What is the Best Type of Interval Training to Burn Fat?

Over the past decade the popularity of interval training for fat loss and performance improvements has been amazing. There have been so many studies investigating the fat loss and performance benefits of using intervals versus long boring cardio - and the results truly speak for themselves.

But as with any type of training, your body will adapt to how it is trained, and different energy systems will be used depending on the intensity and duration of the exercise. For instance, a 400m sprinter might run intervals lasting up to 60 seconds and then recovering for 3 minutes because that type of training would most closely resemble what he/she would be going through in a race.

And in reality there are literally thousands of different interval training protocols to follow depending on your goal and fitness level.

But what about the average person who doesn't want to train for track and field or a particular sport and is solely concerned with burning fat? Is there an interval training protocol that is best to maximize fat loss?

Some people recommend long duration intervals (ie. 1 minute hard, 2 minutes recovery) while others recommend shorter duration intervals that can elicit a much harder effort.

So the answer to the interval training for fat loss question is yes - but it isn't what you think.

Here's why...

Research published in the European Journal of Applied Physiology compared the fat-burning effects of two different interval training workouts, both lasting 40 minutes.

The first consisted of short intervals lasting 6 seconds, with 9-second rest periods. The second workout involved long intervals lasting 24 seconds, with 36-second rest periods.

It's important to note that the treadmill speed was identical during both the short and long interval workouts. Moreover, the ratio between work and recovery bouts was also the same (ie. 2:3 work to rest), meaning that the total amount of time spent running on the treadmill (16 minutes) was also identical.

Despite the fact that exercise intensity and duration were kept constant during both trials, there were large differences in fat oxidation.

In fact, the number of fat calories burned was approximately 3 times LOWER during the long (24 seconds) interval workout.

To understand why the short intervals were so much more effective at increasing fat oxidation, it's important to understand a little more about a substance called myoglobin.

The mysterious myoglobin

Myoglobin is a large protein that binds to oxygen inside your muscle cells. Think of it a little like a "reserve" oxygen supply. As the blood has hemoglobin, the muscles have myoglobin. Both store oxygen.

According to physiologist Olof Astrand, myoglobin is repeatedly used and reloaded during the work and recovery phases of interval exercise.

However, as the duration of the work period increases, myoglobin's oxygen stores are reduced.

Your body needs more oxygen to burn fat as a fuel (compared to carbohydrate or protein). When oxygen supplies become limited, carbohydrates then provide a greater proportion of energy. This is also seen in the "crossover effect" where carbohydrates become the more dominant source of fuel at higher exercise intensities compared to fats which are mainly oxidized at lower intensities.

And furthermore, because lactic acid, a by-product of carbohydrate metabolism, "blocks" fat burning, intense intervals that continue beyond the point at which myoglobin loses its supply of oxygen (usually above 15 seconds or so) rely to a greater extent on carbohydrate as a source of energy and, thus, don't burn as much fat as shorter intervals.

As mentioned, because myoglobin holds enough oxygen to last for 5-15 seconds it would make send to perform short, intense rather than long intervals to promote a greater rise in fat oxidation.

As such, if your goal is to lose fat, then limit your work intervals to a maximum of 15 seconds.

Studies also show that shorter intervals don't feel as physically demanding as long intervals which means that you can get better results without feeling like you're working harder.

How much rest should you take between work bouts?

This depends on the duration of the work intervals and your fitness level. The longer the work interval, the more myoglobin gets used up, and the longer it takes to "reload". Therefore, you'd need more time to recover.

The study alluded to earlier used rest intervals that were 1.5 times greater than the work intervals (6 seconds work: 9 seconds rest).

Based on these findings, a 15-second work interval would require a minimum of 22 seconds rest.

However, if you're new to interval training I would recommend starting with a work-to-rest ratio of 1:3. This means that if you work hard for 15 seconds, give yourself 45 seconds to recover. As you become fitter, then you can reduce the amount of recovery.

Reference:

Christmass, M.A., Dawson, B., & Arthur, P.G. (1999). Effect of work and recovery duration on skeletal muscle oxygenation and fuel use during sustained intermittent exercise. European Journal of Applied Physiology, 80, 436-447