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The scoop on HIIT workouts - Yay or Nay?

With our fast-paced 21st century lifestyles, you've just gotta love HIIT. Who wouldn't love a 7 minute workout that is said to be as effective as a 45 min workout? In between catching up on emails, running to work or dropping kids to and from school a 7 min workout sounds like heaven. It seems someone finally figured out how to defy nature - amazing!

But wait….we're not quite there yet, let me explain why HIIT may not be as magical as it promises to be.

HIIT stands for high-intensity interval training, which involves working out at super high intensity for a short interval then followed by a short rest interval, HIIT is famous for being a great way to get fit, improve strength and lose weight fast.

HIIT came to the scene after a study by Dr Izumi Tabata back in 1996. He took a group of elite athletes ( the fit of the fittest) and divided them into two groups. One group worked out at moderate intensity (70% VO2 for 60 min) and the other at extremely high intensity (170% VO2 for 4 min). By the end of the study, the high intensity group had burned more calories and that set the basis for HIIT. It is important to note that the athletes in this study were elite athletes who could manage to work out at 170% V02 capacity. For an average human 100%VO2 is total exhaustion. Most people do not have the anaerobic or aerobic capacity to be able to sustain a high heart rate long enough to actually maintain a high enough intensity. So in reality it might be very difficult to get a real hit from HIIT.


It is well established that women exhibit several unique anatomical and physiological characteristics that distinguish their responses to exercise. Women are generally smaller than men, have less muscle mass and and more fat mass for any given body size. Blood volume, stroke volume and cardiac output are all lower in ladies, not to mentioned that our fluctuating sex hormones also influence our capacity to exercise. All this to say that while HITT may be a great form of exercise, we cannot expect the benefits to correlate similarly in men and women. With HIIT, men are able to more easily reach a high VO2 capacity which is where the real Hit of HIIT lies. Women would have to work harder and faster to reach similar V02 capacities. The question then becomes whether this extra work is actually necessary?

Previous research has demonstrated that both sexes gain significant health benefits when moving from a sedentary state to low/moderate levels of exercise; however, when intensity further increases to vigorous levels, as observed in HIIT programs, males gain greater health benefits than females. Women appear to benefit from increased duration of low to moderate intensity with limited benefits following higher-intensity exercise.


The current exercise guidelines for adults recommend at least 150 minutes per week of moderate physical activity — or 75 minutes per week of vigorous physical activity. I know you might love your HIIT classes and I am certainly not here to tell you to ditch them, I would like to simply ask you to pay more attention to a few more factors before, during and after exercise.

  1. Cortisol Output

21st century living is often plagued by stress and when we add a high-intensity workout, it continues to push the body’s cortisol output higher and higher. Cortisol is a hormone your body releases to help you handle stress. During HIIT, the brain senses stress, and a cascade of hormones including cortisol is released. The release of cortisol activates the sympathetic nervous system, generating a fight-or-flight response. Cortisol is responsible for physiological changes, such as the quick breakdown of fats and carbohydrates and a rise in blood sugar for immediate energy, and repressing the immune system to focus the bodies’ energy on the potentially life threatening task at hand.

Cortisol release also tends to :

  • suppress your growth processes

  • suppress your digestive system

  • suppress your reproductive system

  • change how your immune system responds - there is an "open window" of impaired immunity after intense exercise so viruses and bacteria might have an easier time invading and infecting the body.

That said, if your hormones are fairly well balanced and you do HIIT workouts once or twice a week, your body only registers a small amount of cortisol, but it you are regularly stressed out, anxious or dealing with hormonal imbalance and proceed to workout intensely on most days, you cortisol levels will stay spiked.

  1. Injury

According to the American Academy of Orthopaedic Surgeons, women are two to 10 times more likely to suffer a knee injury than men, thanks to a biomechanical difference in the angle of the hips to the knees called the "Q-angle.

High-impact movements create a force equal to about 2.5 times your bodyweight, which can put a lot of stress on your joints, ligaments and tendons. This can increase the risk of both acute and overuse injuries.

Interestingly, where you are in your menstrual cycle may also make you more susceptible to injury, with more injuries being common when women are on their periods. Why? Low estrogen at the beginning of the menstrual cycle causes reduced muscle tone and impairs coordination, making you more susceptible to injury, especially in the knees, feet, and ankles.

So to answer the question of how much is too much, my recommendation is HIIT training should be done no more than 2 times a week and this should be done in careful consideration of your unique circumstance to avoid a stress on stress situation.

If you do take up hiit, avoid low carb diets or calorie restriction. A combination of high-intensity training, other stressors and low carb diets can raise cortisol levels and create weight loss resistance. Take up a good supplementation protocol. Consider B-complex, Vitamin C and magnesium to support adrenal function and aid recovery. Eat clean. Remember the importance of refueling your body after such intense workouts and drink lots and lots of water.

Lastly, allow your body sufficient recovery time after training. This allows you to rebalance your hormones and avoid adrenal fatigue. Most importantly make sure you are sleeping well 7-9hrs a night. Poor sleep and HIIT can very quickly catapult to total burnout.


The body always gives us clues to its internal situation, some signs are symptoms that you might be overdoing it are:

  • chronic fatigue/ burnout

  • muscle fatigue or a noticeable decrease in power while exercising

  • changes in mood / lack of physical and psychological motivation

  • changes in sleep patterns or sleeplessness

  • feelings of anxiety

  • repressed immune system and consistent illness

  • Changes in your menstrual cycle patterns - loss of menses, delayed ovulation etc.

  • Increased insatiable appetite

  • Weight loss resistance

  • Increased inflammation

  • Digestive health issues


Girls, I know we are all out to burn as much fat as possible and tone up our bodies, but I am here to be that voice in the back of your head to remind you that there is much more to health. If HIIT leaves you totally flustered with your hormones out of whack and constantly edgy, you may be fit, but that is no definition of health.

I long for us to see health not as a ‘look’ ( read: slim, toned, and perky) but more as a ‘feel’ (read: energy, power and strength). So if you do HIIT simply to look good, sadly that is not good enough. You need to ‘feel’ good from the inside out.


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