The Cortisol Creep: Is HIIT Stressing You Out?
High Intensity Interval Training has gained acclaim as being an efficient way to improve many aspects of physical fitness, but without proper recovery intense exercise can lead to the constant presence of cortisol in the bloodstream and heightened symptoms of physical stress even when exercise is not being performed.
What is HIIT?
Whether you are on your Peloton, doing a Youtube workout, or attending a class at the gym, chances are good that you’ve heard of, and maybe even tried High Intensity Interval Training, or HIIT. A HIIT workout consists of short bouts of intense work lasting anywhere from 10 to 60 seconds, followed immediately by a period of active recovery of the same length or longer. This cycle of hard work and recovery is repeated anywhere from 3 to 10 times depending on the workout or your level of masochism.
Positive physiological benefits from HIIT include: heightened post-exercise metabolism, improved body composition, and improved fasting blood glucose and insulin sensitivity (1, 3, 11). Because of the benefits gained within only a few workouts, HIIT has gained a reputation as being a “magic pill” of exercise.
If you are willing to do the hard work, noticeable changes can be gained in a matter of weeks and you walk away from your high intensity intervals feeling a new level of productivity and power. It only takes a few intervals to experience your body's heightened level of energy which is influenced by a fluctuation of hormones, cortisol being a big contributor (4).
What is Cortisol?
Cortisol is one of the many hormones our body produces to handle stress. During HITT, the brain senses stress and a cascade of hormones are released including Cortisol. The release of cortisol activates the sympathetic nervous system, generating a fight and flight response (4, 16). Historically, this sympathetic nervous system response to danger was the key to our early survival, providing our bodies with immediate energy and power to fight or flee when necessary.
Some of the physiological changes that cortisol is responsible for are: the quick breakdown of fats and carbohydrates and a rise in blood sugar for immediate energy, and repressing the immune system to focus more of the bodies’ energy to the potentially life threatening task at hand (2)
HIIT and Cortisol
Part of what makes HIIT training so effective at turning the body into a lean, fast, and powerful machine is this cortisol response that it generates(4). As your legs start pedaling as fast as possible, your brain receives the message that your survival depends on this interval, at which point cortisol and other hormones are released to send you into the sympathetic nervous system response. The body then makes metabolic improvements following this energetically and hormonally demanding experience(3).
HIIT, Cortisol, and the Creep
The problem with cortisol is when our body has too much of it -- either because of physical or psychological stress -- and it floats freely in the bloodstream, causing negative symptoms to ‘creep’ into your everyday life.
Overtraining syndrome has a number of physiological causes including a raised level of cortisol (5, 6) with prominent symptoms including (7,8,12, 15):
Chronic fatigue
Muscle fatigue or noticeable decrease in power while exercising
Mood swings
Lack of physical and psychological motivation
Changes in sleep patterns or sleeplessness
Feelings of anxiety
Repressed immune system and consistent illness
When your body is being overly taxed by an imbalance of cortisol any of these symptoms can be present, even when you haven’t worked out within the last few days. Everyday tasks such as packing lunches and driving to work might leave you feeling agitated, with your body misinterpreting these as life-threatening stresses.
Ideally, your body should be able to accurately determine when the reaction of flight and fight is most useful and appropriate, but too much HIIT can confuse the brain into signaling a protective response even when our bodies are supposed to be calm or at rest.
Because HIIT solicits such a powerful reaction from our sympathetic nervous system, it's critically important to prioritize recovery.
HIIT and the Importance of Recovery
In contrast to the sympathetic nervous system, the parasympathetic nervous system is responsible for sending the body into rest, digest, and recovery mode (16). Recovery is a crucial component to your HIIT workouts resulting in positive physical changes -- I’m talking about recovery between intervals and recovery days between your next workout (9). The quality of your recovery is also important, and can be enhanced with a number of different practices including (13, 14, 15):
Sleep
Good nutrition and hydration
Meditation and slow breathing exercises
Foam rolling or massage
Abstaining from intense exercise
If our body is constantly in a state of stress the positive effects of HIIT can be reversed, with your hard work working against you. Listen to your body's state of stress, both psychological and physical and if you feel any of the warning symptoms listed, take some additional time away from HIIT. It is important to note that this kind of workout should be performed 2-3 days a week at most, with rest days between each HITT session. Periodizing your workout program on a monthly cycle is a good way to prevent deleterious symptoms of “overtraining”, leaving yourself a few days to a week that do not include HIIT workouts(10).
While HIIT will make your body stronger in many ways, because of the cortisol response it generates, it is perceived by the body as stress. The role of recovery is an essential component to this method of training, as is an awareness of physical and psychological signs of chronic stress.
For more information on monthly fitness and recovery programming that is specific to your body, email Lexy for a free 20 minute consultation call!
This article was originally published April 2021 and can be found on the Healthline website.
Hi, I’m Lexy — a personal trainer for women 40+
I help you feel strong, capable, and at home in your changing body through functional strength, mobility work, and breath-led movement that evolves with you through every stage of menopause and midlife.
