If you don’t have a lot of time to work out, you can still maintain an optimal level of fitness with shorter periods of exercise — if you’re willing to work hard.

That’s what many studies are finding when it comes to a type of exercise called “high-intensity interval training” or HIIT.

You’ve probably heard of it as it’s gained popularity during the last few years as an efficient way to exercise and reach your fitness goals, particularly for busy people. It’s not ideal for everyone, however, so it’s good to know both the advantages and disadvantages before you start.

What Is HIIT?

HIIT is a training technique that involves short bursts of all-out, maximum effort, followed by short recovery periods. The goal is to keep the heart rate up and burn more fat in less time.

This type of intense training — where you work at 80 to 95 percent of your estimated maximal heart rate in the intense periods — requires a higher oxygen intake, which equates to a greater calorie burn. Even during the rest periods, you’re at 40 to 50 percent of your maximal heart rate, so you’re still working while recovering at the same time.

The intense routine raises your metabolic rate to the point where you continue to burn calories even after you finish the workout, which is one reason why this sort of exercise can be so effective when it comes to reaching your fitness goals.

HIIT is also convenient. You can do the workouts anywhere, you don’t need any special equipment, and you have to give up only short periods of time. You can also adapt most any exercise you enjoy — including cycling, swimming, running, group fitness classes and more — into a HIIT workout.

What Are the Advantages of HIIT?

HIIT has maintained its popularity by delivering on many of its promises. The benefits include the following factors.

1. HIIT Burns Many Calories in a Short Time Frame

If time is an issue for you when it comes to exercise, HIIT might be the answer. When you work more intensely, you burn more calories not only during the workout but in the period after too.

One study, for example, compared the calories burned during 30 minutes of HIIT — involving 20 seconds of maximal effort followed by 40 seconds of rest — and 30 minutes of weight training, running and biking. Results showed that HIIT burned 25 to 30 percent more calories than the other forms of exercise.

2. HIIT Continues to Burn Calories After the Workout

If you want to burn calories even after your workout is done, HIIT is the way to go. Studies have shown that this type of training can increase your metabolic rate for hours after exercise, with some indicating that it can increase that rate even more than jogging or weight training can.

Other studies show that HIIT can increase 24-hour energy expenditure just as much as endurance exercise, even though the time commitment is reduced.

3. HIIT May Help You Lose Fat With Less Time Commitment

Diet is the key when it comes to weight loss, but HIIT can help. In one review of 13 studies on exercise and weight loss, researchers found that both HIIT and traditional moderate-intensity exercise helped reduce body fat and waist circumference — but that HIIT required 40 percent less training time and commitment to achieve those results.

Other studies suggest that HIIT may be helpful when it comes to losing the most dangerous type of body fat — that which exists around the abdomen. In one small study in which participants performed HIIT workouts — 20 minutes per session — for 12 weeks, results showed significant weight loss and reduced total fat mass as well as reduced abdominal and trunk fat. The exercise group also decreased unhealthy visceral fat by 17 percent.

4. HIIT Can Improve Heart Health

While most any form of exercise can improve cardiovascular health, HIIT has been found in studies to reduce it in specific ways — mainly by lowering heart rate and blood pressure and improving overall cardio fitness.

In one study on stationary bike training, HIIT decreased blood pressure as much as endurance training, but in less time — 20 minutes per day rather than 30. The training seems to be particularly helpful for those who start out overweight. In normal weight individuals with normal blood pressure, HIIT hasn’t consistently proven to lower blood pressure.

HIIT may also help those who have cardiovascular disease to improve their heart fitness. In one 2018 review of 17 studies, researchers found that HIIT was superior to moderate-intensity continuous training when it came to improving cardiorespiratory fitness overall.

5. HIIT May Help Prevent Type 2 Diabetes

High blood sugar levels can lead to type 2 diabetes and a number of other health complications, including heart disease. All types of exercise help, but HIIT may help safeguard certain individuals against type 2 diabetes better than other types of exercise.

In one review of 50 studies, researchers found that HIIT was similar to regular training when it came to reducing hemoglobin A1c levels, which is an average measure of blood sugar levels, but that HIIT was more effective at improving metabolic health in those at risk for type 2 diabetes.

In another small study, researchers found that HIIT training during a period of two weeks helped individuals with type 2 diabetes to improve insulin resistance.

6. HIIT Is Flexible

Whether you enjoy cycling, running or pushups and burpees, you can adjust any of them to include HIIT.

In a 2014 study, scientists compared HIIT with sprint interval cycling and a HIIT calisthenics protocol consisting of burpees. Each group completed four testing sessions across nine days, with each session separated by 48 to 72 hours. Each session included four repeated 30-second bouts of “all-out” efforts interspersed with four-minute active recovery periods.

Results showed that both training methods created similar benefits in terms of heart rate and other fitness measurements.

7. HIIT May Help You Live Longer

All types of exercise can help you avoid those diseases like heart disease and cancer that tend to shorten life expectancy. However, there is some evidence that HIIT may help reverse signs of cellular aging.

In a recent study by the Mayo Clinic, 72 sedentary adults in two age groups — 18 to 30 and 65 to 80 — went through a 12-week training period involving one of three routines: HIIT cycling, strength training with weights and combined strength training and cycling.

At the end of the 12 weeks, all three groups had gained lean muscle and improved aerobic capacity, but the HIIT group experienced the best benefits at the cellular level. The younger participants had a 49 percent boost in mitochondrial capacity — a measurement of a cell’s ability to take in oxygen and produce energy. The older participants had an even higher 69 percent increase.

Scientists believe that a greater mitochondrial capacity is indicative of a younger body and that keeping cells healthy in this way can reverse some signs of age-related decline.

Potential Disadvantages of HIIT

Considering the above, there are several good reasons to consider incorporating HIIT into your workout. However, you do have to take a few precautions.

1. Don’t Overdo It

In general, it takes longer for the body to recover from a HIIT workout, so most fitness experts suggest you alternate days with it. In other words, don’t do HIIT every day — take breaks in between.

All types of exercise are stressful to the body, but HIIT is even more so. This stress is a good thing as it forces the body to make biochemical changes that improve fitness. But you can overdo it, at which point the body starts to break down rather than build up.

In one study of 35 active exercisers during a three-week period, researchers found that consistently training at over 90 percent of one’s maximum heart rate could result in diminishing returns. Too much HIIT increased the risk of injury and led to body fatigue.

What’s too much? Most experts recommend no more than 20 to 30 minutes when doing HIIT and no more than three times per week — at the most.

To get the best results, balance your HIIT sessions with other types of exercise, including less-intense cardio and strength training. Allow the body to recover between HIIT workouts.

2. Watch Out for Injuries

HIIT is more challenging by its very nature, pushing the body to its limits. The increased pressure can increase the risk of injury compared to gentler forms of exercise, particularly for beginners and those who aren’t physically active, to begin with.

If you’ve been living a sedentary life, it may be best to start your fitness program with easier workouts for a time until you build up your body. If you are fit but haven’t tried HIIT before, go slow to begin with, and choose workouts that put less strain on your tendons, muscles, and joints.

If you’re already cycling or jogging, adding in some high-intensity bursts may be a better way to get started rather than engaging in a full-on callisthenic-type workout.

3. It May Increase Fatigue

Because of that longer recovery time, HIIT workouts may leave you feeling tired, even the next day. If you’re noticing some increased fatigue, it could be that you’re going at it a bit too hard and your body is not ready for it yet.

With time, you’ll build up your muscles and endurance, and you won’t feel the same fatiguing effects, but don’t be surprised if you notice increased tiredness initially.

4. It Can Increase Your Stress

If you’re going through a job change, just lost a loved one or are managing a new health problem, don’t start HIIT. The training increases stress on the body, which is what helps you improve your fitness level, but if you’re stressed out in other areas of your life, the accumulative stress could be unhealthy for you.

Combining too much HIIT with other stresses in your life could have an additive effect. Try beginning with just one HIIT session a week.