The Role of Lactic Acid and Lactate

One element that affects each of these three important components of endurance—brain, muscles, and metabolism—is lactic acid. While it’s produced in muscles, it ends up in the blood, where it converts to lactate.

We’ve all heard of lactic acid—that’s the so-called waste product that’s produced in anaerobic muscles with harder workouts. Lactic acid was thought of as a cause of fatigue and muscle soreness. But a revolution has occurred in this field in the last few years. With a better understanding of physiology, we’ve come to know these compounds as an important part of our overall fitness and health. In particular, lactate is an important source of energy.

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Blood lactate is a normal and important component of our body chemistry. Its metabolism includes production (as lactic acid) in muscles at rest and during all intensities of exercise. Specifically, blood lactate provides us with a variety of benefits; it’s an important source of energy, forming glucose, which helps replace muscle glycogen stores when they are diminished; it’s an important fuel for aerobic metabolism (to help maintain fat burning); and helps spare blood sugar. Lactate does not necessarily increase fatigue during long endurance activities. And, lactate (or lactic acid) is not directly associated with muscle soreness.

Lactate is also an energy source for many other areas of the body. It provides energy for the heart (cardiac) muscle, the liver and kidney, red and white blood cells, and the brain. Lactate is also important for wound repair and healing. The largest mass of tissue in the body, our muscles, uses considerable amounts of lactate. Let’s look at some of these in more detail.

The old view that lactate, beginning with the muscle’s production of lactic acid during hard exercise (due to oxygen debt), is a “dead-end” waste product has changed dramatically in recent years. It’s not simply an anaerobic metabolite. This is the case when oxygen availability is low, but lactic acid is also formed aerobically in the presence of sufficient oxygen.

Lactic acid is produced in muscles even at rest. The amount can increase significantly with increased exercise intensity. When lactic acid levels elevate, body pH—the acid-alkaline balance—is reduced. This is associated with muscle fatigue, but it may not be the cause, which is still not completely known (although the disruption of calcium and phosphorus metabolism and muscle imbalance are key factors in fatigue). Lactic acid is ultimately diffused into the blood and converted to lactate.

Lactate plays an important role in providing energy to muscles as a source of carbohydrate. During moderate intensity exercise, for example, lactate may become more important than glucose for muscle energy, which may help spare blood glucose. Aerobic muscles utilize lactate for energy as well, helping to burn fat. Lactate accomplishes this by returning to the muscle cells from the blood, and converting to glucose. (Lactate conversion to glucose also occurs in the liver.)

Even without food we can quickly restore depleted glycogen in aerobic and anaerobic muscle fibers with lactate. This is an important part of the “fight or flight” mechanism. (Body fat and even protein can also serve as important sources of energy to help replace depleted glycogen stores.)

Lactate is an immediate (less than thirty minutes) fuel source that helps replenish glycogen following exercise, especially hard efforts. Food sources can also contribute, if consumed immediately (within a thirtyminute window) following a hard workout or competition. Complete repletion of glycogen stores during recovery over the next ninety minutes and beyond is more dependent upon the breakdown of body fat and protein; for example, certain amino acids convert to glucose.

Up to half of the energy needed for complete glycogen repletion comes from lactate, and 50 percent or more from the breakdown of fat and protein. However, new research shows these levels may be conservative and that fat and protein may contribute even more significantly to glycogen replacement.

All workouts and races require recovery. The process of active recovery—the cool-down, discussed later—is especially important for glycogen replacement. This occurs quickly in the anaerobic muscle fibers with the contribution of glycogen from the aerobic fibers. Glycogen, lactate, fat, and protein all provide significant contributions to this process.