It’s time to HIIT back on cortisol myths and misunderstandings
“Don’t do HIIT, it increases cortisol!” “Don’t have too much caffeine, it increases cortisol!” “If you have a “stress belly” your cortisol levels are too high!” I see post after post on the “evils'' of cortisol for midlife and menopausal women. But honestly, when you scratch beneath the surface, most are aimed at scaring women into buying supplements or tests to reduce cortisol and “balance our hormones”.
It’s time to have a serious talk about stress and its related hormone, cortisol, which contrary to this popular (and I’d argue predatory) narrative is absolutely necessary, and is not an evil danger that threatens to impede our health and longevity, or should prevent us from doing anything more strenuous than going for a walk. Ready for Action?
Cortisol is a steroid hormone your body uses to mobilize the resources it needs for action, specifically to respond to stress, which we commonly know as the “fight or flight response.” When you experience stress–be it physiological like illness, injury, and trauma or psychological like work anxiety–the hypothalamus and pituitary glands in your brain send an SOS signal via the hypothalamic-pituitary-adrenal (HPA) axis to your adrenal glands atop your kidneys to secrete cortisol.
Once released into the bloodstream, cortisol helps the body respond to stress by increasing fuel (blood glucose), alertness, cognition, and improves cell responses to inflammation; all in anticipation of the body having to run or fight.
But it’s not just threats that cause cortisol to rise. So do the rhythms of life. Blood levels of cortisol vary throughout the day, but generally are higher in the morning, peaking around 30 minutes after waking up, and then fall throughout the day. This is called a diurnal rhythm. In people who work at night, this pattern is reversed, so the timing of cortisol release is clearly linked to daily activity patterns. Should you experience a stressful episode, extra cortisol is released to help you respond appropriately. But your levels are never–nor should they ever be–zero. You need cortisol for life.
The majority of the body’s cells have cortisol receptors, and cortisol plays an important role in regulating and supporting various bodily functions and systems, including cardiovascular, metabolic, homeostatic, cellular health, and central nervous system. Most of your body’s cells contain glucocorticoid receptors, and their function is anti-inflammatory and immunosuppressive.
The Difference Between Acute & Chronic Stress
Though exercise is a form of stress, short stressors like exercise are good for our health, because they essentially make us more stress resilient. It’s like how training in general makes us stronger and fitter. By placing a bit of stress on our system, it adapts, so we can overcome subsequent stressors more rapidly and effectively, which is essential for improvements in overall health.
Where we run into trouble is when stress becomes chronic from factors like poor sleep, demanding jobs/family/lifestyle, chronic dieting and low energy availability; and overtraining/under recovery. This chronic stress leads to dysregulation of cortisol, which in turn is associated with many chronic diseases, such as cardiovascular disease, metabolic syndrome, diabetes and so forth.
For midlife women, perimenopause can also be considered a form of chronic stress because the body is upregulated to a greater sympathetic (i.e. fight or flight) drive due to the fluctuation of sex hormones and the change in receptor regulation. This is why you’ll sometimes (too often times, really) hear that perimenopausal women should not do high intensity exercise or sprint interval training, because it “adds” to our already stressed out state. But again, exercise is not the enemy here; and when done well, it’s really the hero of this story.
The Cortisol Exercise Connection
There’s a lot of big misunderstandings of how cortisol responds in the body in response to exercise and exercise intensity. So let’s set the record straight.
Cortisol is an essential hormone for exercise. When we start exercising, the body perceives this as a major physiological stressor. We know there are significant, acute, and immediate changes to metabolism (muscles need fuel!), thermoregulation (need to offload the heat working muscles are producing), and cardiovascular (heart and breathing rates increase, changes in blood flow to increase blood to the working muscles). The increases in cortisol seen during exercise are critical to the control and regulation of these systems and thus exercise performance capacity.
There are significant differences in cortisol production depending on the type of exercise we’re doing. Long bouts of aerobic exercise keep you in that sympathetic “fight or flight” state for extended periods of time, so, as you’d expect, you have an elevated cortisol response. Though resistance training with short recovery periods will also increase cortisol, overall, aerobic exercise stimulates greater production of cortisol when compared to resistance training.
Of course, the body is adaptive. So while we see that blood level of cortisol increases in proportion to exercise intensity once the workload is above a critical threshold (50–60% VO2max), this threshold increases as we become fitter. So, at the same absolute intensity, our cortisol response will be lower after a consistent few months of training. In steady state, aerobic exercise, cortisol will initially increase and plateau; and again, the concentration at this plateau is dependent upon the exercise intensity and duration. There is a marked difference in cortisol concentration between 45 minutes and 120 minutes of endurance exercise; with greater concentrations occurring in longer duration. That’s why researchers measuring cumulative cortisol exposure in hair samples, actually find higher cortisol levels in long-distance runners, triathletes, cyclists. They’re just building up more over many hours of steady, endurance exercise.
Hard, short, supramaximal exercise intensities like HIIT and SIT can cause cortisol to spike significantly (even more so than endurance exercise), especially during the rest intervals (because the work interval is so short, it’s not immediately detectable). It stays elevated during the post-exercise recovery period, because we need it for exercise adaptation. Here cortisol increases catabolism at the cellular level, where we have muscle damage from exercise. This helps increase the circulating amino acid pool. These free amino acids are then used at the basic cell level to rebuild and repair damaged cells (primarily muscle) with insulin, testosterone, and IGF-1.
But those short, higher spikes in cortisol elicit an overall positive training adaptation, especially for women. Women have a greater response of growth hormone and insulin, as well as increased signaling to build skeletal muscle to sprint training as compared to men; and as compared to endurance training, despite the fact sprint training elicits a greater post-exercise cortisol peak.
So let’s pull that all together. In simple terms, a single bout of exercise, regardless of mode, triggers cortisol increases. But we need to consider our overall change in baseline, in terms of our adaptive response. In women, research has demonstrated that regular, moderate intensity endurance exercise creates an adaptive response to increase circulating cortisol (cortisolemia) overtime, whereas regular high intensity interval training lowers basal cortisol concentrations. The adaptation occurs because of the difference in physiological stress and how the body overcomes repetitive stress exposures.
So the rhetoric of “women should not do HIIT because it increases cortisol” does not follow through with the entire story, and it creates misinformation and misleads women to assume that low intensity exercise is the only way they should exercise.
Let’s finish with the complete story.
Yes, HIIT does increase cortisol (like all exercise), but because it is such a strong stress to the body, the body responds in kind to reduce baseline cortisol levels with chronic exposure to high intensity. So there’s no need to essentially make yourself less fit, strong, and healthy by avoiding high intensity exercise! Embrace those sprint training sessions not just for less stress, but for a fitter, healthier life!
