Is cortisol friend or foe?

Cortisol is associated with our fight and flight response to a natural or environmental threat.

Cortisol is a steroid hormone released by the adrenal glands in response to messages from the region of the brain called the hypothalamus via the pituitary glands. This important systemic pathway is known as the hypothalamic-pituitary adrenal axis (HPA for short).

Image credit: https://physoc.onlinelibrary.wiley.com/doi/10.14814/phy2.14644

Evolutionary function

Cortisol is considered as our fight or flight hormone, released after encountering an external stressor (typically no tigers involved). It's job is to increase blood flow and release fuel to our muscles and our brain so we can act. Given this primary and evolutionary function, cortisol plays a big role in our blood pressure and blood sugar regulation. Once the stress passes, cortisol levels reduce and our body returns to systemic homeostasis (resting state).

Circadian Rhythm

Cortisol release also follows a circadian rhythm characterised by a sharp rise in the early morning hours and a gradual decline throughout the day. This is known as a diurnal variation. A rise in cortisol in the morning prepares the body for the upcoming active period by promoting the natural synthesis of glucose in the liver. Peak levels are ordinarily reached around 30 min after waking and this is known as the cortisol awakening response (CAR). Levels decline throughout the day and are expected to be lowest as we are in the first stages of sleep.

Image credit: Circadian Cortisol Secretion Pattern (Lovallo & Thomas, 2012)

The Autonomic Nervous System

Another interesting feature of cortisol is that the activity of the HPA axis is intimately connected to the activity of the autonomic nervous system (ANS) and its two branches - the parasympathetic and the sympathetic nervous system. Both ANS systems have regulatory control over organs throughout our body, but have opposing effects. Together they operate under the influence of the hormones and neurotransmitters acetylcholine, adrenaline and noradrenaline working in opposition to achieve physiological harmony. While the parasympathetic nervous system promotes rest, repair, digestion, reproduction, the sympathetic nervous system reduces the activity of these systems down to allow necessary blood flow and fuel to reach our brain and our muscles.

Hyper-arousal

When we are in a state of chronic or persistent stress, our body is in a state of hyper-arousal and this can have a negative effect on our circadian rhythm and the physiological responses regulated by the ANS.

In Summary

Cortisol is an important protective mechanism essential to the working of our body in many situations however cortisol is designed to be released intermittently and when released over a prolonged period it can trigger imbalance and dysfunction across a range of physiological systems including digestion, tissue repair, blood sugar regulation, cardiovascular health and mental health and can contribute to weight gain and fatigue.

References:

Lowrance SA, Ionadi A, McKay E, Douglas X, Johnson JD. Sympathetic nervous system contributes to enhanced corticosterone levels following chronic stress. Psychoneuroendocrinology. 2016 Jun;68:163-70. doi: 10.1016/j.psyneuen.2016.02.027. Epub 2016 Feb 26. PMID: 26974501; PMCID: PMC5656452.