What Happens to Stored Carbohydrate (Glycogen) During Exercise?

The hormone picture that develops during exercise is similar to the one discussed regarding a fasting period; however, there are relative differences. Epinephrine is released from our adrenal glands as a direct effect of exercise.

Quite simply, the greater the exercise intensity, the greater the epinephrine release. Epinephrine stimulates the breakdown of muscle cell glycogen (see Table 4.5 and Figure 4.3). This makes glucose available for the muscle cells hard at work. Epinephrine also promotes the breakdown of glycogen to glucose in the liver. Some of this glucose will then circulate to working muscle to provide support. Cortisol may also be released in response to moderate to intense exercise, particularly as the exercise becomes prolonged (for example, endurance cycling and running). Cortisol will also support the breakdown of glycogen as well as gluconeogenesis in our liver.

What Is Glycemic Index?

As expected, the level of circulating glucose increases after eating a carbohydrate-containing meal. But to what level, and will different foods having the same amount of carbohydrate result in the same increase in blood glucose? This kind of information surely would be of interest to many people, especially those managing their blood glucose levels (such as in diabetes).
 

As shown in Figure 4.4, the level of glucose circulating in the blood increases after eating or drinking a carbohydrate-containing food or beverage and then is reduced back toward the normal fasting level. This response is often referred to as a glucose tolerance curve and it can be used to assess how well a person’s body is able to take glucose out of the blood and use it for energy and to build stores.
 

Since different foods will produce different glucose tolerance curve patterns, scientists developed the glycemic index. Simply put, glycemic index is a measure of the power of carbohydrate-containing foods to raise blood glucose levels after being eaten or drunk. In addition to people managing their blood glucose levels, glycemic index has become popular for many people trying to lose weight which will be discussed this in more detail in Chapter 11. See Table 4.4 for standard levels for glycemic index and load.

Glycemic index is a measure of a food’s ability to raise the level of blood glucose.
 

For a long time it was assumed that because starch was more structurally complex than simpler sugars, starchy foods would be digested more slowly and therefore absorbed more slowly and evenly after a meal. On the other hand, foods containing simpler sugars (for example, soda and candy) would be digested and absorbed more rapidly, leading to a faster and greater rise in blood glucose. However, the relationship between different foods and blood glucose turned out to be more complex, which is why the determination of glycemic index for individual foods has been helpful.

How Much Glycogen Is in Our Body?

Our liver can store up to 6 to 8 percent of its weight as glycogen for about 75 to 100 grams total. Meanwhile, only about 1 percent of the weight of skeletal muscle cells is attributable to glycogen. However, since the total amount of skeletal muscle in our body far exceeds our liver, muscle will contribute much more to our total glycogen stores. Skeletal muscle can contain about 250 to 400 grams, which is about four-fifths of our total glycogen stores. Since carbohydrate provides 4 calories per gram the potential energy from glycogen is typically 1,400 to 2,000 calories, not very much. As you may expect, people with more muscle resulting from exercise training will have more body glycogen owing to increased muscle mass. In addition, their muscle will adapt to double and even triple the amount of glycogen it can store.
 

Interestingly, even though carbohydrates contribute approximately one-half of the energy in our diet, our body composition is not reflective. That’s because only 1 percent or less of our body weight is composed of carbohydrate. This means that carbohydrate is stored with limitations, most of which is in our liver and skeletal muscle as glycogen. Other tissues, such as fat cells and the heart, contain a little glycogen as well; however, the contribution to our total body glycogen stores is very small. Since glycogen stores are relatively small there must be another means of storing the excessive energy from diet derived carbohydrate.