What is stress?
Stress kills by stealth. It works below the surface, slowly and quietly over time. It exacerbates the effects of most diseases. It delays recovery from illness and it makes us age more quickly. Like nicotine it seems to affect all the systems in the body.
Stress also makes us feel miserable, anxious or depressed. We don't cope with problems or perform well in daily tasks. We indulge in self-comforting behaviour and relate badly to other people. Stress is not just an inconvenience. It invariably accompanies poor health and mental disorders. Yet we still find it hard to estimate the degree of risk or know what do about it.
We all seem to know what stress is until we try to define it. It turns out that we use the word in vague and often contradictory ways, and its biological effects can't be easily measured. This is why doctors are often reluctant to take stress seriously as a scientific concept.
In this chapter I will explain the key biological concepts relating to stress. These are:
- Homeostasis – the way the body regulates itself for good health;
- The Autonomic Nervous System, through which stress and relaxation operate;
- Chronic stress and its long term consequences, and;
- Cortisol, the key hormone in stress.
FOUR POINTS
But first, here are four initial points to keep in mind.
Firstly, a stressor is different from a stress response. There is no automatic cause and effect relationship between an external trigger and the body's response to it.
Secondly, an acute response is different from a chronic response. An acute fight or flight response only lasts for minutes. A state of chronic overarousal can last a lifetime.
Thirdly, the stress response is not inherently bad. The body and mind need to be challenged regularly for good health.
Fourthly, we all have very individual responses to stressful events. This makes it very difficult to quantify the effects of stress in any one person.
The term stress is often used in contradictory ways both by experts and the general public. We typically use the word to describe both the stimulus, such as being overworked, and the result, such as insomnia. This leads to a logical fallacy, namely that the cause of stress is stress.
We can overcome this by distinguishing a stressor from a stress response. A stressor is any event that stimulates an alarm reaction within us. Stressors are typically other people, work, problems, noise, over-crowding or being too busy. Being sick or lonely or overweight can be stressors in their own right. While it is easy to identify major stressors, we often fail to notice the accumulating effect of minor ones.
We can also distinguish between acute stress and chronic stress. The term 'stress response' describes the way the body reacts to any challenge. We commonly think of it as the fight-or-flight response or the adrenalin rush. Yet we also use the word 'stress' to describe the quite different state of chronic, low-level pressure – the grind of a long, hard year, for example. It is worth distinguishing these two. An acute response is governed by adrenalin. A chronic response is governed by cortisol.
We tend to forget that stress is not inherently bad. We confuse good, necessary stress, a state of high arousal, with bad, counterproductive stress. In fact, arousal is necessary for everything that requires energy and attention, including learning and memory. In this context, the word stress shouldn't carry any negative connotations at all.
We need stress. Adrenalin and cortisol wake us up and get us going in the morning. Our muscles need to be used vigorously or they will atrophy. Aerobic exercise is good precisely because it pushes our hearts and lungs to the limits of their capacity. Our immune systems need to be repeatedly exposed to pathogens to maintain a robust defence. The brain needs challenges on a daily basis, or the neurons lose their richness of connections. We need good physical stress and mental challenges to keep us feeling alive and healthy.
Arousal and the stress response are good. The problems come only with excess and a failure to balance it out with sufficient rest and recovery time. This leads us to our next definition. Stress is a state of biological imbalance caused by chronic over-arousal.
Because each of us is unique, we can have very individual responses to the same stressors. What is nothing at all to one person can produce a colossal but completely genuine stress response in someone else. Some people need to climb Everest to feel alive. Others are panicked by the sight of a dog or a email.
The stress we feel is ultimately a personal evaluation. It comes from your mental perception or from your body's perception of a particular situation. Others may have good reasons for assuming that you are stressed, but only you can tell.
It comes down to our personal sense of competence. I have no trouble giving public talks, but I would be very stressed if I had to babysit a child. This gives us another definition: Stress is when we feel we don't have the physical or mental resources to cope. It is when we feel we are not in control of our situation.
Unfortunately, we are not good at assessing our own stress. We usually don't want to know. Stress typically makes us tense up, numb out and block out. It goes hand in hand with denial and escapist behaviour. When stressed, we tend to focus on action to fix or avoid the problem, and we may barely notice its effect on our bodies and moods. We often assume that we can't be stressed if we are managing to get everything done. While failure is bad enough, even success and coping well are no guarantees against stress. People have usually been stressed for years before they finally admit it to themselves.
HOMEOSTASIS
Until in the 1920s this word 'stress' was only used in an engineering context. It was adapted as a biological term by Walter Cannon and Hans Selye, the first stress researchers, and it rapidly moved into popular usage. Cannon also invented the term 'fight or flight' but he is most respected for his theory of 'homeostasis'. This idea is so crucial to understanding stress that I will now describe in some detail.
Cannon coined this term to describe the body's remarkable ability to regulate itself. In his book The Wisdom of the Body he says "The co-ordinated physiological processes which maintain most of the steady states in the organism are so complex and peculiar to living things that I suggest a special designation for then, namely: homeostasis."
Homeostatic systems regulate the correct levels of sugar, oxygen, hormones and nutrients in the blood. They maintain body temperature, muscle tone, heart rate, balance, arousal and relaxation. These systems all need to work harmoniously for the body to be healthy. If any one of them start to fail, we fall ill. Cannon hypothesised that stress, by exacerbating these imbalances can bring on an illness, or actually cause one, or make the recovery from illness more difficult. This brings us to another definition: stress is what seriously disturbs the body's ability to regulate itself.
The body maintains homeostatic balance through what is called 'negative feedback'. For example, our bodies always remain close to the ideal temperature of 38 degrees regardless of the weather. If the body gets too hot, thermoreceptors send messages to the hypothalamus, which is the heat control centre in the brain. This assesses the incoming data, and sends counteracting signals back to the body. This is the negative feedback. The blood vessels in the skin dilate, we sweat and so we lose our excess heat to the air. When the ideal temperature of 38 degrees is restored, the feedback stops.
Each of the body's countless homeostatic regulators fluctuates around an ideal set point. Resting heart rate is about 70 beats a minute. The acidity of the blood needs a pH of 7.4. These set points however, are nowhere near as fixed as was once thought, and deviations are quite natural. In other words, homeostasis is not so much a fixed point as a dynamic equilibrium within a permissible zone. Levels fluctuate adaptively according to circumstances, but they can only be stretched so far or for so long. Flexible as these set points are, the body exerts enormous gravitation pull to maintain them. Our lives depends on it.
Most fluctuations are tiny, but some can be quite large. In a crisis, the heart rate may double from 70 to 140 beats a minute, and the blood flow to the leg muscles can quadruple. This is perfectly natural so long as they return to balance fairly soon thereafter. Conversely, if the acidity of the blood drops from 7.4 to just 7.3, the nerve signals that govern muscular contractions will go haywire, and we can die within minutes.
Some fluctuations are very slow while others are lightning fast. Many homeostatic systems follow the sleep/wake cycle and make one big oscillation a day. Temperature and cortisol levels rise in the morning and decline at night. Others systems do exactly the reverse. At the other extreme, the enzymes that govern the regulatory processes within cells may be switched on and off thousands of times a second.
Many set points rise and fall in an orderly fashion in response to changing conditions. When food enters the small intestine, digestive enzymes will increase to a certain ideal level. As it passes through, they drop back again. Likewise, there is a smooth, economical way for muscles to tense and relax during normal activity without causing strain. In other words, many so-called set points are more about an ideal curve or sequence of actions than an ideal number.
Homeostasis used to be conceived as the rest phase of the rest-activity cycle, the still point to which all systems ideally return. This led to the fallacy that rest is good but that activity is intrinsically harmful. We now see homeostasis as the smooth, co-ordinated functioning of every part of the rest-activity cycle. It is quite possible to feel relaxed, balanced and in control while running a marathon or presenting a lecture. This dynamic stability is a sign of energy economy and good health, not stress.
THE AUTONOMIC NERVOUS SYSTEM
Many homeostatic systems operate independently, but Walter Cannon discovered one that has a global effect. This is the Autonomic Nervous System. It operates in close conjunction with the hypothalamus and pituitary in the brain and the adrenal glands above the kidneys. This particular network is called the HPA axis – hypothalamus, pituitary and adrenals – and it governs the rate at which we burn energy. We need a lot of glucose when we are active or stressed, and the HPA axis makes sure it is available.
The Autonomic Nervous System conveniently divides into two so-called 'branches', each with its own extensive chain of nerve connections. Together they manage the body's cycles of arousal and relaxation, activity and rest. The so-called 'sympathetic' branch increases arousal, muscle activity and energy consumption, and the 'parasympathetic' branch decreases it. 'Sympathetic arousal' is the technical term for the stress response, and relaxation is a state of low arousal.
These two branches lend themselves to catchy phrases and images. Sympathetic activity is called the Stress Response or the Fight-or-flight Response. Parasympathetic activity is called the Relaxation Response, or the Rest, Digest and Repair Response. These two branches of the nervous system are also described as being the body's accelerator and brake. One speeds us up and the other slows us down.
This Stress-Relaxation polarity may look like a homeostatic, on-off system, but this is deceptive. The Autonomic Nervous System is so widespread that it necessarily operates through multiple hierarchies of regulatory feedback. Sympathetic arousal, the stress response, is also far stronger than the Relaxation Response. It also has four or five times as many nerve connections, and its effects are longer lasting. Our stress response is hotwired to be fast and powerful, which is crucial in dangerous situations. In contrast, the relaxation response is slower, weaker and more easily interrupted.
THE STRESS RESPONSE
Hardly any of us will go into a full fight-or-flight response on any day but we can use this mechanism to illustrate how stress affects the body. Fight-or-flight starts with the perception of danger. This triggers off the HPA axis to secrete adrenalin and cortisol, and to release glucose into the bloodstream. The heart rate, blood pressure and breathing all increase dramatically to circulate this energy to where it is needed. The appropriate blood vessels dilate and contract to maximise the flow of blood to the muscles, heart and lungs. The big skeletal muscles tense up in preparation for action, and senses are heightened. This prepares the body for rapid movement.
The body hasn't enough glucose to do everything at once so it has to prioritise. To feed the muscles with the fuel they need, it has to starve other functions. It does this mainly constricting local blood vessels to reduce supply. In this simple but effective way, it winds back the processes of digestion, immunity, repair, sexuality and conscious thought. These on-going functions consume a lot of energy. Digestion takes 10-20% of the body's energy each day. Thinking takes even more. In a crisis, we need that energy elsewhere.
All this is positive and adaptive. The problem comes if we don't return to balance quickly enough. If we stay aroused for too long, this is the kind of pattern that gradually emerges. A constantly elevated heart rate contributes to cardiovascular disease. Rapid breathing leads to respiratory problems and anxiety. Chronically tight muscles leads to pain and injury. High blood sugar levels causes many problems as I will detail shortly. These all destabilise homeostasis by over-stimulating certain systems.
Conversely, heightened arousal impairs other systems by understimulating them. It starves them of the resources they need to function adequately. We don't digest food at all well when we are stressed. Immune function suffers, making us susceptible to disease or infection. The processes of growth and repair, which work best in deep sleep, are inhibited. Libido fades and the rhythms of the reproductive process are disturbed.
Stress also affect the mind. It weakens our capacity for deliberate thought and damages memory. We lose self-awareness and operate on automatic pilot. Since stress is our perception that we are not coping well, we are also likely to feel miserable, anxious and over-vigilant. In coming chapters, I will explain these effects in more detail.
THE EFFECTS OF CHRONIC STRESS
The first proof that stress is damaging to health came from Hans Selye. In his era the germ theory of disease was king. This is the view that a particular disease is caused by a unique pathogen, and that if you can kill the pathogen, you cure the disease.
What Selye discovered to his great surprise, was the opposite of this. Rather than one stressor causing one particular illness, Selye found that any number of different stressors had a similar convergent effect. Stress, as anything that upsets homeostatic balance, could cause illness and death in itself, just as Cannon had predicted it would.
Selye spend years torturing thousands of small animals to death and then dissecting them. He found that regardless of the methods he used, his animals all died with similar symptoms. Firstly their adrenal glands were enlarged, indicating that their stress response was working overtime. Secondly, their thymus and lymph glands had atrophied. This indicated that their immune systems were shot, and explained why they commonly suffered from bacterial and fungal infections. Thirdly they usually had gastric and duodenal ulcers.
Selye also made the key distinction between acute, transient stress governed by adrenalin, and persistent, low-level arousal mediated by cortisol. This idea is wonderfully explained by Robert Sapolsky in his book Why Zebras don't get Ulcers.
If a zebra is attacked by a lion, the adrenalin rush will massively dilate blood vessels to its heart, lungs and leg muscles, all in the service of explosive speed. It simultaneously shuts down the blood supply to digestion and other functions, which matter little at that time.
A text-book fight or flight response doesn't last very long and doesn't need to. A zebra that hasn't escaped the lion within five minutes is probably dead. So the adrenaline rush spikes and fades away quickly, usually within 10-15 minutes at most. And this is why zebras don't get ulcers. They have a few minutes of high stress, to which they are well adapted, but their adrenalin levels soon drop back to their normal resting state.
Stress only becomes a health risk when low-level persistent stress sets in and when cortisol takes over. In this context, don't think of zebras. Think of baboons, or humans. A group of baboons in its ideal environment doesn't need to worry much about food supply or predators. Baboons get stressed by other baboons. Baboons are aggressive, hierarchical animals, who maintain their status by routinely attacking their inferiors, particularly females and juveniles, whenever they feel like it. As a result, baboons can be mildly anxious all day long and so, unlike zebras, they do get ulcers, heart disease, opportunistic infections and other stress-related problems.
This is more like our human situation. Hardly any of us will go into a full fight-or-flight response in any day. We don't face violence on a daily basis. Instead, we are more likely to suffer from the low-level, unresolvable stress that comes from being social animals, vulnerable to the opinions and emotions of others. It is our families and colleagues rather than our enemies that cause us the most grief.
Over time, our bodies adapt by recalibrating upwards our set-point levels of both stress and relaxation. Instead of relaxing completely whenever possible, we maintain a persistent low level of arousal just in case. Gradually it feels natural and ordinary to be edgy and restless all day long, whether the immediate circumstances demand it or not.
Let me explain why low-level persistent stress is so damaging. A full-on fight-or-flight response shuts off digestion and immune function. It closes the factories completely. In time of low-level stress however, the factories reopen but with reduced capacity. Because the body is still using energy to maintain a mild stress reaction, digestion and immunity will not get the resources they need to function well. Cortisol will limit their supplies of both the energy to run the factories, and the raw materials they need – namely glucose, proteins and fats – to do their work.
When stressed we still digest food. but with interruptions. We still fight off infections, but badly. We still think but inefficiently. We go to sleep but we're restless and easily disturbed. We get tense quickly, stay tense when we don't need to be, burn through our energy fast, and push on feeling exhausted. None of this is life-threatening but it wears us down. It is that 10% of extra tension impairing crucial functions year after year that does the damage.
CORTISOL
I've now mentioned cortisol several times but what exactly is it and what does it do? Cortisol is a hormone produced in the adrenal glands. Its function is to increase the levels of glucose or available energy in the bloodstream. Because we need energy primarily for physical action, cortisol levels rise in the morning, peak about midday and fade away at night.
As well as this one big daily oscillation, cortisol levels can rise quickly in a stressful situation. Adrenalin triggers off the immediate response by burning the available energy in the bloodstream, but cortisol backs it up. Cortisol is far-sighted. It calls up new supplies for the troubles that may lie ahead. This is how we can get stressed by what never happens. The body is producing high levels of cortisol in anticipation of danger, 'just in case'. In fact, many doctors regard high resting levels of cortisol as the basic medical evidence for stress.
Cortisol is crucial for our wellbeing. We need to have energy on tap to do anything at all. So what is the problem with having too much glucose in the bloodstream? The simple answer is that it upsets homeostasis. Too much of one thing means too little of something else, and the delicate rhythms of the body will not work as well they should.
Too much glucose is bad for the cardiovascular system. It make the blood viscous and sticky thereby increasing blood pressure. Sticky blood is also more like to form the platelets that leads to atherosclerosis and vascular damage.
Cortisol keeps glucose in the bloodstream by preventing its uptake by the cells of the body. Because this directly counteracts the effect of insulin, it leads down the path to diabetes.
Cortisol stops the cells of the body from absorbing the proteins and fats they need to maintain their vital metabolic functions. Cortisol is very one-eyed. It has another purpose for those proteins and fats. It breaks them down into yet more glucose.
Cortisol is a sugar junky. It converts everything it can find into sugar. It is like taking out extra credit cards, and mortgaging the house to maximise the cash in your pocket.
In the first stages of stress this seems to work fine. The adrenalin rush and the cortisol backup really do give you more energy for a while. Robbing Peter to pay Paul seems to work. This is when people feel they are still coping well in difficult times, but this kind of overspending can't last forever. It eventually throws the body into that energy bankruptcy that Selye called the stage of exhaustion.
This happens because cortisol damages cell production and enzyme production. The white blood cells of the immune system, and the epithelial cells of the digestive tract, are very short-lived and need to be regenerated in colossal numbers daily. They need to be able to access the amino acids and fatty acids in the bloodstream as building blocks, but cortisol won't let them do so.
In a similar fashion, cortisol inhibits enzyme production. Within our cells, the body maintains homeostasis largely through the action of enzymes. These are proteins made up of amino acids and they are extremely potent. A single catalytic enzyme can trigger off half a million chemical reactions inside a cell within a minute. Without enzymes, all chemical interactions in the body would happen far too slowly for life. Cortisol however, steals the proteins necessary for their production.
Enzymes are also necessary for digestion and immune function. Under stress, the pancreas and liver can't produce adequate amounts of the digestive enzymes that break down food. The white blood cells can't produce sufficient antibodies to combat infections. This means that one of the most obvious, and well-tested, effects of stress is a person's susceptibility to common infections like the flu.
Of course, cortisol is a good hormone. We need it to cope with the demands of the most ordinary day. The problems only come with excess, with habitual overarousal, and the failure to balance it out with sufficient rest and recovery time.
