Many gym-goers have become obsessed with high-intensity interval training (HIIT). HIIT is the type of exercise that causes burning muscles, burning lungs, and profuse sweating. Much of the draw this style of training is the expectation of high caloric burn and fat loss. But is HIIT the most effective way to drop fat?

Let’s begin with a quick lesson on how we use fuel (carbs and fats) for various exercise intensities.

The lower the intensity of the exercise we are participating in, the more fat we use, not in an absolute sense, but as a ratio. I will explain what that means in a moment. The higher the intensity of our exercise, the more glucose (carbs) we tend to use. The reason for this difference in fuel use is as follows. When you are sleeping, you burn a higher percentage of fat than while doing any other activity.

Via Iñigo San-Millán: Basically, once exercise intensity hits a certain threshold, carbs become prioritized as fat is too slow burning a fuel to meet ATP generation demands. ATP is the energy that makes the body work.
You may be asking why you don’t have 6-pack abs if this is true. The reason is that while rest states burn the highest percentage of fat, the total energy burn is so low that the resulting fat burn is minimal.

This energy usage principle is where the “fat-burning zone” concept came from, along with our obsession with steady-state cardio.

The truth is that both (HIIT and Steady State Cardio) are not ideal modes of peak fat burning.

To burn the most significant amount of body fat during exercise, you need to hit the sweet spot in both intensities and staying below the threshold that turns off your ‘fat-burning system.’ For most people, this is between zones two and three, or around 70% of your maximum effort. To put this into terms that make sense, I am referring to uncomfortable but sustainable exercise. If you can talk comfortably while exercising, you’re not working hard enough, but if you can’t sustain your exercise for more than a few minutes at a time, you may be going too hard.

Between zones 2-3 is where we maximize both fat usage and total caloric expenditure
Examples of exercise modes that hit these ‘zones’ are weightlifting with shorter breaks (45-60 seconds) between sets or very short, intense bouts of exercise with more extended rest periods (i.e., 15 seconds of max work followed by a full minute of rest). The latter of those two examples may sound like traditional HIIT, but in my experience, most coaches teach HIIT classes with bouts of exercise that are too long with rest periods that are too short.

Your heart rate will always fluctuate as you exercise, but you want to settle on an average between zones two and three in the waves of intensity.

Calories Still Matter

That isn’t to say that there isn’t a place for HIIT exercise. HIIT has a variety of unique and helpful benefits that give the mode of exercise significant value. The main reason why HIIT is so addictive to people is because of the endorphin rush and high level of satisfaction that comes from kicking your own ass. But if HIIT is the only type of activity you do, or if you do too much of it, you will suffer.

If a person can exercise 4-5 days per week, one to two of those sessions should be HIIT sessions, and the balance should be strength training with shorter rest periods. The balance should go to low-intensity recovery exercises like zone one biking, walking, swimming, and mobility-based exercise. I make this suggestion while assuming that fat loss (not absolute strength) is your primary goal.

Additionally, there is the issue of over-exercising, under-recovering, and the ‘rejection’ of weight loss that often comes with overdoing HIIT, but that is a conversation for another day.

In the end, you need to vary your exercise to get well-rounded benefits and exercise that you can do more consistently with greater recovery will serve your body better than exercise that runs you into the ground each session.

If your primary goal is psychological well-being and the sense of satisfaction that comes with hard work, keep doing HIIT. Do it every day if you like. But if your ultimate goal is to burn the most generous amount of fat over a long-term period, you’d better start mixing it up.

Tommy, Founder of Hybrid Fitness