By Michael Ormsbee, PhD, Florida State University

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Do you struggle to carve out time for exercise? You can actually burn a lot of fat by exercising for far less time per day, but there is one caveat. Professor Ormsbee explains.

Designing High-Intensity Workouts

If you want to cut your exercise time in half while still burning a substantial amount of fat, you must work out very hard. This is where High-Intensity Interval Training or HIIT comes into play—you intermix rest periods with very high-intensity workouts. For example, you might run fast for one minute and then recover at a slow jog pace for one minute and repeat this for cycle for 20 minutes.

First, remember that two major fuel sources are used during exercise: stored fat and stored carbohydrate. The use of each exists along a continuum and is never only stored fat or only stored carbohydrate. 

However, a greater proportion of fat will be metabolized at rest and during lower intensity exercise, while a greater proportion of carbohydrate will be metabolized at higher intensities. This is where the myth that we burn more fat during low-intensity exercise comes into the picture.

It is true that at rest and lower intensities, we will use fat for fuel primarily, but think about this. If the relative proportion of fat used for metabolism dictated actual fat loss, wouldn’t the best exercise be sitting on the couch, when relative fat metabolism is at its highest? 

We know this isn’t the case. So how could low-intensity exercise be any different?

Exercise and Fuel

The best way to think of exercise for fat loss is to consider how much fuel you are burning overall, not which fuel type you are using. High-intensity exercise will burn more fuel than low-intensity exercise. 

Running a mile burns more fuel than walking a mile. When we exercise at higher intensities, we will burn more carbohydrates for energy.

When we exercise at lower intensities, we will burn more fat for energy. However, using fat for fuel during exercise will not result in the greatest overall losses to total body fat over time. Thus, the fat burn zone may not be named appropriately—and it’s misleading.

How, then, will higher-intensity exercise result in greater fat loss? The best way to think of this is in terms of oxygen consumption. We all have an oxygen consumption demand at rest in order to maintain normal body processes.

When you exercise, the oxygen demand is increased to bring oxygen to your working muscles. Just think about the last time you hustled up a staircase and began breathing hard—this is in part due to a greater need for oxygen to help out your muscles. 

EPOC

The greater intensity at which we exercise, the greater our oxygen demand. Once we come down from our high oxygen consumption after exercise, our resting oxygen consumption will also run a little high in the next several hours for a number of reasons. 

It is believed that this elevated oxygen consumption is necessary to replenish certain energy systems and help bring down your elevated body temperature. Your body is taking care of itself to restore order and prepare to do it again if needed.

This time after exercise where you require more oxygen is called excess post-exercise oxygen consumption, or EPOC. Exercise that is really tough results in a greater magnitude of EPOC. 

This period of heightened oxygen consumption is similar to your car’s engine staying warm after you drive. Your body expends more energy after exercise to come back to normal or back to homeostasis. Your body will work on processes like restoring ATP and glycogen that were used during exercise, bringing your body temperature back to normal, and starting to repair damaged muscle tissue.

Thus, with a high-intensity workout, you will burn energy during and after the exercise, too. Just how much you will burn is dependent on, most importantly, how hard you exercise and somewhat on how long you exercise.

If your number one reason not to exercise is your time, then this technique of going very hard but for a short time eliminates that excuse. Tomorrow’s article will cover how to get the most out of HIIT.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.