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Is HIIT bad for you? The downsides of high-intensity workouts

HIIT workouts promise better results in less time, but they can take a toll on your body.

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High-intensity interval training (HIIT) comes with a lot of benefits -- and a lot of potential drawbacks, as it stands. 

The good? 

Studies show that HIIT workouts can burn more calories in less time than other types of workouts, specifically steady-state exercise such as jogging. In fact, one study suggests HIIT can produce the same health benefits as moderate-intensity continuous exercise in half the time. Other research proves HIIT to be a helpful tool for reducing resting blood pressure, increasing VO2 max, losing body fat and other benefits.

Given the benefits and that "lack of time" is one of the most common excuses for skipping out on exercise, it makes sense that HIIT has become a popular form of exercise. 

And the bad? 

Overdoing any type of exercise can spell damage for your body, but with HIIT, it's important to be especially cautious. Recent research has shown that performing too much high-intensity exercise may undo the very benefits you started doing it for in the first place. 

Defining HIIT

A man and woman doing a high intensity workout in a garage gym

HIIT generally involves short bursts of near-maximal effort followed by short rest intervals.

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The term "HIIT" has become pretty ambiguous, and it means different things to different people. The definition of HIIT varies even in the scientific literature that studies this form of exercise. 

In general, HIIT has come to be defined as exercise characterized by short, vigorous bursts of energy followed by short rest periods. A classic and simple example of HIIT is 30 seconds of running, 30 seconds of resting. 

HIIT was initially a way to improve aerobic fitness and was generally only used by athletes to increase their capacity for running, cycling, swimming or other forms of cardio. But in the fitness industry, HIIT encompasses everything from straight conditioning to high-volume weightlifting to CrossFit-like workouts. 

What happens when you do too much HIIT

a woman riding a stationary bike in a garage gym

Too much HIIT can leave your body depleted. 

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"HIIT is very taxing on the body, hence the 'intensity' in the name," says Lee Jay, a personal trainer based in Tel Aviv. "For all of its benefits, HIIT can sometimes cause more harm than good." 

Below are six ways too much HIIT wrecks your body. 

Cortisol levels spike

Exercise, although usually a good stressor, is still a stressor. 

"HIIT can push our bodies to limits that spike our cortisol levels," Jay says. "As the primary stress hormone in the body, cortisol is involved in how our body handles 'fight or flight.' Although short-term spikes can help our body grow stronger, too high an increase over longer periods can result in a number of unwanted side effects, including digestive issues, bloating and weight gain." 

Such intense exercise can also cause lasting anxiety outside of your workouts, Jay points out, as the body's natural stress responses remain heightened due to the intensity. "The key is to achieve an optimal balance in hormone levels by interspersing intense exercise with enough rest and downtime," Jay says. 

Glycogen stores deplete

During exercise, your body first uses fuel that's available for quick processing. First goes free-circulating sugar in your blood and then it uses glycogen, the form of carbohydrates stored in your muscles and liver. 

Your body replenishes glycogen stores during rest, but if you never rest long enough between HIIT workouts, those stores will struggle to become fully replenished. Low glycogen can make you feel slower and weaker during workouts, and it can also negatively affect the way your body recovers from exercise. 

women look tired in a cycling class

More HIIT is not the answer.

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Sleep becomes elusive

Exercise can improve sleep, but too much can cause sleep disturbances. 

"Given its intensity, bashing out a HIIT session too close to bed may not serve you well as your body is running on adrenaline, making it harder to settle down for some shut-eye," Jay says, although the effects of nighttime exercise differ among people. 

More impactful than timing is chronic elevation of adrenal hormones, as I mentioned earlier. "Raising your cortisol levels to a constant high, and without a natural rise and fall, can hinder steady sleep," Jay says. 

If you find yourself unable to switch off or waking repeatedly during the night, it may be time to cut back on exercise, she emphasizes.

Metabolism is disrupted

A 2021 study on the effects of HIIT found something spooky, but not so surprising considering what we already know about HIIT and hormones. 

The volunteers in the study experienced improved health and performance in the beginning of the study, but once they ramped up to doing HIIT workouts five days per week, things changed. Participants showed mitochondrial impairment (meaning, mitochondria weren't producing enough energy to power cells optimally) as well as disturbances in blood sugar and insulin production. 

In short, excessive amounts of high-intensity exercise disrupted their metabolism. 

Joints are taxed

Excessive HIIT really becomes a problem when exercise technique is an issue, Jay says. 

"When form is inconsistent, our joints may become misaligned which puts strain on the wrong parts of the body, leading to unwanted injuries," she explains. 

This is especially true for HIIT workouts that involve plyometrics or other explosive, high-impact movements. Those with sensitive joints or health conditions that affect the joints and bones, such as arthritis or osteoporosis, should take care to limit hard landings.

Very high impact movements, such as box jumps, burpees and jumping lunges, pose a greater risk for pain and injury if done incorrectly. 

three people in a group fitness class do box jumps

High-impact movements are especially taxing.

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Demotivation persists

"Too much intensity can eventually lead to burnout and demotivation to exercise," Jay points out. If you overdo HIIT, you may find yourself dreading your workouts and ultimately skipping them, at which point you're not getting any of the health benefits of exercise. 

Forcing yourself to push through HIIT workouts you don't want to do isn't healthy, either. Instead, keep other, gentler exercise ideas in your back pocket and utilize them when HIIT just doesn't feel right. 

How often should you do HIIT?

Ideally, HIIT shouldn't constitute the bulk of your weekly workout routine. 

Many experts advise to opt for at least one rest or low-intensity day in between your HIIT workouts, amounting to two to three intense workouts a week -- and lasting no more than 30 minutes (rest, warmup and cool-down time included). 

The American Council on Exercise suggests performing HIIT one to two times a week in order to reduce the risk of injury, and to incorporate it periodically for six-week spells in order to maximize its benefits and enhance the results of other forms of exercise, such as strength training.

It's understandable that many people believe adding more exercise to their week will result in more results, Jay says. But the truth is, exercise is only one factor in living a healthy lifestyle. It goes hand in hand with diet, rest and personal well-being. 

a woman does yoga in her living room

Proactively add gentle, restorative exercise to your routine. 

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"If we continuously push our bodies past our capabilities, we're at risk of burnout, losing motivation and injury," Jay says. "In reality, very few of us need to follow a strict program. If your HIIT workouts are making you feel more low than high, it may be time to reevaluate your program."

Think about your goals and look to other forms of exercise to meet these. Low-intensity aerobic workouts, resistance training, yoga, pilates and outdoor activities are all effective forms of exercise that can bring powerful results. 

If you want to maintain some form of HIIT in your routine, Jay suggests incorporating shorter bursts, such as a couple of five-minute interval workouts in between weightlifting or aerobic sessions. Another option is to reduce your HIIT sessions to once a week and supplement with another rest day to give your body time to restore. 

You can also take a few months off of HIIT completely if you already feel like you're overtraining, and slowly reintroduce short intervals back into your workouts.

"Remember, HIIT is not for everyone," Jay says. "We each respond to exercise in our own unique way. Ultimately, if you want to keep your body moving long term, it's more important to stick to what you love, rather than what you think you should be doing."

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.