We all speak of it and experience it, but what is stress? It is defined as a non-specific response by the body to any demand (stressor) that overcomes, or threatens to overcome, the body’s ability to maintain homeostasis (that state of equilibrium of the body’s internal biological mechanisms) (1 – 2). What this means is that regardless of the nature of the stressor (e.g., exercise, starvation, daily commute, work-related issues), the body perceives stress as a biological event and responds in essentially the same way. What differs is simply the magnitude of our response and this task is assigned to the brain that initiates the appropriate ‘fight-or-flight’ response through our nervous and hormonal systems (3-4). An acute bout of stress and the consequent biological response is quite normal and integral to our survival as a species as long as the body has the opportunity to recover between stressors to restore itself back to baseline (5).

Biological Design

To gain a deeper insight into our biological stress response, using an example of our ancestors may paint a clearer picture. The nature of their stress was very different to what we typically experience today. Their primary stressors involved a fight for survival or to the death against a predator or aggressor and the nature of the stress was an intense, acute physiological response (Figure 1). However, after this brief, but stressful encounter, what followed was ample recovery to return to baseline (state of calm – parasympathetic or PNS dominance). This allowed each physiological system (e.g., immune system) time to restore and regenerate itself after fighting to maintain homeostasis.

By contrast, today’s stress generally involves lower-intensity, sustained psychological stressors that sometimes never go away, but accumulate (Figure 2). For example, you might sleep through your alarm and wake up in a panic late for your meeting, skip breakfast, get delayed by a slow commute, arrive late for a presentation, get reprimanded by your boss, then finally make it to your office whereupon you receive a call that your child is sick and needs to be picked up from school – sound familiar? These sustained stressors, although smaller individually, accumulate and deny the body that needed time to repair, recover and replenish.

Nonetheless, in either situation (ancestors v. present-day) the body activates its stress response in similar ways, albeit it at different intensities and while we are familiar with many responses (e.g., increased heart rate and blood pressure, mobilization of stored fats, increased sweat rates), we may be unaware of others that merit concern (Table 1). For example, elevated levels of epinephrine enhance blood clotting ability by increasing platelet adhesiveness (5). By design, this might be needed to stop one from bleeding to death during a survival fight, but think about this sustained effect upon cardiovascular health.

Table 1: Stress Response Influence on Physiological Systems

Ever wonder why we experience dry mouth when nervous, why a dog urinates when scared or why you may need to run to the bathroom before a big race? Consider our survival needs – any energy-consuming events deemed unnecessary (e.g., reproduction, growth, maintenance) during periods of intense stress are automatically shut down to provide needed resources and energy to the systems that facilitate survival (e.g., muscles, skin – thermoregulation). The release of saliva, digestive enzymes and mucus in the mouth, stomach and upper GI all consume energy and are therefore shut down during our stress response. Similarly, systems involved in growth, repair, maintenance and reproduction are also temporarily inhibited under stress in order to provide the needed energy and resources elsewhere. By contrast, smooth muscle contractility of the lower GI and bladder become activated to void unnecessary urine and fecal matter that may slow you down if you need to run to survive. Ever witness an athlete suffer what appears to be a painful injury during competition, yet manages to somehow play through it? Our fight-or-flight response also temporarily decreases pain perception to sustain our fight-or-flight response.

Sustained Stress

So what happens as a consequence of sustained stress that many of us encounter in our daily lives? Cortisol is a key adrenal hormone released under stress that plays an integral role in preserving homeostasis. One essential function of this hormone is to avoid depletion of blood glucose considering how critical this fuel source is for optimal functionality and survival. Increased circulating levels of cortisol that follow liver glycogen depletion or accelerated muscle uptake of blood glucose (i.e., exercise) influence many of our ‘fight-or-flight’ responses, especially those we need to inhibit (Table 2). Temporarily, these alterations are manageable when spaced few and far between, but when sustained indefinitely, they will compromise health, fitness and performance.

Table 2: Sustained Effects of Elevated Cortisol (1, 2, 5, 6)

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