Food supplies calories in the form of carbohydrate, protein, and fat to fuel the body
(see “Is Fat Where It’s At?” for more information). Which substrate the body uses when and how much of that substrate is used is dependent on many different factors. These factors are:

• The intensity of the exercise
• The duration (length of time) of the exercise
• A person’s fitness level
• A person’s diet/ Exercise frequency

Understanding how each of these factors affects substrate use for various exercises (i.e., sprinting versus marathon running) can help you to decide how to fuel your body best.

Factors that affect substrate use:

The intensity level of the exercise:
Exercise intensity is a primary determinant of fuel usage. Low level, light activity uses primarily fat as a substrate, while very intense exercise uses primarily carbohydrate for energy. Most aerobic exercises that are moderately intense use a mix of carbohydrate and fat. For example, marathon running at approximately 75% or your maximum aerobic capacity burns about 70% carbohydrate and 30% fat. As intensity increases, oxygen delivery cannot keep up with the demand for energy, and glucose is broken down anaerobically (without oxygen). This results in lactic acid build up, which decreases the ability to exercise. The level of intensity that lactic acid build-up occurs at varies per individual, but trained individuals have a higher “lactic acid threshold” than untrained, that is, they can work harder with less lactic acid build up.

The duration of exercise:
The duration of exercise is inversely related to exercise intensity. Very intense exercise, such as an all-out sprint, cannot last very long, and uses primarily carbohydrate for energy. As the time of the exercise increases, and oxygen delivery to the muscle increases, a mixture of fat and carbohydrate is used. Carbohydrate must always be available for the body to burn fat. The body has enough muscle glycogen (carbohydrate) stored to last about 1 ½ to 2 hours (depending on the intensity of the exercise). Longer time periods require replacement of carbohydrate, usually in the form of sports drinks or gels. This provides blood sugar for the body to use to continue to fuel exercise. As exercise time increases over 4 to 6 hours, fat supplies 60-70% of the energy needs.

A person’s fitness level:
As you train, over time your body adapts to exercise by:
Improving your muscle ability to extract and use oxygen, allowing for more fat to be used at a given intensity of exercise. This “glycogen sparing” allows you to exercise for a longer period of time before you run out of glycogen. Increasing your muscle glycogen stores (well-trained people can store 20-50% more glycogen in their muscles than their untrained counterparts). This also allows you to exercise for a longer period of time. Increasing your aerobic capacity (your body’s ability to do work). This adaptation is what increases the lactic acid threshold described in the intensity section.

Diet / Exercise Frequency
A diet consistently low in carbohydrate decreases the amount of glycogen that is stored in the muscles, decreasing the amount available to fuel exercise. Exercising hard many days in a row without adequate replacement of carbohydrate can lead to depletion of muscle glycogen. This eventually leads to fatigue during exercise. A diet high in carbohydrates helps to replenish muscle glycogen stores.

Not eating enough carbohydrate before exercise (i.e., 1-6 hours before) can decrease the amount of glycogen the liver has stored and available, which can lead to low blood sugar. Blood sugar is essential for proper brain functioning, and low levels can cause dizziness, lightheadedness, confusion, etc.

Protein will be broken down into amino acids, which are converted to carbohydrate, if the body needs carbohydrate and its stores are depleted. Muscle, organs, and support structures may be broken down to provide the protein required. The immune system, which is made up of protein, may also be compromised.