Set foot in any gym and you’ll no doubt see cardio machines that have charts to help you train in your ‘fat burning zone’. The theory behind the concept is based on studies showing that the percentage of fat used as an energy source is optimized when you train at an intensity between 60 to 80 percent of your target heart rate. But while increasing the percentage of fat burned for fuel sounds like a great way to get lean, it unfortunately doesn’t translate into burning more fat on an absolute basis. The most important aspect of shedding body fat is the total amount of calories burned—not the percentage of calories from fat—and therefore a higher intensity cardio routine is the better choice.
You see, the human body is a dynamic organism and constantly adjusts its use of fat for fuel. This process is governed by a host of factors (including enzyme levels, substrate availability, internal feedback loops, etc.). If you expend more calories than you consume, your body will shift to a fat burning mode and mobilize excess adipose as a source of energy. Thus, from a fat loss perspective, high-intensity exercise burns more fat calories on an absolute basis than lower intensity activities.
Ideally, the activity that maximizes calorie burning is interval training, where short bouts of low intensity exercise are interspersed with high intensity bouts. Interval training not only heightens fat burning during the activity itself, but also increases the amount of calories you burn after the workout is over! This is due to a phenomenon called excess post-exercise oxygen consumption (EPOC). Simply stated, EPOC keeps your metabolism elevated for several hours following training. Moreover, there is an associated increase in the secretion of both growth hormone and noradrenaline—hormones that are integrally involved in the fat burning process—resulting in an increased utilization of fat of fuel. All told, there is both a greater total amount of calories expended as well as a greater amount of fat oxidation following training.
Here’s the catch, though: EPOC is intensity dependent—the harder you train, the more calories you expend following training. That’s why you need to push yourself as hard as you can during your high-intensity intervals, going as close to all-out as possible. For those who have never performed this type of cardio, I generally recommend starting out with an interval ratio of 4:1 (i.e. four minutes at low intensity followed by 1 minute at high intensity) and, as you become more fit, gradually lower the ratio until you are at 1:1.
Although people tend to associate interval training with treadmill exercise, virtually any cardiovascular activity can be employed, including machines such as the stair climber, elliptical machine, and stationary bike. You can also jump rope or perform any number of outdoor activities, too. Just make sure that you are readily able to switch between high and low intensities. In final analysis, you’ll exert a good deal more effort in your training, but the results will be well worth it!
If you’d like to read about the science behind the concept, check out the article I co-wrote with Jay Dawes titled, High-Intensity Interval Training: Applications for General Fitness Training for a recent issue of the NSCA Strength and Conditioning Journal.
Schoenfeld, B., Dawes, J. (2009). High-Intensity Interval Training: Applications for General Fitness Training. Strength and Conditioning Journal, 31(6), 44-46