HIIT training promise better results in less time, but they can take a toll on your body.
It’s true that HIIT training comes with a lot of benefits — but it also has a lot of potential drawbacks.
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 training 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 training has become a popular way of working out.
And the bad?
Overdoing any type of exercise can cause damage to your body, but with HIIT training, it’s important to be especially careful. 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 training
The term “HIIT training” has become pretty unclear, and it means different things to different individual. The definition of HIIT training differs even in the scientific literature that studies this form of workout.
In general, HIIT training 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 training is 30 seconds of exercise bout, 30 seconds of rest bout.
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 training involves everything from straight conditioning to high-volume weightlifting to CrossFit-like workouts.
What happens when you do too much HIIT training
“HIIT training is very demanding on the body, hence the ‘intensity’ in the name,” says Glenn Ang, a personal trainer from RAW. “For all of its benefits, HIIT training can sometimes cause more harm than good.”
Below are six ways that show too much HIIT training can ruin your body.
Cortisol levels spike
Exercise, although usually a good stressor, is still a physical stressor.
“HIIT training can push our bodies to limits that spike up our cortisol levels,” Glenn says. “As the main stress hormone in the body, cortisol is involved in how our body handles ‘fight or flight response.’ Although short-term spikes can help our body grow stronger, too high level of increase in cortisol over longer periods can result in a number of undesirable side effects, including digestive issues, bloating and weight gain.”
Such intensive workout can also cause lasting anxiety outside of your workouts, Glenn points out, as the body’s natural stress responses remain heightened due to the high intensity. “The key is to achieve an optimal balance in hormone levels by interspersing intensive workout with enough rest and downtime,” Glenn says.
Glycogen stores deplete
During exercise, your body first uses fuel that’s available for quick processing. First goes free-circulating blood sugar in your system 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 training, those stores will struggle to become fully replenished. Low level of glycogen can make you feel slower and weaker during workouts, and it can also negatively affect the way your body recovers from exercise.
More HIIT training is not the answer.
Sleep becomes elusive
Exercise can improve sleep, but too much of it can cause sleep disturbances.
“Given its high intensity, bashing out a HIIT workout that is too close to bed time may not serve you well as your body is running on adrenaline, making it harder to settle down for your sleep,” Glenn says, although the effects of nighttime workout 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 good sleep,” Glenn says.
If you find yourself unable to switch off or waking repeatedly during the night, it may be time to cut back on exercise, he 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 training 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 training really becomes a problem when exercise technique is an issue, Glenn says.
“When form is inconsistent, our joints may become misaligned which puts strain on the wrong parts of the body, leading to unwanted injuries,” he 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.
“Too much intensity can eventually lead to burnout and demotivation to exercise,” Glenn points out. If you overdo HIIT training, 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, Glenn 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.
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, Glenn 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,” Glenn 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.”