THE FAT BURNING ZONE
Do you exercise in the "fat burning zone" -- that mysterious place determined by obscure heart rate formulas that tell you exactly where you should be exercising in order to burn the most fat? Perhaps you've noticed such a program on your treadmill or heard your aerobics teacher talking about it.
"Fat burning" is a vernacular expression, shorthand for what exercise professionals refer to as "the beta oxidation of fatty acids." Although "fat burning” sounds like cellulite melting in a furnace of exercise induced body heat, what actually happens is a little different. At any given moment in time, you are burning (or, technically, oxidizing) fuel (food), that is to say, breaking it down for energy. The food you eat gets taken apart by the body, broken down into its smallest components (fatty acids, amino acids and glucose), which provide energy for the body to do anything and everything that it needs to do. That means providing energy to grow nails and hair, to breathe, to make enzymes, to digest food, to exercise, think, sit at the computer, garden, run for the bus and even sleep. The particular mix of fuel that it uses depends on a number of things.
The greatest proportion of your fuel comes from fat when your body is at rest. But the total number of calories burned is tiny. In other words, you're burning a high percentage of a very small number (about one calorie a minute in the average person).
Now, what happens when you begin to exercise? The minute you start to move from a resting position, the "fuel split" changes -- carbohydrates start to contribute a slightly greater percentage of the mix, and fat a slightly smaller one. The greater the intensity of the exercise, the greater the proportion of calories from carbohydrates and the smaller the proportion of calories from fat. But the harder the exercise, the greater the total number of calories per minute. So although your percentage from fat goes down, your total number increases.
This is where your average aerobics teacher gets it all mixed up. By telling you to keep your heart rate low no matter what, they're essentially admonishing you to keep your "fat percentage" high -- but without considering that the less intense exercise also burns a smaller total number of calories.
Exercising at a higher level of intensity has a lot of benefits as well. For one thing, it increases your aerobic capacity and conditions your heart. For another, it burns a ton of calories. For a third, it pushes you, making you fitter and more conditioned -- and a more conditioned person is a more efficient fat burner during her non-exercising hours.
So throw out those fat-burning charts and use the following rule instead: Within the limits of your ability and fitness level, and without causing undue stress or increasing the possibility of injury, exercise as hard and as long as you can, as frequently as you can. Combine that with the right diet and you've got the recipe for success.