Lactic Acid's Bad Rap - maybe not?
The following is an excerpt from The Paleo Diet for Athletes by Loren Cordain, Ph.D. and Joe Friel. The book is due out in 2005 from Rodale Press.

For the better part of a century, athletes and physiologists alike have considered lactic acid a primary cause of fatigue during high intensity exercise and referred to it as a "waste product" of muscle metabolism. But now this way of thinking about lactic acid is being challenged. Recently scientists have learned that this substance we produce in large quantities during exercise is not a cause of fatigue and actually helps to prevent it.

This former misrepresentation started with British physiologist and Nobel laureate Archibald V. Hill who in his lab in 1929 flexed frog muscles to fatigue and discovered that lactic acid accumulated when muscular failure occurred. He concluded that the lactic acid caused the fatigue he noted during the muscles' repeated contractions. What he didn't know, and what we've only recently learned, is that when the muscle is part of a complete biological system instead of being examined in isolation from the rest of the body, lactic acid is processed and converted to fuel to help keep the muscles going. It does not cause fatigue.

Nor does lactic acid cause muscle soreness the day after hard exercise. This is a myth that has been around for decades and refuses to go away despite evidence to the contrary over the past 20 years. That soreness is more likely the result of damaged muscle cells resulting from excessive usage.

So if lactic acid is not the villain we've made it out to be, what does cause fatigue and the burning sensation in the muscles during short, intense exercise bouts such as intervals or races lasting just a few minutes? To get at the answer it's necessary to understand the pH scale, which tells us how acidic or alkaline (base) the body's fluids are in a range of 1 to 14 as hydrogen ions increase or decrease. On this scale hydrogen readings dropping below neutral 7 indicate increasing acidity while those rising above 7 indicate escalating alkalinity. Examples of acidic fluids are hydrochloric acid (pH=1) and vinegar (pH=3) while milk of magnesia (pH=10.5) and ammonia (pH=11.7) are alkaline.

At rest the pH of your blood is around 7.4-slightly alkaline. In terms of your blood, small absolute changes in acid-base balance have major consequences. For example, during a two- to three-minute, all-out effort, your blood's pH may drop as low as 6.4. In biochemical terms this is a huge acidic swing producing a burning sensation in the working muscles and their inability to continue contracting. Fatigue has set in.

If lactic acid didn't cause this drop in pH, what did? The answer has to do with our sources of fuel during such short exercise bouts-glycogen and glucose. Both are carbohydrates but they have slightly different chemical compositions. Glycogen is stored inside the muscle where it can be quickly broken down to produce energy. Glucose, which is stored in the liver and floats around in the blood stream, is called on to produce energy for exercise when glycogen stores can no longer keep up with the demand or are running low. As glycogen is broken down to produce energy it releases one unit of hydrogen. But if glucose must be used for fuel, as when the intensity of the exercise exceeds glycogen's ability to keep up, two units of hydrogen are released. This rapid doubling of hydrogen ions in the system lowers the blood's pH causing the burning and fatigue associated with acidosis. The same amount of lactic acid is released no matter which fuel is used.

Far from being an evildoer, lactic acid is an ally during intense exercise. It does a great deal to keep the body going when the going gets hard. Besides being converted back into a fuel source, when hydrogen begins to accumulate, lactate transports it out of the working muscle cells and helps to buffer or offset its negative consequences.

After nearly 80 years lactic acid's bad boy reputation has now been lifted.


Joe Friel is author of the Training Bible series of books and founder of Ultrafit Associates. He may be contacted via www.Ultrafit.com