Lactic acid: make it work for you!

John Shepherd explains why lactate should no longer be considered the 'bad guy', but rather one of the body's most vital fuels

Slumped on my rowing machine, I was in pain. I’d just completed two four-minute intervals with a 5:30-minute recovery at 95% effort. My heart rate was relatively comfortable within a minute or so of completing the second interval, but my legs and, in particular, my quadriceps were on fire.

Physiologically my heart seemed stronger than my legs, its oxygen-processing capability apparently in excess of the energy that my legs were able to produce. It took 15 minutes for the burning to stop, and for the rest of the evening my legs felt like jelly. I cursed lactic acid, the (presumed) cause of my pain.

My experience of intense CV training got me thinking about aerobic and anaerobic energy metabolism, and particularly the role of lactate and lactic acid. Note that these are two different chemicals that are often mistakenly assumed to be one (of which more later).

Like many athletes and coaches I had been raised on the notion that lactate was bad; after all, why should we need to warm down to clear it out of our muscles if it was good? It was a ‘waste product’ that caused muscle damage and was the consequence only of anaerobic training – such as the workout I had just completed. Recently, however, I have begun to question much, if not all, of this received wisdom.

Let me start by quoting the words of the famous exercise physiologist and running doctor, Tim Noakes, on this subject: ‘In fact, lactate may be one of the most important energy fuels in the body. Let us banish once and for all the bad publicity that lactic acid has attracted for so long and elevate it to its rightful place as one of the most important of the body’s fuels’.(1)

However, before we follow Noakes’ train of thought, let us first consider why lactate has been labelled the bad guy for so long. This chemical was one of the first that exercise scientists were able to analyse, and it is partly for this reason that it has been mistakenly linked with a myriad of exercise-induced physiological responses, including fatigue, cramp and sprains. At one time, it was even suggested that lactate was the prime cause of muscular contraction!

For those of us of a certain age, lactate’s bad guy image was also reinforced by the late TV commentator Ron Pickering’s immortal reference to ‘swimming in a sea of lactic acid’, which was usually applied to 400m runners as they tired down the home straight. As we shall see, Ron would have been more accurate (if less dramatic) had he referred to athletes ‘bathing in an invigorating pool of lactate’ as they headed down the back straight. Indeed, he could have applied this metaphor to virtually any running event or sporting performance lasting more than around seven seconds. However, old habits die-hard and it is perhaps only now that lactate is beginning to shed its bad image. Lactate is actually produced within our muscles at very low exercise intensities, as well as at much higher ones; in fact, it is argued that it is also present in the body at rest. It is therefore not the consequence of anaerobic exercise.

The importance of glycolysis

Crucially, lactate actually helps to produce energy. It is created during glycolysis, which literally means the breakdown of glucose. Glucose is derived from the carbohydrate we eat, and glycolysis kick starts chemical processes within our muscles that produce the energy required for sustained muscular contraction. Without glycolysis we would be unable to sustain exercise for more than a few seconds.

There are two types of glycolysis: oxygendependent and oxygen-independent; and these can be equated to the aerobic and anaerobic energy systems. Each type of glycolysis produces lactate, although the oxygen-independent variant produces it as lactic acid.

Glycolysis results in the production of pyruvic acid (PA) through the breakdown of glucose, which involves more than 10 different chemical reactions. Pyruvic acid (PA) is then used within the Krebs cycle, a complex chain of reactions leading to energy production.

When pyruvic acid (PA) begins to accumulate in our muscles, as a result of what can be a relatively minimal increase in exercise intensity, the enzyme lactic dehydrogenase converts it into lactate. Under moderate-to-high exercise intensities, lactate is converted back to PA, which is then re-used for continued energy production.

The Krebs cycle provides nearly 90% of the energy required for CV exercise in the form of adenosine triphosphate (ATP), the body’s universal energy donor.

Our muscles possess two basic types of muscle fibre, each with a different speed of contraction and potential to contract repeatedly. Both of these fibre types play a crucial role in lactate production, lactate clearance and exercise performance.

Slow twitch fibres are fatigue resistant and are therefore used for sustained exercise; in terms of lactate they are best suited to clearance. Fast twitch fibres have a 2-3-fold greater speed of contraction and in their ‘pure’ form are used for powerful – if relatively short-lived – activity like sprinting; they are better suited to lactate production.

Lactate stacker workouts

Fast twitch fibres can be sub-divided into type IIa and type IIb. The former are considered ‘transitional’ in that they can, with the right training, improve either their fast twitch or their slow twitch capacity.

Lactate stacker workouts are a great way to boost endurance performance. These highintensity interval workouts generate large volumes of lactate very quickly, while their short recovery periods ensure that lactate levels soar again during the subsequent intervals. Very early into these workouts, an ever-increasing amount of energy needs to be fuelled without oxygen, resulting in oxygen-independent glycolysis.

In terms of improving the use and re-use of lactate in our muscles for boosting endurance performance, lactate stacker workouts encourage fast twitch fibres to produce more of the muscle cell protein known as monocarboxylate transporter 1 (MCT-1), which is present in slow twitch fibres in relative abundance.

MCT-1s are important in that they transport lactate into muscle cells, where it is broken down to produce further energy for exercise (see PP 112, Dec 98 and PP 130, March 2000 for more detail). Very simply, the more MCT-1s a muscle has, the greater the rate of lactate clearance and the greater your muscular endurance. Note also that lactate stacker sessions increase the number of mitochondria (cellular energy power plants) and capillaries (oxygenated blood highways) in and to your muscles, so also boosting their potential for sustained, powerful muscular contractions.

Despite their benefits, these workouts may have contributed to lactate’s bad guy reputation. In life it is natural to dislike something that hurts – and if you haven’t performed a ‘stacker’ session yourself, take it from me that it hurts! And you curse the stinging sensations in your muscles, which you believe derive from lactate.

However, when lactate is released into the bloodstream it does not cause pain. If it did, we’d be hurting all over, perhaps all the time, or at least after any form of exercise, even a stroll in the park. Putting this into context, lactate begins to rise, as a consequence of glycolysis, in the untrained at only about 55% of their maximum capacity for aerobic metabolism (VO2max).

So if lactate is not the painful problem, what is? To answer this question, we need to understand a bit of chemistry. Any substance that ionises in solution and gives off hydrogen ions is an acid. When we exercise at a high intensity (such as those encountered during a lactate stacker workout), we set up the right conditions for acid to develop in our muscles.

Although we are still gulping in oxygen, it becomes insufficient to furnish the required amount of energy. This alters the results of the chemical equations taking place, and lactic acid is produced instead of lactate. Specifically, lactic acid is formed when pyruvic acid temporarily accepts two hydrogens (electrons) due the shortage of oxygen. Note, though, that lactic acid returns to lactate once it enters the blood stream.

Lactic acid can be thought of as lactate’s metaphorical cousin. Lactate is the goody-goody, always able to get the energy creation job done (at least up to certain intensities – as high as 80% VO2max for the endurance-trained athlete). Unfortunately, lactic acid ends up getting the rap, despite trying to emulate its cousin; when trying to contribute to energy production at higher exercise intensities, its rate of production ultimately exceeds its rate of clearance, resulting in a loss of muscle power, pain and eventual exercise cessation.

Not a waste product

The pain that accompanies lactic acid is thought to result from the irritant effect of acidic muscles on nerve endings. Lactic acid is also believed to ‘irritate’ the central nervous system, leading to feelings of nausea and disorientation.

Lactic acid, like lactate, is not a waste product. During recovery, when there is a much more plentiful supply of oxygen, lactic acid looses its two hydrogens and reverts to pyruvic acid for use as an energy source. In fact 50% of the lactate produced during a tough workout (remember that lactic acid returns to lactate when it enters the blood stream) is used for replenishing muscle glycogen stores during recovery. Note that glycogen is premium grade muscle fuel derived from carbohydrate, which can be stored in the muscles and liver in limited amounts (up to about 375g).

The contribution lactate makes to glycogen replenishment and post-exercise recovery occurs during what is known as the ‘lactate shuttle’. When lactate is released into the blood stream, the liver uses it to produce blood glucose and glycogen, while the heart and other muscles use it for energy production. For energy sustainability during exercise, the ability of the lactate shuffle to redistribute carbohydrate – as potential glycogen, through the metabolism of lactate – from muscles that are fully glycogenstocked is key.

The lactate shuffle lifts glycogen from muscles that are not being used significantly – eg the arms during marathon running – to areas where glycogen is being drawn significantly – eg the legs – thus helping to sustain energy.

Although we should see lactate as the good guy from now on in terms of its energy contribution, we should not get too carried away with all it promises, particularly when it comes to using it to predict endurance performance through lactate threshold (LT) testing.

Lactate threshold testing

The lactate threshold is the notional point at which lactate levels accumulate within our muscles to the extent that glycolysis proceeds with less and less oxygen and muscular action eventually grinds to a halt. Lactate threshold can be viewed as the muscular engine’s ‘red line’; once the ‘lactic needle’ enters this band, power is gradually lost.

Lactate threshold tests generally take the form of an incremental increase in effort against a controlled resistance, such as treadmill belt speed. Blood samples are taken, usually from the ear, for analysis of lactate accumulation.

However, Watts and associates have noted that: ‘Lactate threshold values will differ with different durations of incremental (test) steps. The criteria used to determine the lactate threshold will affect the threshold value. Lactate threshold values are largely a matter of the protocol used and the criteria established for declaring a threshold. Without such knowledge the absolute value of a lactate threshold is meaningless and unreliable.’(2) When reviewing endurance training research and testing protocols, Berg made similar observations(3).

Santos took the argument further when he specifically examined the use of lactate threshold as a predictor of endurance performance for half marathon times(4). Eighteen long-distance runners performed a total of 33 half-marathons, together with an equal number of incremental field tests (4 x 2,000m) to establish the relationship between running speed and blood lactate levels. Basically the researchers wanted to discover how fast the runners needed to run to be able to stay within comfortable – for achieving fast half marathon times – lactate levels.

Speeds used in the field test ranged from 4.2 to 5.8 metres per second, with a progression of 0.4 m/second each step. Following each loading level, blood samples were taken and analysed. At first it seemed that the step tests were valuable predictors of half-marathon times, with test speeds corresponding to lactate concentrations of between 3.0 and 5.5mmol, reflecting halfmarathon speed. Even higher correlations were found at lactate levels of 4.5, 5.0 and 5.5 mmol running speeds.

However, when the athletes actually raced, these strong correlations fell apart; 70% of the athletes’ final competition times fell outside the level of prediction based on the lactate levels of 4.5 – 5.5 mmol achieved during the supposedly predictive step testing.

Predictive limitations

In an attempt to further explain lactate levels and lactate threshold’s shortfalls when it comes to endurance event performance, Noakes writes: ‘Lactate is a natural product of carbohydrate metabolism during exercise. As the rate of energy production rises, so more carbohydrate is used and as a result, more lactate appears in the bloodstream. Hence a rising blood lactate level only indicates that more carbohydrate is being burned. It does not mean that the muscle’s work is becoming more anaerobic.’(5)

Thus attempts to correlate event performance with a notional lactate threshold are ultimately doomed to failure. Noakes suggests that better predictors of endurance performance are time trials, race results at shorter distances and self-analysis (for suitably experienced athletes).

By now some of your misconceptions surrounding lactate should have been cleared up. The reality is that lactate (or lactic acid) is neither a bad guy nor a waste product, but a key ingredient in energy production and sustainability.

So now, when you experience the pain of a highly beneficial lactate stacker workout, you should not curse lactic acid but rather pat it on the back for the attempt it has been making at keeping your muscles working and the contribution it will be making to your postworkout recovery. Lactate only blots its newfound copybook when used to specifically predict endurance performance.

John Shepherd

References
Noakes MD Lore of Running (4th edition) P157, 159 and 163 Human Kinetics 2002
Medicine and Science in Sports and Exercise, 30(5), 1998
Sports Med. 2003; 33(1): 59-73
Medicine and Science in Sports and Exercise, 30(5) 1998
Noakes MD Lore of Running (4th edition) P157, 159 and 163 Human Kinetics 2002