The feeling of fatigue that occurs with exercise is probably due to the loss of stored muscle glucose.
With exercise, insulin levels in nondiabetics and people with type 2 diabetes decline, because insulin acts to store and not release glucose and fat. Levels of glucagon, epinephrine, cortisol, and growth hormone increase to provide more glucose. Studies show that glucagon is responsible for 60 percent of the glucose, and epinephrine and cortisol are responsible for the other 40 percent. If insulin did not fall, glucagon could not stimulate the liver to make glucose.
You might wonder how insulin can open the cell to the entry of glucose when insulin levels are falling. In fact, two things are at work here. Glucose is getting into muscle cells without the need for insulin, and the rapid circulation that comes with exercise is delivering the smaller amount of insulin more frequently to the muscle. The muscle seems to be more sensitive to the insulin as well. This is exactly what the person with type 2 diabetes hopes to accomplish when insulin resistance is the major block to insulin action.
One way to preserve glucose stores is to provide calories from an external source. Any marathoner knows that additional calories can delay the feeling of exhaustion. The timing is important. If the glucose is given an hour before exercise, it will be metabolized during the exercise and increase endurance. However, if it's given 30 minutes before exercise, it may decrease stamina by stimulating insulin, which blocks liver production of glucose.
Fructose can replenish you when you're doing prolonged exercise. This sweetener can replace glucose because it is sweeter but is absorbed more slowly and does not provoke the insulin secretion that glucose provokes. Fructose is rapidly converted into glucose inside the body. (See Chapter 8 for more on fructose.)
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...