Richard G. Petty, MD

Burning Fat And Carbs

I was just flipping through a few magazine articles about diet and exercise and it amazed me how each writer was telling the reader that THIS was THE TRUTH about how to exercise and burn calories, and yet many contradicted each other.

This confusion and attempt to insert some order and certainty should not be too much of a surprise: there is still an enormous amount about metabolism that is simply not known. All of the articles were well written and well intentioned. Yet I am sure that many of the writers probably do not realize how much of what they are saying is guesswork!

The whole field of exercise and metabolism remains in a state of flux, though good progress is being made.

In a some new research from the Copenhagen Muscle Research Center at the University of Copenhagen in Denmark has been published in the Journal of Physiology, that helps us to understand a little more about the way in which muscles metabolize during exercise.

During exercise and physical activity, the primary fuels used by muscles are carbohydrate and fat. When you do mild exercise you tend to burn relatively more fat and less glucose. But as the exercise becomes more intense, a higher percentage of the energy demands of the muscles are supplied by glucose, until at the highest intensities almost only carbohydrates are used. One key question for any aspiring exerciser is how and why this happens? Is the shift in fuel source a property of the muscle itself, or does it represent the interplay between what is happening in the muscle and the exercise-related responses in the rest of the body.

The research looked at muscle fuel utilization in the quadriceps muscle during graded exercise done with only one leg. Nine healthy males did the one-leg exercise at 25, 45, and 85% of maximal workload. Their results showed that when only a small mass of muscle is contracting, the shift in fuel source from fat to glucose with increasing intensity does not happen. The explanation is that blood flow and oxygen supply are easily able to keep up with the demand.

The key to understanding the shifts in fuel source do not lie in the muscles at all, but in adaptations in the whole body during exercise. The oxidation of fat during exercise requires a fine interplay between the cardiovascular, neurological, endocrine and muscle metabolic systems.

The research also helps explain why athletes “hit the wall” during events like a marathon: the body can no longer switch fuels efficiently.

There are also some implications for the adaptations made in middle-aged adults who are using exercise to prevent or treat conditions like diabetes or obesity.

What we need to do is use slower training methods with good quality loose breathing to maximize oxygenation of the blood. This in turn allows fat oxidation to be maintained even during intense exercise with a large muscle mass.

And it looks as if we are heading back to the idea that cardiovascular work should be done before we start lifting.

About Richard G. Petty, MD
Dr. Richard G. Petty, MD is a world-renowned authority on the brain, and his revolutionary work on human energy systems has been acclaimed around the globe. He is also an accredited specialist in internal and metabolic medicine, endocrinology, psychiatry, acupuncture and homeopathy. He has been an innovator and leader of the human potential movement for over thirty years and is also an active researcher, teacher, writer, professional speaker and broadcaster. He is the author of five books, including the groundbreaking and best selling CD series Healing, Meaning and Purpose. He has taught in over 45 countries and 48 states in the last ten years, but spends as much time as possible on his horse farm in Georgia.

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