[CoachCorner] Understanding Lactate: A Practical Guide For Endurance Training
Tomorrow I’ll be back to racing.
I haven’t raced in over 5 months (well, 9 months, if I think about the last time I went out for a personal best). I needed this time to come back from a big effort, and to learn more about how my nutrition impacted my health and performance. I then took the time I needed to turn this body and metabolism inside out and become a better long-distance runner. But you know that story already.
This week, during my mini-taper, I took the opportunity to do a little lactate testing, a topic I’d like to cover in broader terms in this blog.
Below, I briefly look at the physiology behind lactate and then explore how we can use lactate testing in a way that's practical and nuanced.
I hope you’ll find it useful.
Lactate 101: What It Is and What It Isn’t
Lactate is a byproduct of glycolysis, the process that breaks down glucose for energy. It’s often misunderstood as simply "waste" or associated only with fatigue, but that’s an outdated view. Lactate is actually a fuel, shuttled between cells and used to our advantage.
However, lactate does accumulate when glycolytic energy production exceeds the body’s ability to clear or reuse it, which typically happens when exercise intensity increases, but not always in a straightforward or linear fashion. The resulting increase in blood lactate levels is something we can measure to better understand our body’s metabolic limits.
Why Measure Lactate?
Lactate gives us a direct window into how our body is working metabolically, which can help us do the following:
Track progress over time:
If our protocol and tools are consistent (see later for recommendations), lactate testing can help us track training progress as our ability to clear lactate at higher intensities should improve, effectively shifting the lactate curve to the right. While a shift to the left likely shows regression, similar values to a previous test do not necessarily indicate a lack of progress, as there is only so much that lactate can capture. For example, our running economy or durability might improve over the years, without marked changes in the lactate curve.
Determine exercise intensity zones:
The data shown above comes from a maximal (VO2max) test. Such a test is not ideal for lactate testing (more comments on this below in the protocol section), and can only be used for relative comparisons between tests, not to set zones, as the stages are too short and there is not enough time for lactate to stabilize. An alternative that I prefer is submaximal testing. In particular, I mostly test intensities between an easy jog and 50 km race-pace, typically stopping after crossing the first lactate threshold, or LT1. This is what I consider the upper limit of Zone 2, or what I call Endurance Runs (see here for more details on how I prescribe training). I like this type of test because it can be done any day without stress and is actionable. To detect LT1, I normally look for the first meaningful rise in lactate above baseline, e.g., 0.2-0.3 mmol/L higher than the first sample. Then, I take the midpoint between the two speeds at which there is such a jump in lactate, as the actual threshold (e.g. ~13 km/h in yesterday’s test, the yellow line below).
Inform pacing:
Similarly to determining exercise intensity zones, we could learn something about the limits of our physiology and adjust pacing accordingly, using lactate testing. For example, for most recreational athletes, long events such as the marathon or ultramarathon likely require an intensity at which lactate does not accumulate in the blood. Well-trained or elite athletes, on the other hand, could further optimize pacing, looking at an increased but stable blood lactate level (above LT1).
That said, lactate is just one tool, not the tool. Like any tool, it’s not without limitations, both technical and interpretive, something I cover below.
Zones and Thresholds
With training zones, we establish ranges of exercise intensities that should result in certain physiological responses when we spend time exercising at those intensities. It is a way to break down continuous measures (like heart rate or lactate) into something possibly simpler to understand or use. This has some advantages, for example, given day-to-day variability and inter-individual variability in general, using zones removes some of the problem/noise.
For myself and the athletes I coach, I use a 5-zone system, where Z3 is the only zone that is actually demarcated by what we consider somewhat measurable thresholds (the first and second lactate thresholds, LT1 and LT2, or the first and second ventilatory thresholds, VT1 and VT2). Thus, we have 5 zones and 2 thresholds.
When I talk about the aerobic threshold, I refer to LT1 (or similarly, VT1), i.e., the top end of zone 2, where we can see the first small rise in lactate during an incremental test (more on this later). Typically, training up to the aerobic threshold is considered easy from a cardiovascular point of view. However, for well-developed athletes, training near the aerobic threshold is quite demanding from a metabolic and mechanical point of view, and therefore often limited.
When I talk about threshold workouts, I refer to LT2 (or similarly, VT2 or critical speed or power, or maximum lactate steady state, or the anaerobic threshold), i.e., the limit between zones 3 and 4. While I used “LT2” a few times here, or the second lactate threshold, I do not use lactate to define this intensity. I define this as the intensity that you can sustain for a 1-hour race (e.g., half-marathon pace for an elite athlete, or 10km pace for a novice athlete), and often call it the second threshold or simply threshold. In my experience, it is often easier and more accurate to capture this second threshold with workout data and mathematical modeling (e.g., critical pace or speed) than with lactate data.
To LT2 or not to LT2
LT2, the so-called "second threshold," requires a few more words. I think here we need to be more careful in how we use this concept. In theory, LT2 represents a point where lactate accumulation becomes exponential and unsustainable. In practice, even in controlled lab conditions, this threshold is difficult to detect reliably and can only be approximated with different mathematical models that hardly ever agree with each other. By definition, we can’t sustain very hard efforts long enough during testing to expose this inflection. While it is easy to spend long periods of time at low intensities, we cannot do the same at these higher intensities in which lactate supposedly behaves differently, and as such, we need to extrapolate and make many assumptions. Personally, I rarely test lactate at high intensities as we can get a very good understanding of an athlete’s ability to perform at “threshold” based on their regular workouts. On the other hand, what happens at lower intensities is more interesting (and stable!) and hard to capture from low-intensity external load data, as it can decouple from internal load (e.g. we think we are running slow, but we are not), hence the utility of sub-maximal testing for LT1, or the “aerobic threshold”.
Protocols for LT1
Personally, when I talk about lactate in endurance training, I’m usually interested in seeing how the lactate curve changes over time (years), and in LT1. For LT1, or the First Lactate Threshold, I consider the first meaningful rise in lactate above baseline. I typically define it as the first consistent increase greater than 0.2-0.4 mmol/L from the baseline or from the first sample. Some athletes might start at 0.7 mmol/L, others at 1.4 mmol/L — and that’s exactly why absolute values should not be used to define LT1 (you can still find papers using 2 mmol/L as LT1, this way of thinking is also outdated and of no practical utility when working with individuals, as you might get people that start near those numbers, as well as people that don’t get there even at intensities that they can barely sustain for half an hour). As it often happens in physiology, the relative change is what matters. LT1 marks the upper limit of low-intensity training and is often a bit higher than the intensity at which fat oxidation is maximized. This is what I consider the top end of Zone 2 (just my choice, not a “universal truth”).
There are two types of tests that I often use to capture LT1:
A submaximal incremental test, typically done in the lab.
A training session done at constant effort.
Submaximal Incremental Test Protocol
Warmup
6-8 minutes for each stage, to allow for lactate levels to stabilize.
VO2max tests often use 2-3 minutes for each stage, as we need to get to max in a relatively short time, and therefore are not useful to determine sustainable training intensities and zones, despite what the lab might be telling/selling you. You can still compare lactate curves between VO2max tests done with the same protocols, but if you’d like to do something more with the data, then it is better to test with long steps. Long steps come with trade-offs, as previously discussed, we can only test lower or moderate intensities, but that’s where there is most practical utility in the data, typically.
Increase speed by 0.5 or 1 km/h each stage (or 25-50 W), depending on your level and the level of resolution you are looking for.
I often increase speed by 1 km/h, which is rather low resolution, but makes for a short test (30 minutes, and I’m done). Once you know where your LT1 is, you could add resolution only around those stages to better capture the first rise in lactate.
Measure lactate shortly after each stage.
Stop when you see a jump in lactate higher than 0.2-0.4 with respect to your baseline or lactate at the first stage.
Constant Effort (Outdoor) Tests
An alternative to the previous protocol that I have used a lot in the past, especially when I had no access to a lab or no treadmill / indoor trainer, was to do constant effort training sessions on different days, slightly increasing the effort on each day. For example, we could go out for a run at a certain pace for about an hour, then test. The day after, we do the same but run a little quicker, and test. This is also an easy and low-stress way to capture your body’s response to a sustained effort at a given intensity, and can be used to determine the point of blood lactate accumulation.
Over the years, I’ve tested lactate in both incremental protocols and real-life runs. The latter — spot checks after holding an effort constant for a while — have proven especially insightful. This kind of test reflects how lactate behaves under actual training conditions and helps answer the most important question: Is this intensity sustainable?
This is not only useful in the context of LT1 but for any slightly higher intensity in which we aim for stability, e.g. because we are hoping to maintain that intensity for a (very) long endurance event.
Determinant vs Limiter of Performance
In the context of the question above, I think lactate is especially useful for ruling things out, as a limiter of performance. If it’s high during an easy run, that’s a red flag; we might be pushing harder than we think, and therefore picking an unsustainable intensity for a long endurance event. But if lactate is low, that doesn’t automatically mean you can sustain that intensity for a given amount of time. While this seems obvious, it wouldn’t be the first time someone told me their overly optimistic marathon time based on a single lactate test. There are many other limiters — economy, durability, muscle resilience, thermoregulation, etc - that need to be considered. Typically, only race specificity in training - hard, long workouts at race pace or race simulations - or racing itself, can answer these questions.
Other Practical Considerations
There are a few additional points that I think are important to consider and discuss when it comes to lactate. In particular, below, I cover the impact of the following when it comes to lactate measurement and interpretation:
Metabolic Flexibility and Nutrition Context
Athlete Level and Training Volume
Heart Rate Zones and Thresholds
Metabolic Flexibility and Nutrition Context
Metabolic flexibility — our ability to use fat or carbohydrate depending on the situation —, as well as our nutrition in broader terms, can change how our lactate profile looks. For example:
Athletes on a low-carb diet might show lower lactate values at submax intensities, not because they’re more aerobically efficient, but because they’re simply producing less lactate due to reduced glycolysis.
High-carb athletes might reach higher lactate levels sooner, especially if they're not well fat-adapted.
With periodized nutrition, these dynamics can change across the week. That’s not a problem — it’s a feature. But it does mean you should interpret lactate values within the context of recent dietary intake and fueling strategies. In this context, chronic dietary changes can override acute choices, as I have seen in my own data (e.g., no change in lactate when on a periodized nutrition regimen, even after carbohydrate intake).
Additionally, lactate max values are typically suppressed during ketogenic or low-carbohydrate diets, and this is an important consideration when interpreting test results. This is something I recently tested to make sure I was still able to oxidize carbohydrates the way I used to, even after switching to periodized nutrition and eating lower amounts of carbs on the days in which training does not require them. By testing lactate after shorter high-intensity sessions, e.g. a VO2max workout, we can determine if we are still metabolically flexible, despite a greater focus on fat oxidation. In my case, lactate was higher than when on a high-carb diet at the end of a VO2max test, which tells me metabolic efficiency is spot on, and periodizing carbohydrates did not lead to any issue from that point of view.
Athlete Level and Training Volume
For athletes training at high volume, we often don’t need lactate to confirm that we’re below LT1. We’re usually running so easy that intensity isn’t an issue — we’re limited by fatigue, soreness, or simply the need to recover. In these cases, lactate is even less of a prescription tool. But when volume is lower, or if you find yourself drifting into moderate intensities too often, lactate can be useful. A simple test — holding a steady pace for 30-40 minutes and taking a single lactate measurement — can reveal whether you’re creeping above LT1. If you are, it might explain why progress stalls despite consistency. In this context, a lactate profile might be better than heart rate to determine a cap for low intensity exercise, as this intensity might be at different percentages of our maximal heart rate, depending on a number of individual factors (training history, fitness, genetics, etc.). However, testing should be done in realistic settings, both for lactate and heart rate, which again, is not always practical.
Using Lactate to Determine Heart Rate Zones and Thresholds
A topic somewhat linked to what I just discussed above. It’s common to use lactate testing to determine heart rate zones. That makes sense — heart rate is easy to measure, non-invasive, and available in real time, while lactate isn’t. But there’s an important limitation to keep in mind: if we use heart rate as a proxy for lactate, we’re assuming the relationship between the two is fixed. And it’s not.
That relationship can and does change over time, especially with consistent endurance training. After months or years of structured work, we might be able to clear lactate more effectively, even at higher intensities or higher percentages of max heart rate - and this is what’s interesting. In practical terms, this means that the lactate response we saw at, say, 150 bpm a year ago might look very different today — lactate might rise later, or not at all, at the same heart rate. But if we haven’t tested lactate again, we’d never know.
This is why I think lactate and heart rate are most useful when they’re used together, not just once, but repeatedly over time. The value isn’t in the single number you get on a given day, but in how that number changes with training. These nuances matter, and we miss them when we treat heart rate as a fixed stand-in for lactate beyond the initial test. Additionally, when testing in the lab, heart rate tends to be impacted more than lactate (e.g, by temperature, stress, etc.) and therefore often does not translate well to heart rate values during our typical training outside of the lab, hence the relationship between the two is better captured using the second protocol I discussed above, i.e. constant effort tests done outdoor, in realistic conditions.
There’s More to Endurance Performance than Lactate
Once again, lactate can be a useful parameter, but as the title of this section says, there’s more to endurance performance than lactate.
For example, durability and running economy can improve over time even without significant changes in the lactate curve, and I think this is an important point to keep in mind when interpreting test results. Running economy—essentially how much oxygen is required to sustain a given pace—can get better through neuromuscular adaptations, improved mechanics, or changes in tendon stiffness, none of which necessarily affect lactate production. At the same time, durability, or the ability to maintain efficiency under fatigue, can also increase, allowing an athlete to hold the same pace for longer without a change in the underlying metabolic markers. This means that even if LT1 or the whole curve remains the same, the athlete may be able to perform far better simply because they can sustain that level of effort longer, with less cost. These adaptations don’t always show up in the lactate data itself, but they’re meaningful in training and performance. It’s a reminder that not all progress is reflected in shifts in the curve—sometimes, it’s about what we’re able to do with the physiology we already have. As I mentioned earlier, typically, only race specificity in training - hard, long workouts at race pace or race simulations - or racing itself, can answer these questions.
Final Thoughts
Lactate testing can be a useful tool to individualize training, especially when it comes to identifying LT1 and monitoring changes over time. But like everything else in physiology, it should be used with context and nuance.
Looking at the point in which lactate production is higher than its removal seems to be one of the most meaningful things we can do with this data (as opposed to looking at absolute values, or fitting data to curves hoping to find a ‘second threshold’). My recommendation is to focus on trends and relative changes, measure in realistic conditions, and interpret your data within your training and nutrition context.
Personally, once in a while, I test simply because I’m curious or interested in seeing what’s changing (or not changing) with training (or detraining!), as it’s also part of my work, but I have hardly ever used the data to make large adjustments. This is mostly due to having trained for many years, training high volume (and therefore far below LT1 on most non-workout runs), knowing at this point how certain intensities should feel, and having done enough races and race simulations to understand what to expect or what I can do for a given distance. Yet, the occasional test can nudge me in the right direction, showing me that there is some extra room for pushing my limits at a certain time, or that I should be more conservative on another occasion.
Twenty-four more hours and we'll put this to the test at the 50 km di Romagna. Excited to race.
I hope this blog gave you some useful pointers, and thank you for reading!
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Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching. He is a certified ultrarunning coach.
Marco has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, Endurance Coach at Destination Unknown, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine.
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Hi Marco, great post. Considering the "accuracy" of portable lactate devices, do you take 2 readings at each step or just go with one? I own a "The Edge", but something like a "Super Compact GL" would likely be way more accurate, although in no way suitable for outdoor testing. I was wondering if the accuracy of the portable devices is enough to meaningfully look for 0,2 to 0,4 jumps at low lactate levels (1.5- 2 mmol/L)...
Thanks Marco for sharing your thoughts and insights. I’ve been proving lactate after long steady efforts just to see if it’s low or high, just to confirm if it can be a sustainable pace for long. That way I know which pace I should train when training specific training. I see lactate as a good tool for this.