By Tim Nguyen, M.S., CIP Berkeley Center Health and Wellness Coordinator
Many of us might have heard or believe the notion that we are “burning fat” when we are working out or participating in vigorous physical activity.
While the notion is not completely false, most people forget the important fuel source that sustains us while we perform activities like running or bicycling. It all comes back to our fundamental and familiar fuel source, that is, carbohydrates.
As you may be aware, carbohydrates are the primary and immediate fuel source that supplies us with the energy for daily living and physical activities.
Carbohydrates (aka carbs) can be found in many types of starchy food including pasta, rice, bread, flour, corn, and beans, a many more. When we consume too much carbs, our body automatically converts the excess carbs into fats, or adipose tissues, and stores them mostly under our skins and surrounding our organs. When we are deficient in carbs, our body begins to breakdown the fat tissues to smaller components for fuel. But that’s not the full story behind the breakdown of fat!
At rest, the body physiologically burns mostly fat– approximately 80 percent of the total caloric expenditure. The body utilizes carbohydrates the remaining 20 percent. In other words, fat is the main fuel source at rest and carbohydrate is only a secondary fuel source. This concept may lead you to wonder what happens when one exercises? Is this partitioning of fuel source similar to when one exercises?
As the intensity of exercise increases, our body slowly switches to using more of carbohydrates and less of fats. Yes, this means that as we are working at our maximal aerobic capacity, we are utilizing solely carbohydrates and none of fat. This concept in exercise physiology, formally called the Crossover Concept, was originated by Dr. George Brooks at UC Berkeley several years ago.
Now, does this give one the justification to not exercise since we burn more fat when we are at rest? NO! In fact, our body burns even greater amounts of fat at rest AFTER an intense exercise than what we normally would without exercise. If you are confused, let’s compare a regular exerciser with a sedentary individual. Since a regular exerciser is adapted to high intensity activities and therefore has higher lung and heart functions than his/her sedentary counterpart, it’s takes a longer period of time for the exerciser to reach maximal aerobic capacity than a sedentary person, relatively speaking. On the other hand, due to low fitness level, the body of a sedentary individual switches to carbohydrates utilization earlier than the regular exerciser. This means that there is a period where the regular exerciser still burns fat while the sedentary individual already starts using carbohydrates.
The point I’m trying to make here is that our body burns mostly fat when we are at rest. At the onset of exercise and as the intensity of the exercise increases, the body gradually crosses over to using carbohydrates. The more fit or trained an individual is, the better they are at burning fat since not only do they have improved cardiovascular and pulmonary systems, they also are able to stay “at rest,” relatively speaking, a bit longer than those who lead a sedentary lifestyle! I hope this give you more reasons to exercise!