Dr. George Brooks, a Professor from the Department of Integrative Biology at University of California at Berkeley, described the dynamic production and use of lactic acid in metabolism in his "Lactate Shuttle Theory." This theory describes the central role of lactic acid in carbohydrate metabolism and it's importance as a fuel for metabolism.

6. The body uses lactic acid as a biochemical "middleman" for metabolizing carbohydrates.

Carbohydrates in the diet are digested and enter the circulation form the intestines to the liver mainly in the form of glucose (blood sugar). However, instead of entering the liver as glucose and being converted directly to glycogen, most glucose from dietary carbohydrate bypasses the liver, enters the general circulation and reaches your muscles and converts into lactic acid. Lactic acid then goes back into the blood and travels back to the liver where it is used as building blocks for making liver glycogen. Your body produces much of its liver glycogen indirectly from lactic acid rather than directly from blood glucose.

Scientists call the process of making liver glycogen from lactic acid the "Glucose Paradox". The theory was formulated by famous biochemist Dr. J.D. McGarry and his associates. It shows the importance of lactic acid in carbohydrate metabolism.

7. During endurance races, such as marathons and triathlons, blood lactic acid levels stabilize even though lactic acid production increases.

This occurs because your capacity to produce lactic acid is matched by your ability to use it as fuel. Early during a race, there is a tremendous increase in the rates that muscle uptake and use glucose and breakdown glycogen. The increased rate of carbohydrate metabolism steps up production of muscle lactic acid, which also causes an increase in blood lactic acid.

As your body directs blood to your working muscles, you can shuttle the lactate to other tissues and use it as fuel. This reduces lactic acid levels in your muscles and blood, even though you continue to produce great quantities of lactic acid. However, you often feel better during the race or training. This relief is sometimes called "second wind".

Scientists use radioactive tracers to follow the use pattern of fuels in your blood and muscles. Their studies show that during exercise, lactic acid production and removal continue at 300-500 percent of resting rates, even though oxygen consumption has stabilized at submaximal levels.

8. The heart, slow-twitch muscle fibers, and breathing muscles prefer lactate as a fuel during exercise.

In the heart, for example, the uptake of lactate increases many fold as the intensity of exercise increases while uptake of glucose remains unchanged. These tissues suck up lactate at a fast rate to satisfy their energy needs.

9. Lactic acid is a very fast fuel that can be used to athletes' advantage during exercise.

The concentration of both glucose and lactic acid rise in the blood after a carbohydrate-rich meal, but the blood lactic acid concentration does not rise much because it is removed so rapidly. The body converts glucose, a substance removed from the blood only sluggishly, to lactate, a substance removed and used rapidly. Using lactic acid as a carbohydrate "middleman" helps you get rid of carbohydrates from your diet, without increasing insulin or stimulating fat synthesis. During exercise, you won't want an increase in insulin because it decreases the availability of carbohydrates that are vital to high performance metabolism.

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