We are often asked how short, intense workouts can provide better fat loss benefits than traditional long duration, low power workouts in the so called "Fat Burning Zone." The "Fat Burning Zone" is supposed to be exercise done at around 50% VO2 max. That means, working at half (50%) of your aerobic capacity (V02 max). In other words, slow and easy. Jogging or other slow paced exercise on "cardio" machines is often proclaimed to be the most efficient way to lose weight. But, it's not.
Do you know anyone who jogs for miles every week and cannot lose weight? The people you see who religiously do 30-60 minutes on the elliptical machine look about the same as they did months ago, don't they? Why aren't they getting fitter? And why are we, CrossFitters, getting such amazing results while our workouts last a mere 20 minutes?
We've all seen the charts on cardio machines and on the walls of traditional gyms that erroneously show that the most effective fat burning exercise occurs when the heart rate is in the range of 55% to 65% of maximum. These charts are in exercise science books everywhere. This inaccurate obsession is based on research which shows that when we exercise at 50% of VO2 max (low intensity cardio) our body draws about 50% of its energy from stored fat, and that when we go to higher intensity, 70% or more of VO2 max (CrossFit), we only draw about 33% of our energy from stored fat. All this is true (Romijn et al and other researchers have come to similar conclusions). But, we burn more calories overall with a more intense workout. A smaller percentage of a bigger total can be better than a larger percentage of a smaller total. Would you rather have 1% of a million dollars, or 50% of a thousand?
Now, if you go on an all day hike or four-hour bike ride, you will burn a significant amount of body fat (that is, if you don't sustain an overuse injury in the process). Possibly more than during our 20 minute workouts. But if you don't have 4-8 hours every day to work out, high intensity is the way to go (assuming a level of intensity that is safe for you). Obviously, "high intensity" is a different value for everyone. My grandmother's high intensity will be drastically different from a college athlete's high intensity; it's all relative.
But there is more to the story than just the amount of fat burned during the exercise session. High intensity exercise raises your metabolism for a longer period of time post exercise than slower paced aerobics. After finishing an intense (but short) workout, your metabolism may be higher for up to 14 hours afterward. But at the end of a slow, steady jog, your body will be back to it's steady state within an hour and you will be right back where you started. A higher metabolism post workout means you are burning more calories, even if you're sitting at a desk most of the day. Not to mention, you will have significantly more energy!
More vigorous workouts will give you greater fat loss and better overall fitness than slow and easy workouts. If you have limited time to workout, interval training will help you reach your goals faster. However, you should never jump right into a high intensity exercise program. You must begin by learning the techniques and movement patterns. Only then can you begin to gradually increase the intensity level of your workouts. Your body will need time to adapt to the stimulus, as it is vastly different from watching tv on a stairclimber. But, you will not be disappointed with the results!
Supporting Research:
Tremblay, et al: Impact of exercise intensity on body fatness and skeletal muscle metabolism
Romijn et al: Relationship between fatty acid delivery and fatty acid oxidation during strenuous exercise
