Simply looking at the amount of fat burned during exercise is shortsighted. Fat burning must be considered over the course of an entire day.

Q: I was told that low-intensity cardio is better for fat burning than high-intensity cardio. True or not?

A: Not!

It is true that aerobics performed at a low intensity (60 to 80 percent of max heart rate) burn a greater percentage of fat calories than higher intensity activities (which is why low-intensity cardio is called training in the “fat-burning zone”). The problem is, burning a greater percentage of fat doesn’t necessarily equate to burning more total fat calories. Consider the following as an example:

Suppose you walk on the treadmill for a half-hour and burn a total of 150 calories. Of these calories, say 60 percent comes from fat. This would mean that you’ve burned a total of 90 fat calories from the low-intensity cardio. Now, let’s say you run on the treadmill for that same half-hour, which might burn 300 calories with only 40 percent coming from fat. Well, the percentage is lower, but you’ve actually burned 120 fat calories in that same half-hour!

What’s more, not only is fat burning maximized during high-intensity activity, but there is also a bigger increase in excess post-exercise oxygen consumption (EPOC) – the amount of calories burned once the activity is finished. EPOC is intensity dependent … the more vigorous the exercise, the greater the after-burn. If you’re really training hard, this can keep your metabolism elevated long after you’ve stopped working out and have a significant effect on the amount of fat metabolized.

It’s also important to note that the human body is a very dynamic organism and continually adjusts its use of fat for fuel. Substrate utilization is governed by a host of factors (including enzyme levels, substrate availability, internal feedback loops, etc.) and these factors can literally change by the moment. Thus, simply looking at the amount of fat burned during exercise is shortsighted. Fat burning must be considered over the course of an entire day – not on an hour-to-hour basis – to get a meaningful perspective on its impact on body composition. And on this basis, high-intensity cardio always comes out ahead.

Bottom line: High-intensity exercise burns more fat calories on an absolute basis than lower intensity activities, making it the preferred choice if you’re looking to optimize fat loss.