Physiological systems burn any available fuel source when energy is needed. Fat, protein and carbohydrate are stored in the body and contribute to the production of energy. Each type of activity causes its own metabolic reaction with optimal energy sources, but a poorly fueled body will use muscle protein as energy if required. Understanding the basics of fuel utilization aids in proper dietary choices and creation of goal-specific exercise programs.
Categories of exercise are based on oxygen utilization and include anaerobic and aerobic exercise. Three sources of energy are fat, carbohydrate and protein and each contributes according to the category of exercise being performed. Most cardio exercises -- like walking, jogging and cycling -- are considered aerobic in nature due to their long-duration time frame at a less-than-maximum intensity. Traditionally, cardio is expected to burn carbohydrate as a primary energy source.
Protein contributes less than 5 percent to total energy production during exercise, according to the “Health Fitness Instructor’s Handbook.” Instead, an intensity-dependent combination of fat and carbohydrate is used during cardio activity. Higher intensity results in carbohydrate being the predominate fuel, but low stores of carbohydrate will increase protein utilization.
Discrimination between a carbohydrate and fat calorie is not important regarding weight loss. High-intensity cardio predominately burns carbohydrate and lower intensities promote fat utilization. But, the burning of 3,500 calories, regardless of source, results in a one-pound weight loss. Fat loss occurs with carbohydrate use because the body gathers calories from fat during rest.
Sustaining long-duration, low-intensity cardio is easier than performing similar duration high-intensity cardio; therefore, lower intensity promotes longer periods of exercise. Conversely, high-intensity cardio burns more calories per minute than low intensity does; therefore, high-intensity cardio promotes more calories to be burned in less time. Using personal preference and advice from a fitness professional can create a cardio exercise plan that works best for you.
Utilizing muscle protein as an energy source breaks down key metabolic tissue. Muscle tissue promotes body composition by burning more calories at rest than any other. A person with more muscle mass will use more calories at rest and has an advantage in physical activities. Maintaining a balanced diet ensures proper fuel utilization during exercise and is a key aspect to any fitness or weight-loss program.