What Really Happens to Your Body When You Do a HIIT Workout Every Day

It's easy to become addicted to high-intensity interval training (HIIT) workouts. They're short, and they melt fat and build muscle. Plus, they give you the ultimate exercise high that'll leave you feeling like you ran a marathon and back. While it's tempting to get into the HIIT habit every day, it's not always the best idea for your body.

When you do HIIT, your body releases cortisol — the stress hormone — which "causes increased heart and breathing rate, pulse rate and blood pressure," says Sherry A. Ross, MD, a board-certified obstetrician-gynecologist and women's health expert at Providence Saint John's Health Center in Santa Monica, California. This makes it a good physical stressor because it activates your body's fight-or-flight response without saddling you with health issues.

But if you do too much of it, HIIT can actually keep your cortisol levels sky-high when compounded with other life stressors (hello, COVID-19 pandemic). And that's where you get into trouble because it puts your body in a chronic state of stress, which can lead to weight gain, heart disease and diabetes.

Whether you're joining a group Zoom workout or doing a quick one on your own, here's exactly what happens to your body when you do HIIT every day.

HIIT, at its essence, is a cardio workout, so it naturally increases your heart rate and the demand for oxygen in your blood.

"During HIIT, your heart works harder, meaning your blood pressure and heart rate increase at rates higher than low-intensity, steady-state exercises," Satjit Bhusri, MD, founder of Upper East Side Cardiology, tells LIVESTRONG.com. "The higher cardiac output can result in increased arterial dilation," which expands the blood vessels and increases blood flow.

This increased demand for oxygen during exercise, followed by rest, helps the heart become more efficient. So the heart not only does a better job of pumping blood, but it can pump ‌more‌ blood with each beat — all of which can decrease strain, and ultimately, lower blood pressure.

In addition, HIIT increases your energy and stamina levels, which are associated with a reduced risk of a heart attack, Dr. Bhusri says. In fact, HIIT is a great way to meet physical activity guidelines and decrease your risk of cardiovascular disease, according to a July 2019 article in the ‌World Journal of Cardiology‌.

While HIIT can do great things for your heart, you want to avoid doing it every day. "The key is to do a variety of exercises and not do the same exercises every time," Dr. Bhursi says.

If you have a heart condition, check with your doctor before you start a HIIT routine. You might have to adjust the intensity of your workout, he explains. Signs that you might be pushing yourself too hard during exercise include shortness of breath, chest pain and lightheadedness, according to Harvard Health Publishing. If you experience any of these symptoms, you should stop exercising and call your doctor.

Dr. Bhusri recommends using a heart rate monitor to help keep track of your exercise intensity. "Start exercising at a lower-intensity program and build up to higher intensities as you can tolerate it," he says.

"During HIIT workouts, your body activates fast-twitch muscle fibers," says Shayla Cornick, owner and chief happiness office of CYCLED! Studio. FYI, you have two main muscle fibers: type I (slow-twitch) and type II (fast-twitch).

Your type II muscle fibers, which are denser and larger [than slow-twitch], "are used for short, powerful exercises that take you near exhaustion," Cornick says. Think explosive movements like sprints while running or cycling, burpees and box jumps — all of which are classic HIIT movements.

While fast-twitch muscle fibers generate power quickly, they also fatigue faster and require more recovery. That's why you can't bust out squat jumps like there's no tomorrow. And after going hard for a brief time, your body needs about a minute of rest to replenish fuel for your muscles before it's ready to handle another round, according to the American Council on Exercise (ACE).

While the foundation of HIIT is cardio, tapping into your fast-twitch muscle fibers also encourages muscle growth and strength, Cornick says, giving you both cardio and strength benefits.

But be careful not to go overboard; it's best to do HIIT only two to three times a week, says Tara A. Nicolas, a Nike trainer and instructor at Fhitting Room, a HIIT-based studio in New York City.

"Everybody is different with unique strengths and weaknesses. Not everyone can push themselves to such an extreme on a daily basis without injury. Meaning for some, doing HIIT three to five times a week feels like a walk in the park while others may need a little less like two to three times a week," Nicholas explains.

Taking time off (think: 48 to 72 hours) gives fatigued muscles adequate time to recover. That said, if you're mixing up muscle groups, say, arms on Monday and legs on Tuesday, doing HIIT training on back-to-back days could be fine. Just listen to your body and take a recovery day if you're not feeling up to it.

There's no doubt that HIIT is a calorie crusher. Known as excess post-exercise oxygen consumption (EPOC), your body goes through an afterburn effect when you do a HIIT workout because it creates an oxygen deficit.

After an intense workout, your body needs additional oxygen to return to its normal metabolic state, and in this process, burns more calories 24 to 48 hours after your workout is over.

"The higher the intensity of the workout, the more oxygen is required to recover, which means your system is working to catch up long after you've left the gym," compared to steady-state cardio, Nicolas says.

While you're in the afterburn zone, you're also torching tons of fat. The goal of the high-intensity portion of your workout is to go from the aerobic to the anaerobic zone, which helps burn fat, explains Rachel Henderson, MD, a non-surgical sports medicine and orthopedic physician at the Andrews Sports Medicine and Orthopedic Center in Birmingham, Alabama.

In fact, a June 2017 review in ‌Obesity Reviews‌ showed that three weekly sessions of HIIT helped reduce overall fat and waist size in people with obesity as well as those who are overweight as much as moderate-intensity exercise, but you can do HIIT in less time and get the same fat-loss effects.

Still, you want to avoid doing HIIT every day to avoid overtraining and injury. According to the ACE, doing too much high-intensity exercise, or any form of exercise, too often without proper recovery can also lead to metabolic issues, including overtraining syndrome and lactic acid buildup — all of which can put major distance between you and your goals.

There is plenty of research that shows that working out can help support your immune system, but doing too much of it, especially HIIT every day, can backfire.

According to a May 2017 study in the ‌Journal of Applied Physiology‌, high-intensity exercise without proper recovery can cause your overall immunity to drop, leaving you more susceptible to infection. If you continue to train with low immunity, you open the opportunity for more infection.

"You have to find a balance so that your body is able to work efficiently and effectively," says Annie Mulgrew, CITYROW vice president and founding instructor. "Having a balanced fitness routine that includes rest days and is coupled with a balanced daily food routine, plenty of water and other hydrating liquids, like non-caffeinated tea and broths, will keep your body healthy and able to fight off sickness."

Because HIIT forces you to work to max exertion with minimal rest, you need to take some recovery days in between your sessions to allow your muscles to re-charge. Doing HIIT two to three times a week is enough to reap its benefits without going overboard.

When done correctly, HIIT is generally safe, but it has some inherent risks for injury, Dr. Henderson says. Because HIIT is often full of plyometric movements, it can put some added pressure on your joints.

According to Dr. Henderson, the most common injuries associated with HIIT workouts "occur in the lower extremities, such as knee or ankle sprains and muscle or tendon strains, which can also occur with other high-impact activities, like running and jumping."

"There is also a risk of back and shoulder injuries, which can be associated with repetitive bending or lifting," she says.

According to a July 2019 review in ‌The Journal of Sports Medicine and Physical Fitness‌, injuries during traditional HIIT-based exercises (like burpees) are largely due to a lack of flexibility, mobility and core strength.

To help keep injuries at bay, "it's important to maintain good form or neuromuscular control, particularly if fatigued," Dr. Henderson says. "Start 'low and slow,' then build speed and endurance over time" is also key. Cornick says that proper recovery and stretching are essential, too.

Because HIIT puts your body under an intense amount of physical demand, injuries are more likely. A simple solution is to not do HIIT every single day. Instead, weave in other types of workouts, like running, yoga and strength training. Be sure to change up the intensity, too, by varying the load, speed and tempo. (LISS, anyone?)

Look, HIIT is great, and it has many amazing benefits. But unfortunately, too much of a good thing can be problematic. If you're constantly HIIT-ing it without proper rest, you're likely on a fast track to overtraining and injury. And constant fatigue and soreness can lead to poor performance, so while you're working harder, you might not be getting stronger.

"Recovery is just as important to receive the health benefits of any exercise routine and stay healthy to avoid injury," Dr. Bhusri says. It's when your body recovers, repairs and adapts that those good-for-you gains from HIIT are cemented.

The bottom line: Mulgrew and Nicolas say it's fine to do HIIT two to three times a week, but any more than that, and you are setting yourself up for failure.