted is correlated with intensity of the stress (Hellhammer, Wust, & Kudielka, 2009)
Upon exposure to, or anticipation of, a stressor, ACTH levels rise within 5 minutes. Cortisol levels then begin to rise within 5-20 minutes of the release of ACTH into the bloodstream, with peak levels achieved within 10-30 minutes (see figure below). The transfer of cortisol from blood to saliva takes place rather quickly, within no more than 2-3 minutes (Kirschbaum & Hellhammer, 1994).
Saliva Sample Collection
There are two primary methods by which saliva can be collected: 1) by means of an absorbant swab placed in the mouth (e.g., a Salivette), and 2) by means of passively drooling into a small container.
Swab technique
A swab is placed in the mouth, typically under the tongue, for 1-2 minutes. If placed in a different location, flow rate may be different and swab may require more time in the mouth. Check with guidelines supplied by the manufacturer of the collection device. After collection is complete the swab should be removed from the mouth and placed in the storage tube for later assay.
SalivaBio Oral Swab (SOS), from Salimetrics.
Passive drool technique
In this method, saliva is allowed to pool in the mouth and then drooled into a collection vial. A straw can be used to help guide the saliva into the vial but it is not necessary.
Unfortunately it can also be somewhat messy as saliva can easily be deposited outside the vial and foam can form, making it difficult for saliva being drooled to enter the vial. Companies such as Salimetrics have, however, devised devices that can make passive drooling more easy and less messy.
The passive drool method puts the onus on the participant to use proper technique in depositing saliva in the vial. Therefore it would not be indicated for young children or for individuals not able to follow instructions.
Which technique is better?
Both techniques get the job done but the passive drool technique is generally considered to be superior to swab absorption.
Requiring only a small vial, it is cost effective, and is appropriate for the collection of almost all saliva analytes. Samples are not subject to contamination from the absorption swab and saliva vials may be frozen for long-term storage. Generally the main problem with the passive drool method is that it can be a bit messy and may entail longer collection times, but with proper technique and better quality collection vials, this can be minimized. For my research, I have relied on the passive drool method and have found it to work fine. I prepared and present to participants a short video of someone (one of our excellent research assistants) demonstrating correct technique to encourage consistent results across individuals.
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