The Body Can Use Glycogen As An Energy Source When Aerobic Exercise Lasts How Long?

Glycogen, the storage form of glucose, is the main source of fuel during exercise. Muscle and liver cells both store glycogen. Muscle glycogen is useful during activity and liver glycogen is broken and down and put into the bloodstream during periods of low blood glucose. Adaptations to exercise influence your glycogen stores and use.

Glycogen For Energy

The first source of energy for metabolism during any exercise is creatine phosphate. Your body uses this molecule to make quick energy before it can switch to long-term energy production. Muscle glycogen is the go-to source for aerobic exercise energy production. Glycogen is right at the source of energy production -- inside the muscle cell -- so there is no work involved in getting the fuel to where it is needed. Glycogen is broken down into glucose molecules that enter the mitochondria and are made into energy.

Duration

For the first 30 minutes of aerobic exercise, energy is mainly provided by glucose. After that, the contribution shifts from mainly glycogen to fat stores. For optimal fat burn, which aids in both wellness and weight loss, you must exercise for at least 40 minutes. According to the University of Michigan, after 40 minutes of aerobic exercise, fatty acids become your body's primary fuel source. There is rarely a time during aerobic exercise though that glucose or fat provides 100 percent of the means for energy production. It is usually always a combination.

Muslce Glycogen Sparing

A limitation to the duration and intensity of exercise is fatigue. Running low on glycogen stores is one reason that you may feel fatigue during exercise. As an adaptation to a regular exercise program, your body makes changes to metabolism. Regular exercise causes an increase in the mitochondria of your muscle cells. This allows for a more rapid use of fat as fuel. Turning to fat as the primary fuel source during exercise allows you to work harder and longer without feeling fatigue. This is known as muscle glycogen sparing. Part of glycogen sparing means that your body relies more on fat for fuel. This mechanism slows the rate of glycogen use.

Muscle Glycogen Stores

Another adaptation to regular exercise is an increase in muscle glycogen stores. When you use your muscle glycogen regularly, the body decides that it better store more. After exercise, your glycogen synthase levels increase. This is the enzyme that triggers an increase in glycogen stores. This adaptation makes it possible for your body to use glucose as fuel for a longer time frame. This means a trained individual may work faster, harder and longer than an untrained person. In combination with glycogen sparing this adaptation helps increase fitness and performance.

Considerations

To take advantage of an increase in muscle glycogen stores, replenish blood glucose so that your body has a source for producing more glycogen to fill them. Consume .7 g to 1 g of carbohydrates per 2.2 lbs. of your body weight immediately following exercise, then eat high-carbohydrate meal one to two hours later.