All personal trainers and their clients should be familiar with what the “burn” associated with repetitive or intense activity feels like. Historically, the blame for such discomfort fell on the build-up of lactic acid, a condition referred to as lactic acidosis, and was thought to inhibit one’s ability to keep going. Research now suggests that acidosis may potentially improve muscle performance during high-intensity exercise. Read on to learn more about this surprising lactate-exercise link!
Lactic Acid Fatigue Fallacy
Since its original discovery, and even up through the 1970s, lactic acid bore the brunt of responsibility for muscle fatigue and tissue damage following intense workouts. For years, researchers in the fitness/physiology industry have tried to suggest that increased lactate/H+ concentrations in skeletal muscle also resulted to less than stellar exercise performance.
In the early 2000s, scientists showed conclusively that lactate imposed little to no detrimental effect on mechanically stimulated muscle fibers. Lactic acid, a byproduct of anaerobic metabolism, certainly does build up as the body produces energy in the absence of oxygen; but experts feel this does not necessarily work to an athlete’s disadvantage.
Lactate Offers Positive Leverage
Several well-documented reports have revealed a somewhat protective effect of lactate exposure or induced acidosis on muscle contractions. It appears that lactate exposure can attenuate severe fatigue in stimulated rat muscle. Furthermore, upon ingesting exogenous lactate, a sprinter may delay the onset of exhaustion. Taken together, these latest findings have led to the idea that lactate/H+ plays an ergogenic role during exercise.
Lactate, Pyruvate, and Energy
The human body prefers to generate most of its energy through aerobic means. As one performs strenuous exercise, respiration increases in an attempt to shuttle more oxygen to the working muscles. Some circumstances, however, demand energy production faster than our bodies can deliver oxygen. In those cases, the working muscles generate energy anaerobically, utilizing the process of glycolysis. When this occurs, glucose gets metabolized into a substance called pyruvate.
In the presence of ample oxygen, pyruvate moves through an aerobic pathway and gets further broken down for additional energy. Under conditions of limited oxygen, however, the body responds by temporarily converting pyruvate into lactate. This lactate then allows glucose breakdown/energy production to continue. As working muscle cells continue performing anaerobic activity, lactate often accumulates at high levels.
Lactate as a Fuel Source
Dr. George Brooks, Professor of Integrative Biology at UC Berkeley, recently published an article in the journal Cell Metabolism, posting data that helped elucidate the misunderstanding of lactic acid. While lactate does play a significant role in metabolism, Brooks and his team pioneered the research that culminated in labeling lactate as a fuel routinely produced by muscle cells.
“It’s a historic mistake,” Brooks said. “It was thought that lactate is made in muscles when there is not enough oxygen. It has been thought to be a fatigue agent, a metabolic waste product, a metabolic poison. But the classic mistake was to note that when a cell was under stress, it produced greater amounts of lactate. Today’s theory stresses that lactate production, in and of itself a strain response, occurs to compensate for metabolic stress.”
After decades of research, Brooks pinpointed at least three main uses of lactate in the body:
- a major source of cellular fuel for function and repair
- a key player in supporting healthy blood sugar levels
- a powerful signal for metabolic adaptation to stress
Brooks’ data indicates that extra lactate present during illness or after injury could help encourage the process of recovery. “After injury, adrenaline will activate the sympathetic nervous system and that will give rise to lactate production,” Brooks said. “It is like gassing up the car before a race.”
The “Lactate Shuttle” System
The human body possesses the ability to store energy in several forms:
- glycogen, derived from ingested carbohydrates and stored in the muscles
- fatty acids or triglycerides, stored in adipose tissue
When the human body requires energy, it breaks down glycogen into lactate and glucose, and/or adipose tissue into fatty acids, all of which are distributed throughout the bloodstream as general fuel. However, Brooks feels that lactate deserves top billing as a major fuel source. He coined the term “lactate shuttle” to describe the processes by which lactate plays a key role in supporting the body’s organs.
Lactate also acts as an energy substrate in fast-twitch and slow-twitch muscle fibers. Newer studies have brought to light the role played by lactate in enhancing oxidative capacity, by tapping into skeletal muscle mitochondrial biogenesis. In the presence of lactate, the mitochondria in muscle tissue utilize this substrate more efficiently than even glucose or fatty acids. In fact, lactate will actually signal adipose tissue to cease its breakdown to eliminate fats as fuel utilization.
Lactate and Brain Energy
The brain, too, seems to benefit from an uptick in lactate. The lactate-neuron and lactate-astrocyte shuttles allow for the utilization of lactate to support cognitive function, particularly as extended aerobic activity often results in a significant drop in circulating blood glucose levels. Clinical trials have corroborated that both the heart and the brain function optimally when fueled by lactate instead of glucose.
If Not Lactate, What Does Cause DOMS?
As for the burning sensation and side pains that had heretofore been ascribed to a build-up of lactic acid, scientists now consider noxious ion metabolites to blame, particularly for post-exercise fatigue and DOMS. The “burn” follows from anaerobic ATP production and the hydrolysis of that ATP, with an end result of increased proton release (in the form of hydrogen ions). An accumulation of hydrogen ions lowers the body’s pH, causing acidosis. This in turn prevents oxygen from binding to hemoglobin in red blood cells. When the body faces reduced oxygen transport to skeletal muscle, ATP (energy) production suffers.
Both hydrogen ions and excess inorganic phosphate (accumulated from the breakdown of ATP) directly affect calcium release, which can negatively influence muscular contraction. Calcium contributes to muscle contraction by binding to a protein that enables contractile filaments of the sarcomere to interact; as this occurs, the entirety of the muscle shortens. Impaired calcium release and/or sensitivity will dampen the contraction, a true indication of muscular fatigue.
Exogenous Lactate Supplementation
Professor Yoshitaka Ohno and colleagues from the Toyohashi SOZO University in Japan recently undertook a study to determine if orally consumed lactate could facilitate muscle hypertrophy in addition to its use as an energy substrate. Their data confirmed that the administration of oral lactate positively influenced both hypertrophy and regeneration of the tibialis anterior muscle in the mouse model. Scientists believe that the potential exists for extracellular lactate to contribute to skeletal muscle plasticity as well.
Should endurance athletes think about supplementing their training regimen with oral lactate? A few select sports beverage companies have begun offering products that tout the advantages. However, fitness professionals await further research and in vivo testing before advocating such supplementation.