2.9: Glucose provides energy for the body’s cells.

The carbohydrate of most importance to living organisms is glucose. This simple sugar is found naturally in most fruits, but most of the carbohydrates that you eat, including table sugar (called sucrose) and the starchy carbohydrates found in bread and potatoes, are converted into glucose in your digestive system. The glucose then circulates in your blood at a concentration of about 0.1%. Circulating glucose, also called “blood sugar,” has one of three fates (FIGURE 2-21).

• 1. Fuel for cellular activity. Once it arrives at and enters a cell, glucose can be used as an energy source. Through a series of chemical reactions, the relatively high-energy bonds in the glucose molecule are converted into lower-energy bonds of other molecules (a process explained in detail in Chapter 4). The change from higher-energy to lower-energy bonds releases energy, and organisms use the released energy to fuel cellular activity, including the muscle contractions that enable you to move and the nerve activities that enable you to think.
• 2. Stored temporarily as glycogen. If there is more glucose circulating in your bloodstream than is necessary to meet your body’s current energy needs, the excess glucose can be temporarily stored in various tissues, primarily your muscles and liver. The stored glucose molecules are linked together to form a large web of molecules called glycogen. When you need energy later, the glycogen can easily be broken down to release glucose molecules back into your bloodstream. Glycogen is the primary form of short-term energy storage in animals.
• 3. Converted to fat. Finally, glucose circulating in your bloodstream can be converted into fat, a form of long-term energy storage.

“Carbo-loading” is a method by which athletes can, for a short time, double or triple the usual amount of glycogen stored in their muscles and liver, increasing the store of fuel available for extended exertion and delaying the onset of fatigue during an endurance event (FIGURE 2-22).

Carbo-loading is usually done in a depletion phase and a loading phase. Six or seven days before competition, the athlete depletes glycogen in the muscles through a super-low carbohydrate intake and exhaustive exercise. Then two days before the competition, the athlete eats a super-high carbohydrate diet and reduces exercise to achieve a higher blood glucose than is necessary, so that much of the excess glucose is stored as glycogen.

Glycogen also plays a role in the initial rapid weight loss people experience when dieting. If you reduce your caloric intake such that your body is burning more calories than you are consuming, your body must use stored energy. The first, most accessible molecules that can be broken down for energy in the absence of sufficient sugar in your bloodstream are glycogen molecules in your muscles and liver. Large amounts of water are bound to glycogen. As that glycogen is removed from your tissue, so, too, is the water. This loss of water leads to the initial dramatic weight loss that occurs before your body resorts to using stored fat, and the rate of weight loss then slows considerably (FIGURE 2-23).