Lactate and lactic acid - dispelling the myths

You work hard in training and get fatigued. You've accumulated lactic acid in your blood and that's why you're tired, right? Wrong, says Dario Fredrick, M.A. - it's a bit more complicated than that.

One of the most interesting challenges that coaches and exercise scientists face is to explain exercise physiology in easy to understand way. A common goal is to provide athletes with useful information to optimize their exercise behavior. Unfortunately, the oversimplification of concepts can lead to misunderstanding. Lactate has suffered this regrettable fate.

The belief that lactate or "lactic acid" as the cause of fatigue in working muscle was a convenient explanation for complex processes. Since the appearance of lactate increases as exercise intensity increases, and as exercise intensity increases, fatigue increases, some assumed that lactate caused fatigue. If we didn't understand heart rate as well as we do, one might assume the same correlation between heart rate and fatigue. Heart rate and fatigue both increase as exercise intensity increases, but we know that heart rate does not cause fatigue. The same is true of lactate.

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This point is heavily debated. Although lactate production increases progressively as exercise intensity increases, the ability to use lactate as fuel varies from person to person and varies with one's trained level, fed and rested state[5]. The ability to use lactate as fuel will also vary the amount that will appear in the blood at maximum sustainable workloads[4]. This evidence suggests that a fixed lactate concentration may not be a valid predictor of performance.

The aerobic pathway is the most complicated given the steps involved, but yields the largest amount of ATP, allowing the use of many types of fuel (fats, proteins, glucose and lactate). The conversion of these fuels into ATP requires various steps within the muscle cell. Oxygen and fuel need to be delivered into the mitochondria (mini-organ "aerobic furnaces" within cells) during this process to produce ATP aerobically. While the aerobic pathway is the slowest ATP producer of the three pathways, it is also the most efficient.

The process of glycolysis results in the formation of lactate and hydrogen. These two products, while produced from the same reaction, disassociate and have different fates in the body. If lactate and hydrogen were to remain a single unit, then it would be lactic acid. It is unlikely however that you would find any lactic acid in the blood. Lactate can remain in the cell for energy or leave the cell and travel to active and inactive muscles to be used as a fuel[1]. The ability to use lactate as fuel, particularly within the muscle itself, varies with the trained characteristics of aerobic muscle fibers, specifically via endurance training[5].

While hydrogen (H+) accumulation resulting from glycolysis can lower pH, increasing acidity, much of the H+ is buffered via the bicarbonate buffer system (H+ + HCO3- <---> H2CO3 <---> CO2 + H2O), and converted to H2O and to CO2 which is eliminated via the lungs. If the accumulation is severe, there is some evidence that it may interfere with muscular contractions, although recent evidence suggests otherwise[9, 10]. Glycolysis is not the only contributor to an increase in acidity and hydrogen accumulation is not the only potential contributor to fatigue. Fatigue at non-sustainable workloads appears to result of an accumulation of other metabolites such as inorganic phosphate[9], as well as the inability of muscle to keep up with the rates and force of contraction through the progressive loss of potassium from inside the muscle cell[8].

For the sake of understanding, athletes have been taught that lactate or "lactic acid" is the root of all evil. However, while easy to relate to athletes, this paradigm has hindered what has really been known about lactate for the last 5-20 years. Lactate is not: 1) present as lactic acid in any appreciable amount in the blood, 2) the direct cause of fatigue at higher workloads, or 3) directly predictable of acidity. Lactate is: 1) a valuable energy source within working muscle, non-working muscle, and the heart, 2) quantitatively the most important contributor to the making of glucose in the liver, and 3) subject to training- induced improvements in its use as a fuel. Just as many 'old school' athletes had to adjust to the idea that rest is good for training, the idea that lactate is 'good' continues to fight a similar resistance.