The calories you burn during exercise come from three energy sources: carbohydrates, fat and protein. The two main types of energy used during exercise are stored fat and carbohydrates. The types of fuel your body uses during a workout depend on the nature of the activity performed. Most exercises burn a combination of two or three fuel sources.
Carbohydrates
Your body converts carbohydrates to glucose. The glucose that you don't need for energy gets stored as glycogen in your liver and muscles. When your glycogen stores are full, excess glucose gets stored as fat. Stored glycogen is the primary fuel used during high-intensity aerobic exercise, which is aerobics performed at 75 percent or more of your maximum heart rate. When you exercise at 80 percent of your maximum heart rate, for example, approximately 80 percent of calories burned come from stored carbohydrates. If you consume simple carbohydrates just before exercising, your muscles will also use the circulating glucose for immediate energy.
Fat
Fat is the primary fuel burned when you rest and when you perform low- to moderate-intensity cardio. Approximately 60 percent of the calories you burn working out, at 65 to 70 percent of your maximum heart rate, come from fat. If you continue exercising after your glycogen stores are depleted, fat is the preferred fuel source. When your body switches to fat for fuel during high-intensity cardiovascular exercise, your ability to maintain intensity is considerably compromised.
Protein
Protein is the least preferred source of energy during exercise. Short bursts of exercise, such as sprinting, rely solely on carbohydrates, while protein provides 3 to 5 percent of the fuel for longer exercise sessions. When glycogen stores are low or depleted, as much as 10 percent of the fuel from exercise comes from protein.
Considerations
Consume enough complex carbohydrates to maintain adequate glycogen stores for exercise. Aim to get roughly 60 percent of your calories from carbohydrates if you work out on a regular basis. Good choices include whole grains, fruit and vegetables. If you train for an endurance event, such as a marathon or triathlon, cut back on your activity the week before the event to ensure that your glycogen stores are optimized. Aim to get 12 to 15 percent of your calories from lean protein and 20 to 30 percent from healthy fats.
References
- Iowa State University Extension: Eat to Compete: Carbohydrate
- Brian Mac: Exercise Intensity & Energy Source
- American Academy of Orthopaedic Surgeons: Sports Nutrition
- Brian Mac: Protein and Endurance Athletes: Timing It Right
- Beginner Triathlete: Protein, Carbs and Endurance Performance: Finding the Right Balance; Nancy Clark
- Bodybuilding: Sports Nutrition Guide Online: Carbohydrates; Daniel Gastelu
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