Whether you're munching on a candy bar or scarfing down your favorite pasta, the carbohydrates you eat are eventually broken down to glucose in your digestive tract. From there, they are either used for quick energy or stored for later use. Learning more about how your body stores and burns the food you eat will help you eat and exercise smarter for better health and performance.
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From Food to Fuel
The foods you eat can be subcategorized into fats, carbohydrates and proteins, and while all three can be used to fuel your body, carbohydrates are the fuel most readily used for physical activity. Carbohydrates come from fruits, vegetables and grains, and are broken down into simpler sugars, fructose, galactose and glucose in your digestive tract. They are then absorbed in your small intestine and travel to your liver, where fructose and galactose are converted to glucose. A small amount of glucose is stored in your liver as glycogen, and the rest is transported via your bloodstream to tissues throughout your body to be used for energy or stored for later use.
Once it enters your bloodstream, the upswing of glucose triggers your pancreas to release insulin, a hormone that works like a key that opens the doors of your cells to let glucose in. If there is space for more glucose in your muscle cells, it's stored there in the form of glycogen, to be drawn on for future energy demands. But if your muscle stores are full, the excess glucose is converted to fat and stored as adipose tissue, or body fat.
Burn Baby Burn
The duration, frequency, type and intensity of your exercise will determine which fuel stores you draw on to meet your energy demands. According to a review by Coyle, published in the "American Journal of Clinical Nutrition," low- to moderate-intensity activity draws on both fats and carbohydrates for fuel. As intensity increases, glycogen contributes more because fat cannot be oxidized quickly enough to meet energy demands. During moderate-intensity exercise, glycogen stores can contribute to energy production for two to three hours. Active people will deplete their muscle glycogen stores within 30 to 60 minutes of high-intensity intermittent exercise. Energy levels can be maintained during endurance activities by ingesting glucose while exercising.
After exercise, the carbohydrates you eat will serve to replenish your muscle glycogen stores within 24 hours. If you continually eat more sugar than you need, and don't exercise to burn off excess fuel, your cells will eventually become insulin resistant, predisposing you to diabetes. You will also gain weight, as excess sugar is converted to body fat. Regular endurance training improves your ability to use fats for energy and spare glycogen. Striking a balance between energy consumption and energy expenditure is the secret to optimal performance and weight management.
- American Journal of Clinical Nutrition: Substrate Utilization During Exercise in Active People
- Human Kinetics: Carbohydrate Intake During Exercise
- Iowa State University: Carbohydrate
- Medicine and Science in Sports and Exercise: Skeletal Muscle Glucose Uptake Response in Trained and Untrained Men
- University of New Mexico: The Physiological Factors Limiting Endurance Exercise Capacity