Lactic Acid 101

By Rick Morris

Lactic acid has gotten a bad rap over the years. It has gotten the blame for muscle fatigue, muscle soreness, oxygen debt and that burn runners feel in their legs. It has been called a “dead end” waste product of energy production that can only be cleared from the bloodstream during rest and recovery.

There has even been a very commonly accepted theory written that accuses lactic acid of doing dastardly deeds. Here is an example from Lehninger’s biochemistry textbook written in 1970. “After a bout of maximal exercise, such as a sprint, a mammal will continue to breathe in excess of the normal resting rate and consume considerable extra oxygen. The extra oxygen so consumed during the recovery period is called the ‘oxygen debt,’ and it corresponds to the oxidation in the liver and heart of some of the excess lactic acid formed during maximal muscular activity. The remainder of the excess lactic acid accumulating in the blood during the sprint is converted to glycogen in the liver; the extra ATP required is derived from that portion of the lactic that is oxidized via the tricarboxylic acid cycle in the liver.”

In simpler language, this theory says that the accumulation of lactic acid causes your body to build up an “oxygen debt”, which is paid back during recovery. This is a theory that many coaches and running professionals still believe to be true. Just listen carefully to running commentators during the broadcast of a road race. Many times they will talk about a runner slowing slightly because they have gone into “oxygen debt”.

While this theory and other ideas concerning lactic acid were state of the art in their time, we now know that the accusations pointed at lactic acid are simply not true. Lactic acid is not a black hearted devil just waiting to jump on our backs and ruin our races or training runs. It is actually an essential substance that allows us to continue to run as well as possible. It is not a “dead end” waste product. It is an intermediate product that is used to produce large amounts of glycogen to fuel your running.

The Three Energy Systems

In order to understand what lactic acid is and the role it plays in fueling our running activities, it will help to have a basic knowledge of the three ways our bodies produce energy. There are three basic systems. They are ATP-PCr, glycolytic and oxidative. Lactic acid is a major player in only the glycolytic and oxidative systems.

The ATP-PCr system is the simplest of the three. This system uses a molecule called phosphocreatine to produce energy very quickly and without the use of oxygen. There are very small stores of this substance in your body, so it can be used only for short duration, high intensity events such as sprinting. There is only enough stores of phosphocreatine in your body to fuel up to about 15 seconds of energy during an all out sprint. Since lactic acid build up is not a concern with this energy producing system, I won’t muddy the waters with further talk of this system.

The glycolytic system produces energy from the breakdown of glucose. Glucose comes from the digestion of the carbohydrates that you eat and also from glycogen (a storage form of glucose) in your liver. Glucose accounts for 99% of the sugar in your blood.  The process involves 12 highly complex reactions that taken together are called glycolysis. I won’t bore you with the details of glycolysis, but the first important point is that glycolysis eventually produces a substance called pyruvic acid and contributes a small amount of energy. This entire process can take place without the presence of oxygen.

The oxidative system picks up where the glycolytic system leaves off. When your body can supply a sufficient amount of oxygen to its working muscles, most of the pyruvic acid produced during glycolysis enters a series of reactions called the Krebs cycle. This cycle produces 90% of the energy to need to sustain medium to long term exercise. However, the Krebs cycle only works when you are exercising at an intensity at which your body can supply sufficient amounts of oxygen to your muscles.  When exercise reaches very high intensities, such as sprinting or 400 to 800 meter races, your bodies need for energy exceeds your ability to provide an adequate supply of oxygen to your muscles. At this point your body must rely on the shorter term ATP-PCr and glycolytic systems.

Lactic Acid

Now, we are finally getting to lactic acid. Recall that glycolysis ultimately produces pyruvic acid. As I mentioned earlier, much of the pyruvic acid enters the Krebs cycle and is used to produce large amounts of energy. The pyruvic acid that remains is converted into lactic acid. When you are exercising at an easy pace, small amounts of pyruvic acid are produced. Your body easily “handles” this substance and small amounts of lactic acid are produced. As your workout becomes more intense, glycolysis really heats up and you start to produce very large amounts of pyruvic acid. All of this pyruvic acid starts to back up and a large amount of lactic acid is produced as a result.

This is where the old school parts ways with the new school. According to earlier beliefs, this build up of lactic acid was a waste product that caused fatigue, burning muscles and could only be cleared during rest and recovery. Now we know that most of the lactic acid produced is used to produce immediate energy for your muscles and also plays an important part in generating additional glycogen to be used for energy.

Lactic acid is not only produced during highly intense bouts of exercise. It is produced at all times, even at rest. A recent study showed that even during low intensity exercise, as much as 50% of the glucose that is converted to energy during glycolysis is converted to lactic acid.

Our muscles produce lactic acid, but they also use lactic acid. Many studies have confirmed that approximately 75% of lactic acid is removed from our bloodstreams through oxidation. That is, it is converted to energy and used to fuel our muscles. The remaining 25% is cleared by conversion to glycogen which is used to produce further energy. So, as you see, lactic acid is far from a “garbage” dead end by- product. It is an important cog in the mechanisms that fuel our running activities.

Lactate Shuttle

We know that energy is produced from glucose by the process of glycolysis. So, how does lactic acid produce energy? An important piece of this puzzle is the lactate shuttle. The “lactate shuttle” was first introduced way back in 1984. The original lactate shuttle hypothesis had the following definition: “the shuttling of lactate through the interstitium and vasculature provides a significant carbon source for oxidation and gluconeogenesis during rest and exercise.” Wow, that’s quite a mouthful. In more understandable terms, this theory said, back in 1984, what we now know to be true, that lactic acid moves freely in and out of muscle cells to provide quick energy and also to produce glycogen.

This theory was not welcomed with open arms. Remember that when this hypothesis was introduced, the accepted theory was that lactic acid was produced only because of a lack of oxygen and that it was a dead end product. It did not make sense to the proponents of the “oxygen debt” theory. Today, thanks to many additional studies, the view is much different.

Current research has shown that the lactate can move freely in and out of muscle cells and into the blood stream. The lactate that leaves a muscle cell may be picked up by a nearby muscle cell or travel through the blood stream to other muscles cells or tissues. The cells picking up the lactate can either convert the lactate back to pyruvic acid, which then enters the Krebs cycle to produce energy or can be converted into glucose or glycogen which enters glycolysis to produce energy.

Lactate Threshold

All runners and coaches talk about lactate threshold. Recall that during low intensity exercise your body is supplying enough oxygen to your muscles to easily clear the pyruvic acid and lactate produced. When you increase your speed to a moderate or hard pace, your energy demands increase. Glycolysis starts to heat up and produce a lot of pyruvate. The pyruvate starts to pile up and as a result more and more lactate is produced. Eventually you reach a speed where your production and usage of lactate is in balance. You are producing a lot of lactate, but are using an equal amount to produce energy. This is considered your lactate threshold (LT). If you were to increase your speed above LT, lactate acid would start to be produced at a faster pace than you can clear it. The accumulation of lactate causes the acidity in your muscle cells to rise. This is the primary cause of fatigue when running at paces above your LT.

There is a very consistent correlation between your LT and running speed. Research has shown that this relationship between LT and pace remains consistent no matter what the level of fitness. Lactate threshold levels are generally about 2.5% below current 10K pace. For most runners, that is around 10 to 15 seconds per mile slower than 10K pace. 5K pace is around 5% faster than LT, which equate to about 20 to 30 seconds per mile slower than 5K pace. Most runners reach LT at around 10 mile race pace.

Lactate Threshold, Training and MCT1

From this relationship, it is obvious that improving LT will improve race performance. So, what is the best way to improve LT? In order to improve your LT, you must train your body to deal with and clear the accumulation of lactate acid. There are a number of systems that must be improved in order to improve LT. Recall that around 75% of the lactic acid is cleared by oxidation. In other words it is used by muscle cells to produce lots of energy. In order for this to work, your muscles cells must have access to a sufficient supply of oxygen and be able to use that oxygen. So, you must improve your body’s ability to deliver oxygen and increase your muscles cells ability to use the oxygen. To improve your cells ability to use the oxygen, you must increase the number of mitochondria in your muscle cells and increase the concentration of aerobic enzymes in the cells. Mitochondria are structures in your muscle cells where all production of energy takes place. The aerobic enzymes are substances that “trigger” the energy production to take place.

In addition to improving the oxygen delivery and the use of that oxygen, you must improve your ability to clear the lactate from your blood and get it into working muscle cells. In the past, it was thought that lactate freely crossed into and out of muscle cells. However, recent research has proven that the majority of lactate is transferred via a carrier protein called MCT1. This carrier protein attached to muscle cells and acts like a “doorway” to shuttle lactate into and out of working cells. So, the more MCT1 you have, the better your cells are at using and clearing lactate.

The accepted training theories of the past said that “cruise” intervals or long, moderate workouts were the best way to improve lactate or anaerobic threshold. But, the fact is, running at moderate paces does not produce the large amounts of lactic acid needed to make improvements. It makes sense that improvements in LT will not be made unless high levels of LT force your body to adapt itself in order to clear the lactate. So, what pace is best? Research shows that workouts that are at or above LT are the best training paces for improving LT.

This means that workouts that range from longer repeats at just below 10K pace to shorter workouts at 5K pace or better are ideal for LT improvements. While these paces are great for improving the ability of your body to deliver and process oxygen and lactate, even faster paces are needed to improve levels of MCT1. Remember that MCT1 is needed to transport lactate into working muscles. While research into MCT1 is in early stages, it appears that MCT1 levels are only increased during very high intensity exercise. This means short repeats at 3K pace or faster. A couple of examples of MCT1 workouts are 3 minute intervals at 3K pace with 2 minute recovery in between or 2 minute repeats at near maximum pace with 4 minute recovery in between.

It appears that training paces faster than LT make the most improvements. But that does not mean that all workouts should be at that pace. You cannot maintain faster than LT paces for long periods of time. So longer repeats at LT pace or slower are necessary in order to train your body to maintain a quality pace for long periods of time.

The scientific knowledge of lactic acid and its role in energy production has increased greatly in the past 10 years and has changed the way coach’s coach and runners train. Training methods that were widely accepted have been challenged and new methods are taking their place. This is a continuing process and as new discoveries are made these changes will continue and new more efficient training programs will continue to be developed. As runners and coaches, we will benefit from these new findings if we keep our minds open to new ideas.