Carbohydrates and fats are the primary fuel source for most exercise.
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Carbohydrates, fats and proteins undergo a synergistic process that provides energy to keep your engines running. The amount of each nutrient used to fuel cardiovascular exercise varies from person to person and the type of exercise performed. Whether you want to reduce body fat or improve your running time for the 400-meter sprint, your performance depends on your ability to metabolize various nutrients efficiently.
Carbohydrates Jump Start
Carbohydrates are your body's first fuel source to jump-start your workout; carbs are stored in your muscles as glycogen. Your liver also stores glucose -- a simple carbohydrate -- and releases it into your bloodstream during exercise. A 1999 study published in the "Journal of Applied Physiology" showed that cyclists who consumed a low-carbohydrate diet increased their performance time by almost 100 percent in cold temperature when they were tested after consuming a high-carb diet. When the same test was done in a hot environment, the same cyclists increased their performance time by about 22 percent. The amount of carbohydrates used depends on the exercise intensity. At 25 percent of your maximum heart rate -- or MHR -- no muscle glycogen is used and a small percentage of blood glucose is used as fuel. As exercise intensity increases toward 65 to 85 percent of your MHR, the demand for carbs increases. Because carbohydrate metabolism requires no oxygen to produce energy and the process is faster than fat metabolism, your body prefers to use carbs as fuel at very-high-intensity exercise.
Fats: The Long-Burning Fuel
While carbs are the kindling to stoke the energy furnace, fat is like the slow-burning log your body uses for prolonged cardiovascular exercise. Unlike carbs, fat metabolism needs oxygen to convert into energy. The amount of fat used for fuel, which is in the usable form of triglycerides, depends on duration and exercise intensity. At low-intensity exercise, fat is the primary fuel source. As exercise intensity increases to 65 to 85 percent of your MHR, the amount of fat used also increases, but the percentage of fat use drops as the body relies more on carbohydrates. Between one to three hours of cardiovascular exercise, your body increases fat use from 75 percent to 85 percent of your energy expenditure. However, as your carbohydrate level in your muscles and liver decreases, fatigue settles in and performance diminishes, even if fat is still present.
Protein for Backup Power
Although protein can be used for energy by converting its structure to glucose, it's not advisable to rely on it for energy. Proteins provide structure for all organ systems as well as your immune system. However, your body will use protein as an alternative energy source if carbohydrate level is low. Protein is still used at a very low percentage for fuel during cardiovascular exercise, even when there are sufficient carbohydrates present. Even so, new proteins are also formed during exercise to counter the rate of protein breakdown. To maintain lean muscle, exercise physiologist Len Kravitz of the University of New Mexico suggests you consume a meal consisting of carbohydrates and protein within 45 minutes after your workout. The carbohydrates spare proteins from being used as energy.
Oftentimes, you don't need to do more than an hour of cardio to improve your fat-burning potential. Your body continues to expend calories at a higher rate than at a resting state for many hours after high-intensity exercise. This condition is called EPOC -- excess post-exercise oxygen consumption -- where your body continues to rely on fats and carbohydrates to fuel the body as it repairs damaged muscle tissues, balances hormone levels and cools body temperature. In a 2011 study published in "Medicine and Science in Sports and Exercise," a 45-minute bout of high-intensity exercise can elevate metabolism for 14 hours in which subjects were burning an average of an extra 190 calories at the 14th hour.