How the body burns fuel during exercise

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DEAR DR. BLONZ: I have a question about burning fat. I have always heard that when doing aerobic exercise, the first 20 minutes is burning glucose and glycogen, then after that the body turns to its fat stores for energy.

If the body stores about 2,000 calories of glucose and glycogen in the muscles, liver and blood, there must be a point at which the body says "OK, enough of this, it is time to burn the fat stores." If not, do we exhaust our carbohydrates before we dip into the fat stores?

Where is the point when the body says "enough" and turns to burning fat? Finally, what happens when we "hit the wall," and what causes exhaustion cramps?

-- C.S., SSG, U.S. Army Medicine

DEAR SGT. C.S.: The answer to your question requires a good deal of biochemistry. I will attempt to tell the story in a way that makes some practical sense. First, it must be appreciated that fat is an aerobic fuel, which means that oxygen must also be present to access its energy.

The body's other fuels are glucose and its storage form, glycogen. These are anaerobic fuels, meaning that they can provide energy without oxygen. This is an important distinction.

Our muscles are primed to burn fat, and it is our most concentrated form of energy. When we begin to exercise, we burn glucose and glycogen, but it only takes a matter of minutes to shift to fat. It is not an abrupt switch, more like a smooth transition that works better as the body becomes more fit.

Think about what is going on when we are at rest. A minimum amount of fats are in the bloodstream, and our breathing and heartbeat are relaxed, taking in just enough oxygen and pumping blood at a rate that fuels the needs of the moment.

Now think about what happens if we were to start to exercise. The muscles respond instantly, but soon after, the pace of our heartbeat picks up and our breathing gets deeper and more rapid. If, by chance, we happen to be out of shape and push too hard, we become "out of breath" and have to pause.

Now let's go behind the scenes to discuss what is actually happening. If you think about it, it is essential that our muscles can respond even when there is insufficient fat or oxygen to fuel the effort. Otherwise, we would be unable to confront or escape danger.

At first, glucose and glycogen are the main energy sources. The brain immediately sends a signal to the fat storage areas to send some fuel, and to the lungs to start breathing faster and deeper to supply more oxygen and start removing carbon dioxide, the exhaust gas from muscular work.

If the muscles are worked too hard, the available glucose is used up before the fats and oxygen arrive on the scene, and the muscle runs out of gas.

The actual speed and ease of the shift from anaerobic to aerobic reflects the level of fitness. Warming up or starting slowly also helps the process, because it gets the signal sent earlier and helps shunt blood flow to the working muscles.

Even after we have made the shift to fat burning, we continue to burn some glucose. This causes lactic acid to be produced in the muscles, which then goes to the liver where it can be recycled back into glucose.

"Hitting the wall" occurs when we ask our working muscles to work beyond their abilities. Glucose attempts to fill the energy gap, but eventually lactic acid builds up and causes painful cramps and exhaustion, and the muscles can no longer work.

All things considered, the human body is an amazing machine.

Ed Blonz, Ph.D., is a nutrition scientist and the author of "Power Nutrition" (Signet, 1998) and the "Your Personal Nutritionist" book series (Signet, 1996). His syndicated column, "On Nutrition," goes to more than 600 newspapers and his articles appear in magazines such as Better Homes & Gardens, Cooking Light, Eating Well, Let's Live and Vegetarian Times. Ed has written seven books on foods, nutrition and health, and has won the James Beard Foundation Award for writing on diet, nutrition and health. Ed also maintains his own award-winning web page, The Blonz Guide.