During cardiovascular exercise, such as running, cycling and swimming, your muscles require large amounts of energy. Carbohydrates and fats provide most of the energy, with proteins making only a small contribution. The exact mix of fuel your muscles prefer depends upon both the intensity and duration of the exercise.
During low-intensity cardiovascular exercise, fats are your muscles' primary fuel. They provide a slow-burning, long-lasting energy source. Even lean individuals store enough fat to fuel many days of low-intensity exercise, such as walking. Fat is also more energy dense than carbohydrates or protein, providing 9 calories per gram. Because you metabolize primarily fat when you work out at low intensity, you might think that low-intensity exercise would be the best way to lose body fat. However, because you burn more total calories when you exercise vigorously, you actually metabolize more total fat during high-intensity exercise.
As exercise intensity increases, carbohydrates form a larger part of your muscles' fuel mix. Carbs don't contain as much energy as fat, yielding only 4 calories per gram. However, they can be metabolized faster, making them an ideal energy source during high-intensity cardiovascular exercise such as interval training. Your muscles store carbs in the form of glycogen, but the amount of stored carbohydrate is much less than the amount of fat. A long, intense bout of cardiovascular exercise, such as a marathon, can exhaust your glycogen stores, causing a decline in your performance.
Although you can metabolize proteins for energy, your body generally prefers to use fats and carbs, sparing proteins for other purposes, such as building muscle and bone. However, during long-duration cardiovascular exercise, your muscles may supplement their energy needs by metabolizing the building blocks of proteins, known as amino acids. The branched-chain amino acids -- leucine, isoleucine and valine -- are the most commonly metabolized amino acids during exercise. Some amino acids, such as alanine, can also be converted into glucose, or blood sugar, to serve as fuel for muscles.
The American College of Sports Medicine released a joint position statement with the American Dietetic Association and Dietitians of Canada in 2009 outlining nutritional recommendations for athletes and physically active individuals. The groups suggest that athletes consume 6 to 10 grams of carbohydrates per kilogram of body weight daily. Fats should be between 20 and 35 percent of total calories. Endurance athletes should aim for 1.2 to 1.4 grams of protein per kilogram of body weight each day.
- Exercise Physiology for Health, Fitness and Performance; Sharon A. Plowman and Denise L. Smith
- Medicine and Science in Sports and Exercise: Joint Position Statement: Nutrition and Athletic Performance
Joe Miller started writing professionally in 1991. He specializes in writing about health and fitness and has written for "Fit Yoga" magazine and the New York Times City Room blog. He holds a master's degree in applied physiology from Columbia University, Teacher's College.