Lactic acid for runners: The truth about lactate
- Training
- Racing
- Fatigue
Thomas Solomon, PhD.
This page explains what lactate and lactic acid really are in running, how they relate to fatigue, and why the old poison-and-waste story does not hold up.
The content is for runners, athletes, and coaches who want to understand the science behind lactate, clear up common myths, and use that knowledge to improve training and performance.
Key takeaway: Lactate is not just a waste product. The body produces it even under aerobic conditions (when oxygen is available), and the body can use it as a fuel and shuttle it between tissues. Basically, lactate is part of normal exercise metabolism.
Key takeaway: The burning feeling during hard running and the slowing down that follows are not explained by lactate alone. The fatigue is bigger than that, and the metabolic strain, the acidity, and the other exercise-related changes all matter.
Key takeaway: Next-day (delayed-onset) muscle soreness is not well explained by lactate, and the training goal is not to avoid lactate. The better goal is to improve the aerobic system, the sustainable pace, and the ability to keep running well before fatigue starts pushing back.
But don't stop there! Scroll down to fully educate yourself on the details, nuances, and nerdy bits.
Listen to the NotebookLM discussion:
If you listen to enough runners, coaches, and commentators — such as those I’ve been enduring with Eurosport's coverage of the Winter Olympics, World Cup cross-country skiing, and Milan-San Remo — you’ll hear the same story again and again: when the pace gets hard, your legs fill up with lactic acid, the lactic acid poisons the muscles, and you need to flush it out.
It is a great story. Tidy. Dramatic. Convincing. Yet — in the way that many exercise myths are tidy, dramatic, and convincing — not really true.
The modern view is much more interesting. Lactate is not just some nasty waste product that appears when oxygen disappears (anaerobic) and your quadriceps start writing their will. Lactate is produced continuously, even under fully oxygen-abundant (aerobic) conditions, and it acts as a fuel, a shuttle, and a signalling molecule in the body (Brooks et al. 2018; Brooks et al. 2020). In other words, the thing runners have spent decades blaming for everything short of bad weather is actually part of how the body powers and regulates exercise (Brooks et al. 2018; Brooks et al. 2020).
What is this wizardry?!
This is really important because, if you misunderstand lactate, you end up misunderstanding fatigue, training, pacing, and recovery too. If you want the bigger picture first, have a look at my article on what fatigue actually is. Fatigue is not one thing. It is a crowd.
What is lactic acid?
The word “acid” doesn’t exactly bring butterflies, ice cream, and rainbows to mind. Acid sounds like the kind of shiz that destroys candy munchers’ teeth or turns you into Two-Face from Batman. Lactic acid, therefore, sounds evil.
But lactic acid gets a bad rap simply because of its name. Poor bastard. And, since your brain has been filled with journalists’ and commentators’ versions of lactic acid, you are probably unaware of just how awesome lactic acid is. In fact, you probably don’t even produce it! (see Robergs et al. 2018)
So, in exercise conversations, when people say lactic acid, what they mean is lactate. That sounds like nerdy nit-picking, but it matters. George Brooks’ reviews make the point clearly: in living human systems, lactate has traditionally been understood as a dead-end waste chemical. But this is wrong. It is the product of glycolysisThe process the body uses to break down (“lysis”) glucose (“glyco”) for energy. It happens in the cells and can quickly help power exercise. Glycolysis occurs in every cell of the body. and a substrate for downstream metabolism, which means it connects the flow between breaking fuel down and using it to make energy (Brooks et al. 2018; Brooks et al. 2020).
The old idea painted lactate as metabolic rubbish. The newer idea treats it more like a courier, and George Brooks argues that lactate fulfils at least three major roles in the body: it is a major energy source, it is the main precursor for gluconeogenesis, and it acts as a signalling molecule (Brooks et al. 2018; Brooks et al. 2020). That’s not the profile of a useless poison. That’s the profile of an important molecule the body is using on purpose.
Is lactate only produced when oxygen runs out?
No. This is one of the big old myths, and it hangs on like the smell of your shorts after a long run.
Lactate is formed continuously under aerobic conditions, not just during oxygen shortage (Brooks et al. 2018; Brooks et al. 2020). In fact, there’s strong evidence that glucose and glycogen breakdown produce lactate under fully aerobic conditions (Brooks et al. 2018; Brooks et al. 2020). That means lactate is not a reliable sign that your muscles have suddenly become airless anaerobic dungeons.
This matters for runners because the old phrase “going anaerobic” has encouraged people to imagine a kind of switch being flicked — one moment oxygen, the next moment “I’m fecked” — when the reality is more of a moving balance between how fast lactate is produced, how fast it is used, and how hard the exercise is. The body is not flipping from oxygen use to chaos. It is adjusting, constantly, in real time (Brooks et al. 2018).
Does lactate cause the burning feeling when you run hard?
This is where the story gets messy… During hard exercise, lactate rises, but the muscles accumulate all sorts of metabolites that are a consequence of hard work: hydrogen ions, H+; inorganic phosphate, Pi; adenosine diphosphate, ADP; and potassium, K+ (read more on that at veohtu.com/peripheralfatigue).
For a long time, folks bundled that whole mess together, called it lactic acid, and blamed it for fatigue. However, while lactate and hydrogen ion accumulation happen during exercise, it remains unclear whether lactate itself is a cause of muscle fatigue in the body. That said, one thing is certain: lactate is not the only cause (Brooks et al. 2018).
In fact, increases in lactate itself probably have little direct detrimental effect on performance, while severe intracellular acidosis (a drop in pH caused by the rise in H+ concentrations) can contribute to force and power fatigue during intense exercise, particularly in fast-twitch fibres (Cairns and Lindinger 2025; Cairns and Lindinger 2025).
So, the simple version is this: the burn is real, but the old line that “lactic acid” is the single villain behind it is basically wrong. That is one reason I wrote Do your muscles slow you down?. Yes, the muscle side of fatigue is very real, but it’s just not as video-game-like as “acid build-up equals game over”.
Does lactate cause soreness after a run?
No. Not in the way runners usually mean.
The classic claim is that a hard run “fills the legs with lactic acid” and that this is why you’re sore for the next day or two, when walking downstairs becomes a bit emotional. But lactate does not fit the timing. Blood and muscle lactate concentrations rise during hard exercise and are then cleared relatively quickly. Delayed soreness arrives later, peaks later, and is better explained by muscle damage and the downstream inflammatory response than by lactate being “trapped” in muscle fibres (Cairns and Lindinger 2025; Cairns and Lindinger 2025).
So, if your legs are wrecked two days after a downhill trail race or road marathon, you can stop blaming lactate.
But why lactate is not the enemy
George Brooks’ lactate shuttle theory argues that lactate moves between “producer” and “consumer” cells, tissues, and organs, serving useful purposes along the way (Brooks et al. 2018). Brooks calls lactate “a fulcrum of metabolism”, which is quite a phrase, but the point is simple enough: lactate is central, not incidental (Brooks et al. 2020).
Lactate is not the enemy for three main reasons:
Lactate is a fuel
Lactate is a fuelLactate is a major energy source for mitochondrial respiration and is used continuously in diverse tissues (Brooks et al. 2018; Brooks et al. 2020). That means the body is not merely producing lactate and regretting it. It is producing lactate and then using it.
Lactate is a shuttle
Lactate is a shuttleAccording to the lactate shuttle model, lactate can move from tissues producing a lot of it to tissues that can oxidise it (burn it for energy) or convert it to glucose via gluconeogenesisThe process the body uses to make (“genesis”) new (“neo”) glucose (“gluco”) from other metabolites, including lactate. It helps maintain blood glucose levels, especially during fasting or long duration / high-intensity exercise. Most gluconeogenesis happens in the liver. (Brooks et al. 2018). This is not waste disposal. This is fuel distribution.
Lactate is a signalling molecule
Lactate is a signalling moleculeLactate can have autocrineAutocrine signalling happens when a cell releases a chemical signal that acts back on the same cell., paracrineParacrine signalling happens when a cell releases a chemical signal that acts on nearby cells., endocrineEndocrine signalling happens when a cell or gland releases hormones into the blood to act on cells in other parts of the body.-like effects, influencing metabolic regulation and adaptation, including cell redox and signalling pathways (Brooks et al. 2020). In other words, like a hormone, lactate helps cells, tissues, and organs communicate with themselves and with other cells, tissues, and organs. Brooks even uses the term “lactormone” to refer to lactate’s ability to function like a hormone (Brooks et al. 2018; Brooks et al. 2020).
So, lactate is not just part of the cost of exercise, it is part of the “conversation” that exercise has with the body.
That’s pretty darn cool.
So why do runners still slow down when lactate rises?
In short: Because fatigue is bigger than lactate.
That does not mean lactate is irrelevant. Rising lactate during hard exercise tells you something important: the metabolic strain is increasing, the balance between production and use is shifting, and the cost of holding that pace is rising. But it does not follow that lactate alone is the thing making you slow down. Even if rising lactic acid and decreasing pH are fatigue agents, they are certainly not the only causes of fatigue (Brooks et al. 2018).
Exercise fatigue happens alongside decreases in glycogen and blood glucose and rises in core temperature, perception of effort, phosphate metabolites, hydrogen ions, and reactive oxygen species, not just acid-base changes (Cairns and Lindinger 2025; Cairns and Lindinger 2025). These simultaneous processes are exactly why I wrote What causes fatigue during exercise?. The answer is not one evil thing but a well-tuned concert of many things jostling for influence — the muscles, fuel availability, heat, perception, pacing, expectations, probably the weather, maybe the several beers you quaffed last night. And, you might have guessed, much of this also overlaps with what’s going on in the brain. Which is exactly why I wrote Does your brain slow you down?.
The bottom line: the body is not a simple machine with one red warning light.
What is the lactate threshold for runners?
Once you stop treating lactate as a doer of evil and the cause of fatigue, lactate threshold becomes easier to understand.
In plain English, “lactate threshold” is a way of describing the exercise intensity where lactate starts to accumulate faster than the body can clear or use it. Different folks define it slightly differently, and I’m not gonna pretend the terminology is always tidy: LT1Lactate threshold is the exercise intensity where blood lactate starts to gradually rise faster than your body can clear it, but you can still keep going for a while. It marks the shift from “this is hard but manageable” to “this is going to catch up with me soon” if you hold it., LT2Lactate turnpoint (LT2) is a higher intensity than lactate threshold (LT1) where blood lactate shoots up rapidly, effort begins to feel hard, and fatigue comes on quickly. It’s the “red zone” where you can only last a short time before you have to slow down, and signals a shift toward more glycolytic (glucose-using) and anaerobic energy use., etc. (I’ll dig into that another time). But, for runners, the practical meaning is straightforward: lactate threshold is tied to the fastest effort you can sustain before things start getting metabolically “expensive”.
The Brooks framework helps here because it turns “threshold” into a story about balance rather than poison. The issue is not that lactate suddenly appears and ruins your day. The issue is that as the effort rises, the production, transport, and disposal of lactate shift in a way that marks a limit on sustainable performance (Brooks et al. 2018; Brooks et al. 2020). For a marathon runner, that matters because the marathon is brutally sensitive to how fast you can run without turning the cost of the pace into a debt spiral. For a trail runner, it matters on climbs, surges, and technical terrain where the effort keeps jumping around. For an ultrarunner, it matters a little differently — not because you are sitting on your threshold all day, but because raising that ceiling makes submaximal running cheaper, calmer, and more sustainable.
That is not magic. It is just good economics for your legs: running economyThe rate of energy expenditure (measured in kiloJoules [KJ], kilocalories [kcal] or oxygen consumption [V̇O2]) per kilogram body mass (kg) per unit of distance, i.e. per 1 kilometre travelled. A runner with a lower energy cost per kilometre has a higher economy than a runner with a higher energy cost. 101.
Can you train your body to use lactate better?
Yes, you darn well can; it’s the outcome of all your training.
If lactate is a normal fuel and shuttle rather than a toxic mistake, then training can improve how well the body deals with it. Lactate is continuously produced and continuously used, and repeated lactate exposure from regular exercise has effects on metabolic regulation and adaptation (Brooks et al. 2018; Brooks et al. 2020). In less jargon-filled blah blah blah, that means the goal is not to teach the body to avoid lactate, but to improve the whole system so you can produce energy, shuttle metabolites, tolerate hard effort, and keep moving at a faster pace before fatigue starts leaning on the brakes. Typically we rely on glucose and fatty acids to fuel our exploits — see How much fuel is available in your body to produce energy during exercise? — but a highly trained runner can also use lactate as an economical fuel to delay the wheels falling off.
Nice.
How runners should actually train instead of fearing lactic acid
Most runners do not need more fear. They need better training logic.
The old “lactic acid” myth encourages people to think the mission is to avoid the burn, flush the legs, or somehow outsmart physiology. But the better training goal is to improve the qualities that actually support performance: aerobic capacity, sustainable speed, running economy, and the ability to handle the stress of hard running without immediately unravelling. The lactate literature supports that logic because lactate is a part of normal exercise metabolism, not evidence that the workout has gone off the rails (Brooks et al. 2018; Brooks et al. 2020).
So, yes, threshold work matters. Faster aerobic sessions matter. Easy volume matters. Recovery matters. The point is not to produce a sensation dramatic enough to make you boast about it later on social media. The point is to build a body that can run faster for longer before the accumulating costs of the effort force you to slow down. That is why How to resist fatigue is really the companion piece to this article. Training is not about dodging lactate. It is about improving the systems that let you hold off fatigue.
Do cool-downs or easy jogs “flush out” lactic acid?
Yes, active recovery (a walk, a jog, a light ride) after a workout can probably help lactate disappear from the blood faster than complete rest. But that does not mean you have flushed out the cause of fatigue, erased muscle damage, or solved tomorrow’s soreness. The key mistake is confusing faster lactate clearance with full recovery. Those are not the same thing. The modern literature treats lactate as something the body is using and clearing as part of normal metabolism, not as toxic sludge that needs to be pressure-washed and deep-tissue-massaged from the calves (Brooks et al. 2018; Brooks et al. 2020).
So if an easy jog after a hard session helps you feel better, great. But don’t build a whole theory of recovery around the phrase “flush the lactic acid out” because that kind of sh*t should be flushed down the loo.
The simple truth about lactic acid for runners — what can you put in your training toolbox?
Quite simply, lactate is not the cartoon villain of endurance sport.
It rises when exercise gets hard. It travels with fatigue. It can sit in the same room as discomfort, slowing down, and that awful feeling that the uphill has become personal. But that does not make it the sole cause of those things. The best current evidence says that while severe acidosis can contribute to fatigue in some high-intensity situations, lactate itself has little direct negative effect on performance, and the whole process of fatigue involves many more factors than lactate alone (Cairns and Lindinger 2025; Cairns and Lindinger 2025; Brooks et al. 2018).
The better question for runners is not, “How do I avoid lactic acid?” The better question is, “How do I train so I can run faster before fatigue forces me to back off?” Once you ask that, everything gets clearer.
Until next time, stay nerdy and keep empowering yourself to be the best athlete you can be by training smart.
Thanks for joining me for another “session”! To receive updates on my new articles, nerd alerts, and training tools, subscribe to my newsletter at veothu.com/subscribe. If you enjoy my content, please help me out!
Reviews and follows train the magical algorithms to promote my content higher up the rankings so that more folks see high-quality information.
FAQ
What is the difference between lactate and lactic acid?
In running and coaching chat, people often say “lactic acid”, but what usually matters in the body is “lactate”. That matters because lactate is not just a waste product; it is a normal part of exercise metabolism, and the body can use it as a fuel and move it between tissues.
Does lactic acid cause the burning feeling in your legs?
Not in the simple way people often say. The burn during hard running is real, but lactate itself is not the direct villain. Hard exercise changes a lot of things in the muscles at once, and lactate is basically part of that bigger picture rather than the whole story on its own.
Does lactic acid cause soreness after a run?
No. Lactate rises during hard exercise and is cleared relatively quickly, while the soreness usually shows up later. So if your legs are sore the next day or two days later, the lactate is not the best explanation for that.
Is lactate bad for running performance?
No, not by default. Lactate is not something harmful that we need to “flush out”. The body can use it as a fuel and shuttle it between tissues; it's a part of normal exercise metabolism. The important point is that rising lactate can signal that the effort is getting more costly, but that does not mean the lactate itself is always the thing slowing you down.
What does lactate threshold mean for runners?
The lactate threshold is a way of describing the exercise intensity where lactate starts to build up faster than the body can clear or use it. For runners, that matters because it helps show the fastest pace you can hold before things get a lot harder. It is not a magic line, but it is a useful idea for training and racing, and coaches are gonna keep talking about it for a reason.
Share this article with your people:
Who is Thomas Solomon?
My knowledge has been honed following 20+ years of running, cycling, hiking, cross-country skiing, lifting, and climbing, 15+ years of academic research at world-leading universities and hospitals, and 10+ years advising and coaching in athletic performance and lifestyle change.
I have a BSc in Biochemistry, a PhD in Exercise Science, and over 90 peer-reviewed publications in medical journals.
I'm also an ACSM-certified Exercise Physiologist (ACSM-EP), an ACSM-certified Personal Trainer (ACSM-CPT), a VDOT-certified Distance Running Coach, and a UKVRN Registered Nutritionist (RNutr).
Since 2002, I’ve conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine.
And, with my personal experience of competing on the track (800m to 10,000m), the road (5 k to marathon), on the trails, and in the mountains, by foot, bicycle, cross-country ski, and during obstacle course races (OCR), I deeply understand what it's like to train and compete — I've been there, done it, and gotten sweat, mud, and tears on my t-shirt.
Disclaimer: I occasionally mention brands and products, but it is important to know that I don't sell recovery products, supplements, or ad space, and I'm not affiliated with / sponsored by / an ambassador for / receiving advertisement royalties from any brands. I have conducted biomedical research for which I’ve received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, the A.P. Møller Foundation, and the Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on R&D grant applications, and I provide research and scientific writing services for Examine.com. Some of my articles contain links to information provided by Examine.com, but I do not receive any royalties or bonuses from those links. Importantly, none of the companies described above have had any control over the research design, data analysis, or publication outcomes of my work. I research and write my content using state-of-the-art, consensus, peer-reviewed, and published scientific evidence, combined with my empirical evidence observed in practice and feedback from athletes. My advice is, and always will be, based on my own views and opinions shaped by the scientific evidence available. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
