The popularity of high-intensity interval training (HIIT) has been on the rise for years. HIIT is a workout that alternates between intense bursts of activity and periods of less-intense activity or complete rest. HIIT workouts can be performed during cycling, swimming, elliptical cross-training, sprinting, and are the basis of most group exercise classes. It can be done anywhere, even without equipment — and can be extremely challenging.
HIIT’s popularity is partly due to the post-exercise period— called “EPOC”, which stands for excess postexercise oxygen consumption. This is the 2-hour period after exercise where the body is restoring itself and thus using more energy to maintain homeostasis.
The vigorous nature of HIIT workouts increases the EPOC, adding about 5-15% to your daily calorie burn. Because of this, HIIT has gained a reputation as being a “magic pill” of exercise. With HIIT, you may see changes within a matter of weeks, which has most people thinking, if a little HIIT is good, then a lot of HIIT is even better!
With HIIT, you can burn a lot of calories in a short amount of time. Studies all show improved cardiovascular fitness and improved muscle tone. Positive physiological benefits from HIIT also include improved fasting blood glucose and insulin sensitivity. It’s easy to get addicted to the of post-HIIT endorphin high. It only takes a few intervals to experience your body’s heightened level of energy, which is influenced by a fluctuation of hormones, especially cortisol.
However, for those who suffer from an already stressful life or adrenal insufficiency, HIIT could cause serious damage and actually reverse physical progress! Intense exercise places additional stress on adrenal functions. Cortisol is released by the adrenal glands under conditions of high mental and physical stress and is the body’s primary catabolic hormone.
HIIT stimulates significantly acute cortisol response and chronically high level of this hormone can increase the risk of a number of health issues— including weight gain, depression, digestive issues, chronic fatigue, sleep problems and brain fog. Studies have found that excess cortisol can stimulate fat gain and cause poor quality sleep.
Cortisol is essential for your body’s fight or flight response; however, it causes fat storage in your abdomen when it’s in excess. Yes, cortisol can cause belly fat, but it’s not a direct reaction to its secretion.
When cortisol is produced, along with adrenaline, there is an increase in the body’s metabolic rate and energy production through glucose. This is to provide the body with extra fuel to get it through whatever the stressful event might be. When cortisol is chronically in excess, glucose/blood sugar that isn’t actually needed as fuel for the body is converted into fat stored in the abdomen; belly fat.
Both too much and too little cortisol can affect thyroid function. Elevated levels of cortisol can suppress the release of active thyroid hormone in a variety of ways. For example, high cortisol may suppress the release of thyroid stimulating hormone in HPA axis or cortisol is required for the conversion of T4 to T3 (active thyroid) and in adrenal insufficiency, cortisol is depleted.
This may lead to conditions such as hypothyroidism. This can cause depression, weight gain, and digestive dysfunction along with a variety of other symptoms. In other words, a person might actually end up gaining weight and ruining their metabolism in the attempt to lose weight!
Done correctly, HIIT can be a useful tool! But not for someone who is not in a healthy place, both mentally and physically. Adding physical stress (even well-intended exercise!) to an already high-stress lifestyle (due to work or personal reason) can actually lead to disease within the body.
If you are suffering from any health issues like gut problems, hormone imbalance, adrenal stress and the like, avoid HIIT until your body is healthy enough to handle it. Never exceed more than 2 HIIT workouts a week to ensure full recovery and avoid damage.