The Lactate Shuttle Theory

Lactate Shuttling Training Cycling

In the late 1990s exercise physiologist Dr. George Brooks, from the University of California, Berkeley, made a discovery that would change our perception of endurance performance forever. 

As a dedicated researcher, Brooks devoted his whole life to a small substrate with a big impact on the body: Lactate. 

For more than 40 years George Brooks studied lactate comprehensively. Most of what we know about lactate today rests on his shoulders. His work is the result of deliberate practice – that is if we pay close attention we can solve problems and move on to better ones. 

Brooks asked the right questions and as a result, he came up with what we call today “The Lactate Shuttle Theory.” To understand his fundamental findings, however, we should first understand Brooks most admired substance a bit better.

What is Lactate?

Lactate is a substrate that is a byproduct of glucose consumption by muscle cells. This is independent of oxygen availability. You always produce certain amounts of lactate. 

During high intensity, however, very high amounts of lactate are produced. This is due to the highly glycolytic (high glucose usage) fast twitch Type-II muscle fibers that are fully recruited at high intensity. Because lots of energy needs to be produced in the form of ATP, lots of glucose needs to be broken down. As a result, lactate builds up. 

But lactate is not a waste product. Quite the opposite: Lactate is the best “glucose re-generator” as about 30 percent of exercise-used glucose comes from lactate “recycling” (back to glucose). From a process called gluconeogenesis.

In reality, the burning in our legs doesn’t come from lactate. It’s merely hydrogen ions (H+). Associated with lactate comes the release of hydrogen ions (H+) that cause a drop in muscle pH, resulting in acidosis. Or to put it straight: That hurts like hell!

What we know today is that the more highly trained a cyclist is or the higher the level of competition is, the lower the blood lactate concentration is and the higher the power output and performance. 

This is an important observation and Dr. San Milan, an exercise physiologist at the University of Colorado, ascribes it to a certain ability: “Lower blood lactate levels observed in top athletes is due to enhanced lactate clearance capacity.” 

It’s exactly what Brooks discovered when he came up with The Lactate Shuttle Theory. 


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The Lactate Shuttle Theory

The Lactate Shuttle Theory describes the body’s function to carry lactate from the working muscles, where lactate levels rise, through the blood to better-oxygenated domains where lactate can be used as fuel. 

One of these places is your liver or the kidneys. Then there’s your heart. Then there’s your your brain. Believe it or not, your brain is lactate addicted. Your brain’s neurons need it. Lactate is essential for long-term memory. Maybe a reason why we remember all these painful workouts so well…

As we can see lactate can be supplied to the blood, where it can be carried to almost every organ to be utilized. But there’s a problem in the equation. This process takes a lot of time. Time we don’t have in a race when the next surge hits. 

The reason well-trained athletes have such low blood lactate levels is their efficiency in clearing it right in the producing muscle itself. This is a great advantage, but a complex mechanism too. Because during bike riding, lactate is mainly produced in the fast-twitch muscle fibers, whereas it’s mainly cleared in the aerobic slow-twitch muscle fibers. This process involves different lactate transporters and enzymes. 

In fast-twitch fibers, we find a high amount of MCT-4 transporters (Monocarboxylate-4) which transport lactate out of these fibers. In slow twitch fibers, we find transporters called MCT-1 which take lactate into these fibers. Then, in the slow twitch fiber lactate is converted to pyruvate in the mitochondria, by an enzyme called mLDH (mitochondrial lactate dehydrogenase) to be finally synthesized to ATP for energy production. 

Our training goal seems obvious: We need to increase the number of these transporters, enzymes, and mitochondria to improve our lactate clearance capacity and with that improve our overall cycling performance by a lot. But how do we achieve that? Let’s talk about that now.

The Top 4 Training Sessions to Make Lactate Your Best Friend

1. Zone 2 Endurance Training

Probably the most boring answer you could receive on how to increase your lactate clearance capacity, but a crucial one: Ride easy for a long period and do it again. As mentioned earlier the purpose of Zone 2 endurance training is to increase mitochondrial density and the number of MCT-1 and mLDH. 

The key here is not just knowing about it, but doing it. Zone 2 endurance training should make up the bulk of your training. You need to find comfort in boredom to master the fundamentals. 

Aim for 2-6 hours in zone 2 and about 50-65% of FTP or your lab-tested endurance zone. If you participate in ultra-endurance events you can ride up to 7 or 8 hours. Zone 2 has proved itself as the most important training zone and as San Milan put it, “With the experience of the past 18 years Zone 2 endurance training has shown to be the training zone eliciting the best results to improve lactate clearance capacity.” This is an invitation to do more of what already works. 

2. Sweet Spot Training

Even sweet spot training shows beneficial adaptations in lactate clearance capacity. The idea of it is to fatigue a certain type of fast twitch fibers so they become more aerobically efficient. Therefore, mitochondrial density increases and we can see increases in MCT-4, MCT-1, and mLDH. Fat oxidation improves as well and with enhanced capillarization in muscles a better oxygen supply is possible, further improving lactate clearance. 

Sweet spot training is done at 88-94% of FTP or below your lab-tested anaerobic threshold. A good starting point is 3×10 minutes of sweet spot. Read more about how to master sweet spot training and get all the adaptations.

3. Over/Under Intervals

Back in 2010, Tim Kerrison took over the role of head of athlete performance at Team Sky (today Ineos). He was part of the overall team transformation, the master of marginal gains, Sir Dave Brailsford caused with the clear goal of getting the first British Tour de France winner. Read more about how Sir Dave Brailsford improved every domain with marginal gains.

One thing Kerrison and the Sky squad remain famous for is their over/under interval training. This is done by spending a significant amount of time above the anaerobic threshold (AT) and then dropping below the anaerobic threshold to recover. And following that rhythm till the end of the set. AT is more commonly referred to as FTP. Even though they’re not the same thing it’s a simple way to build your training zones around. 

The purpose of over/under intervals, recently labeled as lactate clearance or lactate shuttle training, is to flood the system with lactate and then clear lactate below “FTP.” The reason for this is that below FTP your body can clear more lactate than is produced and use it as fuel. 

This type of training is all about teaching your body to clear lactate efficiently and regain glucose for fuel, while under sustained stress. It’s nothing new just wrapped up sexy and exciting today as lactate shuttle or lactate clearance training. A typical workout is 3×12 minutes over/under intervals with a 2 by 2-minute pace change. Do a warmup and an interval at the threshold for 2-3 minutes to flood your body with lactate. After a short break, perform the interval sets changing between 2min at 105-110% of FTP and 2min at 80-85% of FTP.

Another version of a good lactate shuttle training is a 1:4 ratio of over/under. This time with a higher over portion. For example, 3×15 minutes with 1 minute at 120-125% of FTP and 4 minutes at 70% of FTP during the under portion. If you have lab-tested zones use your fatmax zone or LT1 for the under part. 

These workouts not only trigger lactate clearance capacity but also keep cycling training specific. Racing is often very pace-changing. Surges, steep climbs, crosswinds, cobbles, and narrow roads all shape a dynamic structure. With over-under, you’re ready for what’s to come.

4. Steady State Lactate Threshold Intervals

It’s important to note that “lactate threshold” intervals don’t directly improve lactate clearance. Clearance is done by slow twitch fibers and threshold intervals use almost fast twitch glycolytic fibers. That’s why we need the lower intensity pace changes we have at over-under. 

However, intervals done at FTP remain very effective in increasing MCT-4 transporters to get lactate out of the muscle fiber and increase your lactate tolerance (“our turbo”). You also want to be able to produce power at FTP in a race and therefore you need practice. A 3x15min or 2x20min at 98-103% of FTP makes for an excellent workout.

Noteworthy, even intermittent VO2max intervals can improve lactate clearance. For example, Billat 30/30 intervals (30 seconds on, 30 seconds off). This is especially true for trained athletes. Because the 30 seconds off part will be too long to keep oxygen uptake high enough and therefore work better as a lactate clearance workout.


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Conclusion 

The Lactate Shuttle Theory in general is nothing new. Team Sky and its golden boys like Wiggins or Froome used over-unders successfully to create Tour de France victories. 

But there’s a lot more we can derive from it. We know now that lactate is a key fuel for our body and the more we can use the better we will ride. To do so we should focus on improving our lactate clearance capacity. And as we’ve seen we need to do more of what already works. 

For example, we should ride the majority of our training as easy zone 2 endurance. The lactate shuttle remains an “old” fundamental skill wrapped up in something new and exciting. But to make lactate our best friend we need to master the fundamentals. There are no shortcuts, so we better get to work.

Ready to Improve Your Lactate Clearance?

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Footnotes

What is Lactate and Lactate Threshold – by San Millan: https://www.trainingpeaks.com/blog/what-is-lactate-and-lactate-threshold/

Pump your lactate shuttle: make lactate your friend, not foe! https://www.sportsperformancebulletin.com/endurance-training/high-intensity-training/pump-lactate-shuttle-make-lactate-friend-not-foe/

The Science and Translation of Lactate Shuttle Theory: https://pubmed.ncbi.nlm.nih.gov/29617642/

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