HIIT & Cortisol: What You Need to Know

Written By Kaitlyn Owens

If you spend any time on social media, you probably see claims like “HIIT spikes cortisol” or “high intensity exercise ruins your hormones”.

This narrative has led many women to second-guess their workouts, worrying that exercising the “wrong way” might harm their metabolism, hormones, or long-term health.

I wanted to talk about this topic to clear this up: high intensity exercise is not inherently harmful to your hormones.

In fact, when it’s used appropriately (more on this later), it can improve metabolic health, insulin sensitivity, cardiovascular fitness AND stress resilience.

Exercise is a stressor that involves most systems of the body (i.e., cardiovascular, respiratory, immune, endocrine & musculoskeletal) and leads to beneficial adaptations that positively impact our health. We need some intensity and effort in our training to get these adaptations. So telling women to train for “low impact” or “gentle on the hormones” is doing a real disservice to women’s health.

To gain exercise’s positive effects upon health, WHO recommends adults have a weekly volume of 150-300 minutes of moderate-intensity endurance exercise or 75-150 minutes of vigorous activity (which includes HIIT). In addition to this, it’s recommended to include two sessions of resistance training per week to preserve muscle mass & strength.

Now, a lot of us are busy and committing 2+ hours of endurance exercise + two strength workouts just isn’t realistic. This is where HIIT becomes appealing, because you are able to achieve the physical adaptations of prolonged endurance exercise, but in less time.

What is considered “High Intensity”?

Before we dive into cortisol, lets define what we actually mean by high-intensity exercise, because not all workouts fall into this category.

High-intensity exercise is typically performed at:

  • ~80–95% of your maximum effort

  • 7–10/10 on a perceived exertion scale

This typically means…

  • You’re breathing heavily

  • You can’t maintain conversation

  • The effort feels challenging to sustain for long periods

Examples include:

  • Sprint intervals (running or cycling)

  • HIIT classes

  • Circuit-style training with minimal rest

Not all exercise that feels hard is truly high intensity.

For example:

  • Steady-state jogging

  • Pilates or yoga

  • Moderately paced strength training

These fall into low–moderate intensity, even if they feel challenging.

What is cortisol?

Cortisol is commonly referred to as the “stress hormone,” but this label is incomplete.

Cortisol is the primary glucocorticoid hormone produced by the adrenal cortex and plays a critical role in:

  • Metabolism

  • Immune function

  • Blood pressure regulation

  • Stress adaptation

Cortisol secretion is regulated by the hypothalamic-pituitary-adrenal axis.

The pathway:

  • Hypothalamus → releases CRH

  • Pituitary → releases ACTH

  • Adrenal cortex → releases cortisol

This system operates via negative feedback, meaning cortisol regulates its own production to maintain balance.

Cortisol’s role in the body

#1. Maintain Energy Availability

  • Stimulates gluconeogenesis → increases blood glucose

  • Ensures fuel is available during stress or exercise

#2. Mobilize Fuel Stores

  • Promotes lipolysis (fat breakdown)

  • Increases amino acids via protein breakdown

#3. Support Stress Adaptation

  • Provides tissues with energy during physical or psychological stress

#4. Regulate Inflammation

  • Has anti-inflammatory and immunomodulatory effects

Cortisol is part of the mechanism that allows your body to respond & adapt to exercise.

During high-intensity exercise, your body needs rapid access to energy.

To meet this demand, cortisol:

  • Increases blood glucose

  • Mobilizes stored fuel (glycogen, fat, amino acids)

  • Works alongside catecholamines (adrenaline, noradrenaline) to perform and adapt to exercise.

This is a normal, adaptive physiological response.

Why This Increase Isn’t Harmful

The key distinction is between:

  • Acute (short-term) cortisol elevation

  • Chronic (long-term) dysregulation

Remember that HPA pathway we talked about earlier? When you exercise, the HPA becomes activated, leading to the release of endorphins & cortisol.

So one workout activates this system, but this is normal physiology.

Now, the intensity of the exercise does determine the cortisol response during. Research shows:

  • Workouts <60% VO2 max: minimal cortisol response

  • Workouts > 60% VO2 max, cortisol increases linerally with intensity

But research shows duration matters too! If you are exercising >90 minutes at a least 40% VO2 max, cortisol increases.

But here is the most important part.. cortisol returns to baseline ~150 minutes after you finish exercise. It’s a temporary rise to support your body during your workout.

How This Adaptation Is Beneficial

#1. Improved HPA Axis Regulation

  • More efficient cortisol response

  • Faster return to baseline

#2. Lower Baseline Cortisol Over Time

  • HPA axis stimulation by regular exercise leads to reduced resting cortisol levels over time.

#3. Improved Stress Resilience

  • Better ability to handle physical and psychological stressors

#4. Enhanced Metabolic Efficiency

  • Increased mitochondrial density

  • Improved glucose uptake (via GLUT4 translocation)

#5. Better Insulin Sensitivity

  • HIIT has been shown to improve insulin sensitivity in both healthy and insulin-resistant individuals.

When High-Intensity Exercise Might Not Be the Right Tool

It’s not that HIIT is harmful, but when high training volume AND requency of high intensity exercise is coupled with energy deficits and under-recovering, there ARE potential negative outcomes for hormones.

  • Overtraining: occurs when training load » recovery capacity. If you are regularly exercising, high-intensity training should not be the ONLY type of exercise you do. Over-doing high intensity exercise isn’t going to have

  • Under-eating/low energy availability: when energy intake does not match expenditure, the body percieves energy scarcity. This is where hormonal adaptations occur (↓ reproductive hormones, ↓ thyroid output). Eating a very low-carb diet for several days can make your body release even more cortisol during moderate exercise than it normally would.

  • Sleep deprivation: sleep is essential for cortisol regulation, nervous system recovery & muscle repair

  • Not prioritizing recovery: nutrition, sleep, rest days & stress management. Without these pillars of recovery, the body can stay in a chronically stressed state, where beneficial stressors can become overwhelming.

References

Athanasiou, N., Bogdanis, G. C., & Mastorakos, G. (2023). Endocrine responses of the stress system to different types of exercise. Reviews in endocrine & metabolic disorders, 24(2), 251–266. https://doi.org/10.1007/s11154-022-09758-1

Caplin, A., Chen, F. S., Beauchamp, M. R., & Puterman, E. (2021). The effects of exercise intensity on the cortisol response to a subsequent acute psychosocial stressor. Psychoneuroendocrinology, 131, 105336. https://doi.org/10.1016/j.psyneuen.2021.105336

Hill, E. E., Zack, E., Battaglini, C., Viru, M., Viru, A., & Hackney, A. C. (2008). Exercise and circulating cortisol levels: the intensity threshold effect. Journal of endocrinological investigation, 31(7), 587–591. https://doi.org/10.1007/BF03345606

Hackney A. C. (2006). Stress and the neuroendocrine system: the role of exercise as a stressor and modifier of stress. Expert review of endocrinology & metabolism, 1(6), 783–792. https://doi.org/10.1586/17446651.1.6.783

Hackney, A. C., & Walz, E. A. (2013). Hormonal adaptation and the stress of exercise training: the role of glucocorticoids. Trends in sport sciences, 20(4), 165–171.

Kaitlyn Owens
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