The Human Stress Response: Chronic Stress & Chronic Disease

Image courtesy of bottled_void

Image courtesy of bottled_void

We are currently facing an epidemic of chronic disease, like never seen before. There’s plenty of evidence to suggest that many of these diseases are lifestyle related, and that stress plays an important role. 

This article begins with an extract from a short story, showing the human stress response in action. Feel free to skip ahead. We then explore what the human stress response is, why it happens, and the main hormones involved. After that we take a look at the difference between acute stress and chronic stress, and the implications of chronic stress on human health…


The sun was beginning to disappear from the red sky, as dusk started turning to night. He was alone. His eyes felt heavy and his legs numb, but he carried on forward.

His spear, barely used over the month long hunt, now served solely as a walking stick as he stumbled wearily through the forest. He didn’t notice the bitter cold drawing in. The combination of hunger and exhaustion left him in a dreary daze, but the thought of home kept him moving.

The light continued to fade. As he pressed on, his blistered feet were suddenly met with the sensation of cool running water. Dropping his spear to the floor, he cupped his hands in the stream. He took a sip, the first drink to pass his dry lips in days. He dipped his head under the flowing water to soak his wounds, trying to wash away the dried blood and haunting memories from the past month.

Raising his head from the stream, he took a full, deep breath. Even in his wearisome state, he realised that the forest is too dangerous to stay in one place for too long. Time to get moving, he thought to himself.

As he turned to pick up his walking stick, it was then he heard the crack of the twigs in the nearby undergrowth. His ears perked up. He had grown up with the sounds of the forest, the familiar hum of the insects and chattering of small mammals. He knew the forest came alive at night, and the noise only got louder. But this was different.

hunterHis eyes narrowed and pupils dilated as he scanned his surroundings. He began to see the outline of the individual trees and bushes in the dense forest, as if they had suddenly appeared out of the darkness. The dreary daze had suddenly been replaced by an intense focus. First he saw the thin tail, sleeking gracefully through the gap in the trees, several metres away. The hairs on his back stood on end. A bead of sweat trickled from his brow, and ran slowly down the side of his neck.

The tiger slowly appeared from the brush, its head dipped. Each step was graceful and calculated, its giant paws softly padding the undergrowth. Its claws were drawn and teeth glaring, ready to strike.

The hunter felt a rush of adrenaline through his body. His knees were bent slightly, his stance strong, balanced. His spear felt light as he raised it above his shoulder. His grip was firm, yet his arm felt relaxed. Despite the inherent danger looming before him, the hunter felt a quiet satisfaction. This situation was familiar. Fight or flight.

This time, like many times before, he would stand his ground. His breath was heavy and his heart rate rising. He could hear it pound in his chest, drowning out the persistent hum of the forest.

An unknowing amateur would mistake this for worry, or panic. The hunter knew better. His body was preparing for battle. The beast stopped, just a metre away from him. Tunnel vision set in. They locked eyes, each waiting for the slightest movement from the other.

In reality, the violence was finished in a split second, but for the hunter time seemed to disappear. The tiger crouched, tensing its hind legs before launching towards him. Claws drawn and mouth snarling, the beast let out an almighty roar as it soared through the air.

He remained still, eyes locked on his target, waiting for the right moment to strike. His mind was completely focussed. He no longer concerned himself with the events that had recently passed, or with the daunting tasks that lay ahead in the future. All he had was the present moment. As the tiger drew nearer, he lowered his spear ready. He took a deep breath, filling his lungs with air. Timing would be crucial.

The tiger entered the hunter’s range. He thrust his spear towards the heart of the giant beast. The momentum from the leap brought the tiger crashing forward, landing on the hunter with a mighty thump. They both lay motionless on the cold jungle floor.


The stress response can thought of as your body’s reaction to an external (or internal) source of stress, otherwise known as a stressor. It is a completely natural process, where our body essentially recognises that something crazy has gone down and has sent us deviating away from equilibrium. It then cleverly attempts to restore us back to a normal balanced state.

Neuroendocrinologist and professor at Stanford University Robert Sapolsky (2009) uses the term ‘allostasis’ when explaining the stress response; which is a means by which we achieve stability through physiological changes, or changes in behaviour. He concisely describes a stressor as ‘anything that throws your body out of allostatic balance, and the stress-response is your body’s attempt to restore allostasis’.

When a stressor is encountered, our sympathetic part of the autonomic nervous system takes over in order to prepare the body to react, usually in the form of fighting or fleeing.

The endocrine system jumps into action, and a part of the brain called the hypothalamus secretes activating hormones, signalling the release of further hormones from the pituitary and adrenal glands. Levels of Adrenaline increase within seconds, along with a slightly slower increase in Glucocorticoid levels, a set of steroid hormones.

One of the most important and commonly known Glucocorticoids is Cortisol, needed in small amounts in the body to stimulate the release of energy stores, maintain blood pressure, and limit inflammation during stressful events. Many other hormonal changes occur, but it is levels of Adrenaline and Glucocorticoids that drive the stress response. Whilst these hormone levels increase, levels of Testosterone and DHEA fall.

The process is probably much more complex than I can begin to wrap my head around, but this basic explanation will do for now. The main thing to note is that these hormonal shifts act to cause a number of different physiological changes in the body, which can be useful in the short term, but potentially very damaging if prolonged…


The stress response originates as a crucial survival technique. Long ago before the modern inventions of mobile phones and high rise buildings, we lived in the wilderness. The world was much different to the one we live in today. We were more concerned with avoiding injury and finding food than avoiding traffic jams or paying our taxes on time.

Many dangerous predators roamed the land, and in some parts of the world we also had to hunt to survive. We had to keep our wits about us at all times, and have a mechanism to help us deal with any surprise acute stressors, which were usually in the form of life or death situations.

For the vast majority of animals on this planet stress is still an acute, short term event. For the most part it requires the immediate activation of the muscular system.

When the hunter in the story above encountered the tiger, a series of reactions began in his body, essentially preparing him to ‘fight or flight’. The stressful encounter is either dealt with quickly, or you are dealt with quickly. Pretty simple.

In this case the stress response is incredibly useful, acting to keep us alive so we can carry on our lives and perhaps someday pass on our genes.

Some of the main physiological changes that come about during the stress response include:

  • Increased energy availability due to the release of energy stores and cessation of further energy storage. We utilise fat from adipose tissue, glycogen from the liver, and amino acids from the muscular system to fuel the upcoming battle or chase.
  • Increased blood pressure, heart rate and breathing rate. This allows us to transport nutrients and oxygen at a faster rate, and generate a greater force of muscular contraction.
  • Constriction of blood vessels in some areas of the body. This increases the efficiency of blood clotting to minimise blood loss if a wound is opened.
  • Increased perspiration, to prevent us overheating whilst we’re fighting or running for our lives.
  • A numbing of the pain receptors. Often we see this in competitive sports, and it’s especially true in martial arts. The adrenaline is flowing, and often times competitors will only realise they are injured well after the fight has finished.
  • Increased sensory sensitivity and sharpening of cognitive skills. Tunnel vision may occur and pupils dilate, allowing you to focus more intently on the task at hand.
  • Inhibition of long term bodily processes: digestion, growth, repair and reproductive processes cease as they are not needed in the heat of the moment. You can’t be worrying about digesting your lunch or building your muscles when your face to face with a predator. The immune system is also suppressed; you’ve got more pressing things to deal with.

In an ideal situation, first the sympathetic nervous system takes control to prepare our body. We then execute a response to the stressor, utilising our released energy stores by fighting or fleeing. Once the stressor is dealt with, the parasympathetic nervous system would then take over. This part of the autonomic nervous system is associated with rest and recovery, and is responsible for returning the body back to equilibrium by reversing the above adaptations.

Problems can start to occur when stress is repeated or constant. Our sympathetic nervous system remains activated and we do not complete the whole stress response. We remain in a dangerous, activated state. If we cannot escape the stressor or fool our body into thinking we have responded to it, acute stress can quickly become chronic.


The human experience is very unique when compared to the rest of the animal kingdom. In general, we have managed to distance ourselves from the threat of predators, temperature extremes, and the need to hunt to stay alive. Our modern lives are instead ripe with a whole host of unusual stressful events.

We react with road rage when stuck in traffic, we stay up all night panicking over looming work deadlines, and we replay in our heads images of awkward social situations, over and over. We’re constantly being battered by an assortment of emotional, social and psychological stressors.

Adaptive traits take hundreds of thousands of years to develop, whereas our environment has changed at an alarming rate. If we consider the existence of human beings on a geological time scale, civilizations and societies really are a brand new concept. Our bodies have simply not had the chance to keep up with our surroundings.

Essentially, we are primates living in a world built for business people.

Unfortunately our primate bodies can’t tell the difference between these modern day stressors and being attacked by a wild animal. The same metabolic response occurs, causing the same physiological changes. We can even activate this response in anticipation of a stressful event, and often the event never happens.

When we keep telling out body that it needs to react to a stressor, Cortisol and Adrenaline levels continue to rise, and Testosterone and DHEA levels stay lowered.

Stress hormones are supposed to usually fluctuate naturally throughout the day, following a circadian rhythm with highest levels in morning and lowest at night. In this modern world, the natural rhythms are constantly being disrupted. Instead of a gentle ebb and flow, we often experience drastic highs and lows, or even flat line levels (which can be just as detrimental to health).

Normal cortisol ebb and flow.

Normal cortisol ebb and flow.

When our sympathetic nervous system remains active and these unnatural hormonal patterns continue for an extended time period, a whole range of health problems can present themselves. Modern day stressors may seem less scary than a tiger jumping out of the bushes, but for your health, you may stand a better chance against the tiger…


If we look at the time frame of human existence, we can see that there has been a very recent shift in disease patterns. Slow accumulating diseases such as cancers, heart disease, and diabetes are now considered far greater a risk than many infectious diseases. There is growing evidence to suggest that these slow accumulating diseases are related to our modern lifestyles, and there has been shown to be clear links with chronic stress.

Chronic stress and its related diseases emerge because we so frequently activate the physiological system which was initially developed for responding to acute physical emergencies. We leave our sympathetic nervous system in charge for months or even years, and sometimes cannot turn it off.

“If you experience every day as an emergency, you will pay the price”.

Sapolsky (2009)

The time frame may be variable from person to person, but it seems our efforts to retain a balance in our body will eventually wear us down. Chronic stress has been associated with a whole range of different health problems:

  • When we are constantly stressed, we turn off long term processes. Switching off growth and repair can lead to a problems such as a loss in bone density (leading to osteoporosis), muscle wastage, and even dwarfism in children.
  • Halting digestion means we cannot properly obtain nutrients from our food, and increases the risk of IBD, IBS, ulceration and cancers of the gastrointestinal tract.
  • Sustained circulating cortisol levels raises glucose levels, which in turn raises insulin levels in order to combat the high blood sugar. Over time, this leads to excess abdominal fat storage, and causes insulin resistance and diabetes to develop.
  • Reproductive disorders can occur due to shifts in hormones. In males, sperm count can be significantly reduced, and testosterone levels fall rapidly. In women, menstrual cycles can become irregular or halt completely. Both sexes can experience a reduced interest in sexual activity.
  • When we constantly tell our body to mobilize energy sources rather than store them, fatigue can become a problem. Muscle wastage and weakening of tendons and ligaments can occur as the muscular system is stripped of amino acids.
  • Sustained stress keeps your appetite raging as your body demands energy to deal with the stressor. It induces fat storage, particularly around the midsection. Obesity has been unquestionably linked to an increased risk of cardiovascular disease and diabetes.
  • The risk of developing ‘Metabolic Syndrome’ or ‘Syndrome X’ increases when faced with chronic stress. The condition is characterised by insulin resistance, abdominal fat accumulation, chronic inflammation, and an increased risk of heart disease, hypertension and diabetes.

Add to these and increased risk of dementia, depression, mood swings, irritability, headaches, upset stomach, and a loss of concentration; things are not sounding too positive. Some even argue that chronic stress is unavoidable in modern society. There are now more stressors than ever before. The average person works more hours, exercises less, and gets less sleep.


To summarize, lets quickly go back over the main points:

  • During the stress response, the sympathetic part of the autonomic nervous system prepares us for intense physical exertion, usually fighting or fleeing. It is very useful in dealing with acute stressors, such as hunting for prey, or avoiding predators.
  • Our primate bodies prepare the same way when we face modern day stressors, but we are often unable to mount a complete stress response, leading to chronic stress.
  • Remaining under the control of the sympathetic nervous system presents many long term health issues, including obesity, diabetes, heart disease, autoimmune diseases, reproductive disorders, dementia, muscle and bone wastage.

If you’ve made it this far, you might be feeling a little stressed about stress…

I agree that it all sounds a little daunting and pretty bleak. But don’t fret too much; I think there’s still hope. Coming up soon we’ll take a look at different ways we can combat chronic stress, and how we can maybe start to use the stress response to our advantage.

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Luke Jones

Luke Jones is a mover, blogger and wellness enthusiast. He spends his time exploring and sharing ideas in mindful movement, healthy living and adventure.

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