Understanding lactic acid.

Every marathoner has read about the dangers of lactic acid build-up. But mounting evidence on the body's various energy systems tells a slightly different story. Improved training strategies and marathon performance will inevitably follow a better understanding of the relationship between lactic acid, energy made available during long distance races, and fatigue.

We now know that, under the right circumstances, lactic acid is a fuel. Whether it is deliberately produced by the body to burn as such, or whether this is a case of the body making the best of a troublesome byproduct remains up for debate. What is known is that lactic acid (or lactate) is one of many fuel sources, and the subject is definitely more complicated than "lactic acid is bad and causes fatigue."

To begin with, the soreness we experience in the days following intense, prolonged exercise is almost certainly not caused by lactic acid. Lactic acid leaves the muscles within an hour after anaerobic exercise. Muscle soreness often peaks after 72 hours post-race. The cause of this soreness is tissue inflammation and muscle microtears due to intense exercise. However, to be sure, during miles 18 to 22 of a marathon there can be a shutting down of muscle contraction, followed by intense cramping and soreness (the Wall). This is lactic-acid related, but there is more to the story than we thought.

To better understand the new, nuanced view of lactic acid, it helps to review the two main energy systems the body uses in long distance running, the aerobic and anaerobic systems. ATP is a high-energy molecule and the main source of energy for muscle contractions. When we exercise aerobically, ATP is derived from the oxygen that we breathe in. Glucose (in the blood) and glycogen (in the muscles and liver) is nevertheless also required to convert this oxygen into energy. In the absence of oxygen, ATP is derived from the breakdown of glucose and glycogen but by a different metabolic process. When the amount of ATP needed cannot be derived quickly enough with traditional oxygen intake and conversion to energy, the body then relies on the anaerobic energy system, which creates as a byproduct lactic acid.

Remember, in the aerobic energy system, oxygen is required to create the chemical conversion of glucose and glycogen into useable fuel. A common misconception is that the depletion of muscle glycogen directly causes lactic acid build-up and therefore the fatigue and cramping associated with the Wall. But long after the body has used up all of its glycogen stores, a marathoner may continue to run without hitting the Wall. The body relies heavily on fat burning in the later stages of endurance events--after about one hour of running the ratio of energy derived from fat burning to that derived from carbohydrates (glucose and glycogen) is three to one.

It is not the triggering of the fatty-acid metabolism that causes the onset of the Wall. It is exercising above your anaerobic threshold. And even this is now understood to be more complicated than it seems.

The aerobic energy system is vastly more efficient than the anaerobic system. Without oxygen, the body requires a striking 18 times the amount of glucose to derive the same amount of ATP as it can aerobically. And fatty acids generate a particularly large portion of the energy in the aerobic system. The anaerobic system relies only on glucose.

It's entirely possible, then, to run out of glycogen and continue to produce ATP aerobically. This is the desirable state of affairs in marathoning, and is known as running just below your anaerobic threshold. It isn't the burning up of muscle glycogen stores that causes the Wall. Lactate builds up when you run above your anaerobic threshold. The practical effect in marathoning is to say, "I'd better not run out of glycogen," but this is only because you are also no longer running just beneath your anaerobic threshold.

Above the anaerobic threshold, the volume of carbon dioxide production exceeds the volume of oxygen consumption. The removal of carbon dioxide (through exhalation) can no longer maintain blood acidity within reasonable limits. The rapidly rising hydrogen ion concentration and falling blood pH cause the fatigue and cramping we know as the Wall. Labored breathing is a sure sign of this process, and therefore monitoring it will help you determine how close you are to the threshold. For average marathoners, running at about 65% but perhaps up to 75 % of your maximal heart rate will keep you just below the threshold. This is how you avoid hitting the Wall.

Recent findings suggest that muscle cells use carbohydrates anaerobically for energy, producing lactate as a byproduct, but then burning it, too, as an additional fuel source. Proper training helps people not so much clear the lactic acid from the blood as use the lactic acid before it can build up and cause muscle fatigue. Mitochondria are the cellular "energy factories". We now understand that they use the lactic acid produced by burning glucose anaerobically to create more ATP. We also have evidence that proper training increases the size and number of cellular mitochondria; by building these powerhouses up inside the muscle cells, your body can process far more lactic acid for fuel than a non-trained runner.

The muscle cramping associated with the Wall is the result of overexcited muscle receptors experiencing severe fatigue, and this can occur if the athlete is not properly trained. It is not due to the lack of oxygen alone. Lack of oxygen simply pushes one to utilize the anaerobic energy system. How your body handles this experience is the result of training: If your mitochondria are ready to effectively process the lactate, you will not hit the Wall and in fact find an additional energy source in lactic acid.

The body splits lactates into lactic acid and hydrogen ions. The hydrogen ions interfere with electric signals from muscles and nerves, causing pain and impaired contraction. So what is the functional difference between this and lactic acid being a foe? The answer is that through training you have some control over how well you process lactic acid. Employing overdistance and high-intensity workouts such as interval training will help you speed lactic acid removal from the muscles.

Some researchers, most notably George A. Brooks of UC Berkeley, who for years has questioned the bad name lactate has received, believe coaches end up training athletes in the best way possible to increase their mitochondria, though they may not understand this mechanism and are really clinging to the concept of running just beneath the anaerobic threshold. Through trial and error, coaches have learned that athletic performance improved when athletes worked on running longer and longer distances, for example. This strategy increases the mass of the muscle mitochondria, sometimes by as much as double, letting runners burn more lactic acid and allowing the muscles to work harder and longer. Dr. Brooks has found a shuttle protein that mitochondria use to bring lactate into them.

High-intensity and prolonged sub-maximal training, then, can adapt you to remove--by use as fuel--greater and greater amounts of lactic acid using both cardiovascular and mitochondrial systems. It's good news for marathoners. And as our understanding of the complex and interlocking energy systems of the body increases, we are sure to continue to better hone marathoning strategies and keep the dream alive of one day altogether eliminating the Wall.

(Am. J. Physiol. Endocrinol. Metab., 2006, Vol. 290, No. 6, E1237-E1244; Active Living, 2006, Vol. 15, No. 4, pp. 1-6; The New York Times, May 16, 2006, "Lactic Acid is Not Muscles' Foe, It's Fuel" by Gina Kolata; The Complete Guide to Running by Earl Fee, 2005, Meyer & Meyer, New York, NY, pp. 17-37, 152)