Are High-Intensity Workouts Making You Fat?

High Intensity Interval Training is great, but you can get too much of a good thing. Assess the downside so you can avoid it.

High-Intensity Interval Training promises incredible results in a short amount of time. And for once, a fitness trend lives up to the hype. It actually works as advertised—in most cases.

HIIT workouts push your body near its max capacity. Depending on the type of workout, you'll improve your conditioning, burn calories and get stronger at the same time. Sounds great, right?

But there can be too much of a good thing. Because HIIT is so effective, people tend to do it too often, causing problems that might interfere with weight loss goals.

We spoke with Dr. Mike T. Nelson, an exercise physiologist and owner of, to learn how much HIIT is too much.

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What Are HIIT Workouts?

High-Intensity Interval Training refers to any workout in which you perform intervals comprised of short bouts of high-intensity exercise interspersed with brief periods of rest. You're not working the whole time like a traditional running workout, but you can work at a higher intensity for longer durations thanks to the rest intervals.

HIIT workouts come in many forms. For example, you might do 30-second intervals—the length can vary—with a shorter, equal or longer rest period, depending on your level of fitness and your goals. The type of exercise can vary, too. A HIIT workout might include Sprints, Burpees or any number of intense bodyweight moves.

Why Do High-Intensity Interval Workout Work?

HIIT became popular because people saw incredible results.

This is due to a few factors. HIIT introduced people to a new style of training that's more intense than workouts they had previously performed. But more important, HIIT confers more physiological benefits than traditional cardio workouts.

HIIT workouts typically burn more calories in less time than jogging and other cardio workouts. Yes, cardio workouts burn calories, but only when you're working out. With HIIT workouts, the intense work done by your muscles shifts your body into Excess Post-Exercise Oxygen Consumption, or EPOC. During this period, your body burns calories at a higher rate to repair muscles and recover.

Also, HIIT workouts are often designed to build—or at least maintain—muscle mass by including strength moves. The more muscle mass you have, the more calories your body uses on a daily basis, ultimately helping to burn fat. In contrast, consistent use of cardio workouts may actually break down muscle tissue.

"Higher intensity work is generally better," says Dr. Nelson. "You maintain muscle mass, burn some calories, and more fat is going to be used because of EPOC." He explains that in a perfect world, someone looking to shed weight would do high-intensity work, then recover from their workouts to make progress. But there's a catch.

Overdoing High-Intensity Interval Training

Workouts put stress on your body. That's how you make progress. HIIT workouts are designed to maximize stress on your energy systems. There are many different protocols, but they all have the same general goal—to improve your overall fitness.

Extremely fit individuals are better at handling this stress, allowing them to work at higher intensities. This is the same concept as weightlifting. Someone who's stronger has a greater capacity to lift weight, which is the stress.

Problem is, your body has a limited capacity to endure stress. "Everyone wants to do HIIT stuff every day, because the literature shows you use more fat in the long term," Dr. Nelson says. "And that's generally true. But I've seen a lot of people burn out and overreach."

Other factors outside the weight room can add to the stress. "Overreaching usually happens if a person has a stressful lifestyle. Their sleep is junk and general stress is high," explains Dr. Nelson. "If you tell them to do five or six high-intensity training sessions a week, they sort of implode on themselves."

When stress causes you to overreach or burn out, a few things happen.

Your body's cortisol levels increase to release more energy; this may also increase your appetite. This can signal your body to break down muscle protein for energy —the exact opposite of the result you're working to achieve. You might also retain more water weight, have general fatigue and suffer poor performance in your workouts or athletic events, which makes it very hard to do consistent training.

Dr. Nelson says this is extremely variable. Some people might be able to crush four to six HIIT workouts a week and do fine. Others are maxed out after one or two sessions. It depends on what you can handle. And that is up to you to determine.

What Should You Do?

The key is simply not to overdo it. But this is not as simple as it sounds due to the many factors that go into your ability to handle stress.

As a general guideline, do no more than three HIIT workouts per week on non-consecutive days. Couple them with three low-intensity running or bike workouts, keeping your heart rate between 110 and 130 beats per minute. According to Dr. Nelson, this produces a high fat burn during exercise you can sustain over the long-term—a key for maximizing your results. Most feel much better the next day after these sessions, too.

And if you ever feel burned out, take a break.

Dr. Nelson shared a story about one of his clients who consistently trained hard and pushed her body. She was going on vacation and asked what she should do for training. Dr. Nelson told her to just relax, go on some walks, eat well and have fun. She reluctantly agreed. When she came back, she lost five pounds in a week.

A lot of things could have contributed to this, and the weight loss was likely not all fat. A change in water weight could have contributed. But the moral of the story is, if you push too hard with HIIT, you might not reach your fat loss goal.

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