All cellular activity is fueled by adenosine triphosphate, or ATP, which resides in each of your cells. When you sprint for a cab, run to a base in softball, play tag with your kids or start to exercise at high intensity, you use ATP's small supply of stored energy.
If your cab takes off without you, you hit a triple or your child is especially fast, your body will draw power from glucose converted from carbohydrates stored in your muscles. This can carry you an additional 40 to 120 seconds at lower intensity. (If you eat low-carb, you may sputter out earlier due to inadequate glucose stored in your muscles.)
Next, if you decide to walk instead of taking that cab, your aerobic system will kick in. Energy that is aerobic (meaning "with oxygen") uses oxygen from your blood to burn fat and glucose in varying percentages. First, it burns whatever glucose remains in your muscle, then it goes to a second source: your liver. Because aerobic exercise causes your muscles to switch to burning glucose AND fat, it's the best type of exercise for weight loss. However, your exercise needs to last longer than a few minutes for fat burning to kick in.
Pure glucose is your muscles' fuel of choice, however, so when your body runs low on the glucose stored in your muscle (normally after 45 to 60 minutes of exercise) and it switches over to a mixture with a higher fat-to-glucose ratio, you'll feel it. You may feel you're out of gas. That's when athletes say they have "hit the wall."
But you aren't out of gas. Your aerobic energy system can power you virtually forever. For this reason, if you are planning to work out longer than 90 minutes, you need to restock your glucose stores as you exercise.
Exercise improves all levels of your energy delivery. It ups glucose storage in your muscles and liver. Without aerobic fitness, you have fewer mitochondria (the power plants within each cell), and you burn primarily glucose and little fat. With aerobic exercise, your mitochondria increase in size and number, and the enzymes within them gain greater efficiency in producing energy. This means you burn more fat, which reserves your stored glucose longer and allows you to produce more energy for a longer period of time.
We find that aerobic athletes have up to 200 percent more mitochondria than average people. Mitochondria are important -- the more you have, the higher percentage of fat you utilize at any given speed.
When you're starting out, aerobic activity trains all of the energy sources. But as you advance you will work on each energy-delivery mechanism deliberately.
( Eric Heiden, M.D., a five-time Olympic gold medalist speed skater, is now an orthopedic surgeon in Utah. He co-authored "Faster, Better, Stronger: Your Fitness Bible" (HarperCollins) with exercise performance physician Max Testa, M.D., and DeAnne Musolf. Visit www.fasterbetterstronger.com.)