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).

Figure 2.21: What happens to sugar in your blood? Depending upon whether energy is needed, glucose can be released for immediate use or stored on a short-term or long-term basis.

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Q

Question 2.4

What is “carbo-loading”?

“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).

Figure 2.22: Some competitive athletes carbo-load to maintain high energy levels.

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).

Figure 2.23: Water weight. Water molecules bound to glycogen account for much of the weight lost early in a diet.

TAKE-HOME MESSAGE 2.9

Glucose is the most important carbohydrate to living organisms. Glucose in the bloodstream can be used as an energy source, can be stored as glycogen in the muscles and liver for later use, or can be converted to fat.

Why do people lose more weight by burning a gram of glycogen than by burning a gram of fat?

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