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Medical Author:

Richard Weil, MEd, CDE

Richard Weil, MEd, CDE

Richard Weil, MEd, CDE, is an exercise physiologist and Certified Diabetes Educator, and is director of the New York Obesity Research Center Weight Loss Program at St. Luke's Roosevelt Hospital Center in

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Biologic Basis of Aerobic Exercise

A. Oxygen Delivery

Breathing increases during aerobic exercise to bring oxygen into your body. Once inside your body the oxygen is (1) processed by the lungs, (2) transferred to the bloodstream where it circulates to the heart, and then (3) transported to the exercising muscles via the circulatory system, where it is used by the muscle to produce energy.

B. Oxygen Consumption

"Oxygen consumption" describes the process of muscles extracting, or consuming, oxygen from the blood. Conditioned individuals have higher levels of oxygen consumption than deconditioned individuals ("couch potatoes") due to biological changes in the muscles from chronic exercise training. For example, a deconditioned individual might have a maximal oxygen consumption of 35 milliliters (ml) of oxygen per kilogram of body weight per minute (ml/kg/min), whereas an elite athlete like Lance Armstrong has a maximal oxygen consumption of 92 ml/kg/min! Values like this are expressed as VO2 (volume of oxygen consumed) and can be measured with special equipment in a laboratory.

C. Burning Fat

A higher percentage of fat is burned during aerobic exercise than during anaerobic exercise. Here's why. Fat is denser than carbohydrate (fat has nine calories per gram and carbohydrate has four), and so it takes more oxygen to burn it. During aerobic exercise, more oxygen is delivered to the muscles than during anaerobic exercise, and so it follows that a higher percentage of fat is burned during aerobic exercise when more oxygen is available. When less oxygen is present, like during anaerobic exercise, a higher percentage of carbohydrate is burned.

Keep in mind that both fuels are almost always burned simultaneously, except during the most intense, short-term bursts of energy, like sprinting and weightlifting. It's the percentage of fat and carbohydrate burned that changes during a workout depending on the intensity, but you almost never burn just one exclusively. You burn fat while you're at rest, and you burn it during virtually every moment of exercise. It's a myth to think that it takes 20-30 minutes of exercise before your muscles start burning fat.