The Three Types of Fuel for Exercise

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1. Carbohydrate (4 kcal/g): Carbohydrate is the main source of fuel for many of the metabolic process of the body. Before using it for fuel, the body converts carbohydrate into a monosaccharide called glucose which is released into the blood for the muscles to use. When resting, carbohydrates are converted to glycogen, the complex storage form of glucose. Glycogen is stored either in the liver or the muscle. Typically, the carbohydrates stored in liver and skeletal muscle is limited to about 2,500 to 2,600 kcal which is approximately the number of calories needed to complete 25 miles of running. (1)

It is becoming common knowledge that consuming carbohydrates during exercises lasting 1-4 hours can improve performance. Unfortunately, it is unclear why performance improves. Performance improvements occur due to the following possible mechanisms:
  1. Preservation of liver glycogen
  2. Promotion of glycogen synthesis during exercise
  3. Increased reliance on blood glucose for energy late in the exercise bout
  4. Enhanced central nervous system function
Complex carbohydrate is the best form of carbohydrate to consume... but why?
  • Increases muscle glycogen (the storage form of carbs)
  • Improves performance
  • Delays fatigue
  • Causes less stomach problems and indigestion
  • Leads to lower blood sugar and insulin levels
  • Provides other beneficial nutrients found in fruits and vegetables (vitamins, minerals and fiber)
Sources of complex carbohydrates:

  • Fruits
  • Vegetables
  • Whole-grain (breads, rice, cereal, etc.)
  • Legumes (beans, lentils, etc.).


2. Fat (9.4 kcal/g): Fats can provide an amazing amount of energy. For an individual who has about 12% body fat, he or she would have about 70,000 kcal of energy available to burn. That's enough energy to run 28 marathons. So why shouldn't we eat as much fat as possible? Like carbohydrates, fats have to be broken down from triglyceride into glycerol and free fatty acids. The difference is that the energy released from fat takes too long to release compared to carbohydrate. It cannot support high intensity exercise like carbohydrates can.

3. Protein (4 kcal/g): Protein is the survival mode/ repair nutrient. When carbohydrate and fat stores are depleted, protein is metabolized which contributes to muscle loss. When carbohydrate and fat levels are within safe zones, protein helps to repair muscle damage caused by exercise. A cool property of this unit can be found in milk. When protein is available with carbohydrates, glycogen synthesis is enhanced following intense aerobic exercise.
Resources:
1 Wilmore, Jack H., David L. Costill, and W. Larry Kenney.Physiology of sport and exercise . 4th ed. Champaign, IL: Human Kinetics, 2008. Print.

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