By Al Sears, MD
Don't make the mistake of thinking that burning fat during a cardio workout will keep the fat off. Long-duration cardio will stimulate your body to build back any fat burned. And if you do it long enough, the unnatural stress will give you a stooped, weak, arthritic, tired, and old body shape.
Your body makes metabolic "decisions" based on its reading of your environment - which means that the sustainable benefits of exercise happen after you stop exercising. During long-duration workouts, your body thinks to itself, "I'm burning fat. I need to make sure I have enough fat - at all times - so I don't run out of fuel during these stressful periods." As a result, your body will actually start to make and store more fat as soon as you finish the workout. This is your body's defensive response to an unnatural condition you created.
To burn fat and keep it off, you need to give your body the right challenge - a challenge that triggers your body to stop, not encourage, fat production.
Let's take a quick look at what your body uses for fuel when you exercise.
When you put your body in motion, its first fuel source is ATP. This "high-octane" energy source doesn't last long - only for the first two minutes. To keep going, your body burns carbohydrates stored in your muscles and liver. This lasts for the next 10 to 15 minutes. Once you pass the 20-minute mark, you're predominantly burning fat.
Look at the chart below:
Body Fuel With Varying Activity
1 - 5 %
5 - 8 %
2 - 5 %
This chart shows that at low-intensity activity, your body derives most of its energy from carbohydrates and only 15 percent from fat. But when you step up your activity level to moderate, you increase the energy burned from fat to 55 percent of the total. If you increase your activity to high-intensity, you dramatically reduce your dependency on fat and derive nearly all your energy from carbs.
The relationship between moderate-intensity exercise and fat burning has led many to the logical conclusion that you should exercise at moderate intensity, because that's how you burn the most fat. But long-term practice of this strategy actually causes problems.
Hours of Tiresome, Boring Exercise ... Just to Build More Fat
You can use this strategy to lose weight and to get reasonably lean. But because your body learns to store energy as fat, you can only get lean after you sacrifice muscle and other high-energy burning tissues. Unless you cut calories in addition to exercising, your body will try to build more fat and give up more lean tissue until, sooner or later, your body wins.
Another consequence of longer-duration cardio? As many of my patients have found, this unnatural activity can cause joint degeneration. Plus, if you persist through middle age and beyond, this type of exercise can accelerate some very negative effects of aging. It lowers testosterone and growth hormone, boosts destructive cortisol levels, and robs you of muscle, bone, and internal organ mass and strength.
Short-duration exercise, on the other hand, actually increases levels of growth hormone. Case in point: Researchers from Loughborough University in Leicestershire, England tested growth hormone levels in sprinters and endurance athletes. On average, the sprinters had three times as much growth hormone as the endurance runners.
Remember that the most important changes from exercise occur after, not during, the exercise period. If you take a look at the table above, you'll see that the body burns an even higher percentage of energy as fat - 60 percent - while resting. (The "cardio" proponents seem to overlook this fact.)
The way you exercise affects your metabolism for several days. This is good news. It means that all you have to do during exercise is stimulate the adaptive response you need. Then your body will continue making the important changes while you rest.