Last Updated on July 23, 2022 by Heather Hart, ACSM EP
If you’ve been around the running world long enough, you’ve more than likely heard the terms “lactate”, “lactic acid”, or “lactate threshold” thrown around.
And while many of you might be familiar with the effort or associated pace of your lactate threshold, you might not understand exactly what it means on a physiological level.
What is lactate? Is it the same thing as lactic acid? Should you care about your lactate threshold?
Well, wonder no more. In this three part series, I’m going to cover all-things-lactate, including what it is and what it does, all-too-common “lactic acid misconceptions” still floating around the running community, and what you, as a runner, should be doing as far as training to improve you lactate threshold.
It is my hope that at the end of this series, you have a much better understanding of the role of lactate…and never use the term “lactic acid” in reference to running ever again.
What is Lactate?
Lactate is a byproduct of anaerobic glycolysis, a normal part of energy production within your cells. It serves as an energy source, an important buffer to help maintain the normal pH of cells, and as a cell signaling molecule for various bodily processes.
It is not, as so many once thought, an evil, caustic, waste product responsible for sore legs (more on this soon).
How Does Lactate Form?
Our bodies need energy to move, and you know, run. This energy comes from the food we eat. However, our cells can’t simply take that GU Stroopwafel you eat mid race and directly use it for energy.
Oh no, our bodies are far more
high maintenance complex than that.
We have a number of different energy systems that our bodies use to take that Stroopwafel (or pizza, or whatever you prefer to fuel with) and create a form of energy the cells can actually use, called adenosine triphosphate (ATP).
One of those systems, glycolysis, is the process of partially breaking down carbohydrates (either as glucose or glycogen) into molecule called pyruvate. This process happens in the cytoplasm of a cell, doesn’t require oxygen to occur.
(Note: since we’re talking about running, in this entire post, I’m referring to cellular respiration that occurs in skeletal muscle cells.)
One molecule of glucose is broken down into 2 molecules of pyruvate, 2* molecules of ATP for your body to use as energy, 2 molecules of water, and 2 molecules of NADH (nicotinamide adenine dinucleotide [NAD] + hydrogen [H], a coenzyme important for metabolism) at the end of glycolysis.
Bear with me on this chemistry stuff…I promise to keep it as simple as possible.
(*Technically, one molecule of glucose produces 4 molecules of ATP during glycolysis, but requires 2ATP for the entire reaction to occur in the first place. So we’re net 2 ATP energy molecules)
At the end of glycolysis, pyruvate has two directions it may continue in:
Aerobic Glycolysis (Oxidative Phosphorylation)
When you’re running at a slow to moderate effort, where you’re breathing at a normal rate, and likely utilizing lots of those awesome slow-twitch-type-I muscle fibers (woo, endurance!), pyruvate can continue on through oxidative phosphorylation, commonly referred to as aerobic glycolysis. This involves the Krebs Cycle (also called the Citric Acid Cycle), and the electron transport chain (ETC), occurs in the mitochondria of the cell, and it yields a whole bunch of ATP.
Aerobic glycolysis is also called “slow glycolysis”, because it takes a while, relatively speaking, to create all of that awesome ATP. But more important to note, is that aerobic glycolysis requires oxygen for the process to continue.
Now, let’s imagine instead of a casual, “conversational” running pace, you’re going to go for a one mile time trial, running as hard as you can (I can feel my ultrarunners cringing at the thought). Because you’re not only moving faster, but recruiting more of those fast-twitch-type-II muscle fibers to run that hard, your body is going to need more energy at a faster rate in order to sustain that intensity.
So your body keeps pushing glucose through the first part of glycolysis, creating pyruvate at a fast rate. But at these harder intensities, there will come a point where you simply can’t utilize oxygen fast enough to push all of that pyruvate on through the Krebs cycle and ETC.
When this happens, your body shifts to relying on anaerobic glycolysis as the main energy system to produce energy. This process creates energy relatively quickly (which is why it’s often referred to as “fast glycolysis”), so it can temporarily help keep up with the energy demands of running harder.
(Unfortunately, it doesn’t produce as much energy as aerobic glycolysis does, which is one of the reasons why we can’t just run our half mile pace for an entire marathon.)
What happens in anaerobic glycolysis is that that instead of shuttling pyruvate into the mitochondria to continue on with the Krebs cycle & the ETC, pyruvate is instead turned into lactate, via the lactate dehydrogenase reaction.
And that’s a very simple explanation (I hope?) of how we produce lactate.
What’s the Purpose of Lactate?
So you may be thinking to yourself…but why lactate? What’s the point? I’m glad you asked. You see, lactate was once thought to be a waste product of metabolism. It’s now understood that lactate has a number of benefits, and is actually an integral part of ensuring that energy production can continue.
Lactate is a Buffer
Remember how that first part of glycolysis gave us 2 molecules of pyruvate, 2 molecules of ATP 2 molecules of water, and 2 molecules of NADH?
Well, I mentioned that we can’t have pyruvate backing up, unable to be shuttled into the mitochondria because of a lack of oxygen. The formation of lactate helps buffer the pyruvate (once the lactate is cleared from the muscle tissue…more not act soon).
But we ALSO can’t have that NADH accumulating either. An accumulation of hydrogen ions in your cells, tissues, or blood during exercise will cause a drop in pH…which can ultimately lead to a slew of unwanted side effects.
This drop in pH is known as metabolic acidosis, and is responsible for:
- Fatigue, burning, or soreness felt in muscles during high intensity exercise
- The inhibition of ATP production
- Interference of muscle contractions on a cellular level
Let’s get even more specific:
- The normal resting pH of muscle is 7.1
- An intracellular pH of less than 6.9 inhibits the action of an enzyme called phosphofructokinase, which slows the rate of glycolysis and ATP production.
- At a pH of 6.4, glyogen breakdown stops, causing a rapid decrease in ATP.
- Further, a drop in pH directly interferes with muscle contractions (possibly by inhibiting calcium binding or interfering with cross bridge recycling)
The lactate dehydrogenase reaction, where pyruvate is turned into lactate, consumes the hydrogen ions, therefore buffering the tissue and helping to remove the hydrogen ions from accumulating from the process of glycolysis.
Therefore, lactate actually HELPS maintain a normal pH in the body, and helps keep you running.
Lactic Acid vs. Lactate:
Welcome to one of the great debates of bioenergetics: is it lactic acid or lactate? Well, first of all it’s important to know that structurally, lactic acid and lactate are NOT the same chemical.
Now, it’s pretty well established that lactic acid cannot exist in the human body due to the very small window of pH in which the body operates. Therefore, the use of the term “lactic acid” is outdated.
Many researchers believed that lactic acid was the product of glycolysis, but because it was a weak acid, lactate immediately dissociated (dropped like a hot potato) the hydrogen ion. And that “dropped” ion is what led to acidosis of the tissues (and thus, burning, tired legs, etc.)
However, newer research suggests that the chemical reactions of glycolysis (specifically, the phosphoglycerate kinase reaction) leave no hydrogen ion existing to dissociate from lactate in the first place. In this case, it’s believed that all accumulation of hydrogen ions in exercise comes from the process of breaking down and using ATP (ATP hydrolysis) outside of glycolysis.
Either way, the consensus by both sides of the argument is that lactic acid does not exist in human skeletal muscle.
Lactate as a Fuel Source
Fun fact: lactate can – and readily is – used as a fuel source for your cells, in multiple ways!
- Intracellular: once you slow down your running or exercise to the point that more oxygen becomes available, lactate can be converted back to pyruvate, and then enter the Krebs cycle, and be converted into ATP.
- Extracellular: lactate can be shuttled through the blood to other parts of your body (besides skeletal muscle) that can use oxidize lactate into a fuel source. Your brain, heart, kidneys, and even adipose tissue all readily use lactate.
- Cori Cycle: lactate can be shuttled into the liver, where it is converted to glucose through a process called gluconeogenesis. That glucose can either be released back into the blood to be used by skeletal muscle (or whoever needs it), OR, it can actually be converted into glycogen and stored as energy to use later.
See? Lactate is a friend, not an enemy.
What is Lactate Clearance?
Lactate clearance is exactly what it sounds like: the process of your body “clearing” lactate out of the blood and muscle cells. While we now know that lactate is a great, useful molecule, we can’t have it just accumulating in the cells.
Lactate is cleared by the number of methods we mentioned above, including recycling the lactate back to pyruvate and oxidizing it into energy within the cell, shuttling the lactate to the liver to undergo the Cori cycle, or shuttling the lactate to one of many organs that will use it as fuel.
It’s important to know that these clearance processes are oxidative: meaning, they too need oxygen to occur.
Factors Affecting Lactate Clearance:
There are a number of factors that can affect how well your body clears lactate, including:
- mitochondria density
- muscle fiber type
- muscle fiber recruitment
- blood plasma concentration
- hormonal response
- training status
These factors can be greatly influenced by training status / fitness level. Research shows that both endurance and anaerobically trained athletes have faster lactate clearance rates than untrained people.
What Does Lactate Threshold Mean?
The normal amount of lactate present in the blood during rest is around 0.5 to 2.2 mmol/L (“millimoles per liter”).
Yes, our bodies are always producing, and clearing, lactate, even at rest.
As exercise intensity increases, our energy demands increase. And as our bodies work hard at creating more energy, we begin to rely more on anaerobic glycolysis, and less on aerobic glycolysis. This results in the production of more lactate.
The lactate threshold is a specific point in that increasing intensity where the blood lactate begins an abrupt and exponential increase above the baseline concentration, and correlates to the point when aerobic glycolysis cannot keep up with the demands of exercise.
The lactate threshold corresponds pretty closely to the ventilatory threshold, and is often used as a marker of the anaerobic threshold.
In untrained individuals, the lactate threshold occurs around 50-60% of your VO2 max (maximal oxygen uptake). However, in aerobically trained athletes (like runners), the lactate threshold typically occurs around 70-80% of VO2max
Onset of Blood Lactate Accumulation (OBLA)
At even higher intensities of exercise beyond your lactate threshold, a second inflection point in the rise of blood lactate level occurs.
This is called the onset of blood lactate accumulation (OBLA), and it signals the point where the rate of lactate production exceeds the rate of lactate clearance.
For most athletes, the OBLA typically occurs around the point when lactate volume in the blood reaches 4mmol/L.
Why Lactate Threshold / OBLA Matters to Runners:
The abrupt accumulation of lactate signals that lactate production is suddenly greater than lactate clearance. When you aren’t able to clear lactate – either by recycling it for energy within the same cell, or shuttling it elsewhere to be used – the lactate begins to accumulate.
And when the lactate accumulates, so do the hydrogen ions, which leads to the decreased production of ATP and general “this really sucks” feelings of metabolic acidosis. Eventually, you simply do not have the energy resources to sustain that intensity of running or exercise.
So your body forces you to slow down (or completely stop). When that happens, your energy demands drop, so you are able to use oxygen to clear up that accumulating lactate (and thus hydrogen ions), and bring your body back to a happy homeostasis.
You cannot run at a pace that exceeds your lactate threshold for very long. Therefore, the higher your lactate threshold, the longer you can run at faster or harder efforts.
How Training Affects Lactate Production & Clearance:
Good news, everyone! Training at both high and low intensities can help both increase your lactate threshold, and improve you ability to clear lactate.
Remember: we are ALWAYS creating lactate, even at rest. This is because we are never using only aerobic or only anaerobic means of energy production for our cells. So the goal of training isn’t simply to create less lactate, it’s also to be able to clear lactate faster.
This can be done with both high AND lower intensity training. That’s right, even your long, slow runs help improve your lactate clearing capacity. This is because lactate is mainly cleared by slow twitch muscle fibers, which have a higher mitochondrial density than fast twitch muscle fibers. And training those slow twitch fibers increases the number of mitochondria even more.
Further, lower intensity exercise can help improve the amount of MCT-1 (monocarboxylate) transporters, which help shuttle lactate to other parts of the body, and mLDH (lactate dehydrogenase) enzymes, which help catalyze lactate back to pyruvate again.
Training also improves things like hormonal response to exercise, which can affect everything from enzyme regulation in ATP production, to your body’s ability to repair and remodel skeletal muscle.
Now that you (hopefully) have a better understanding of what lactate is, and what it does, head over to the next post in this series:
Clearing 9 Lactate & Lactic Acid Myths in Running
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