Endurance Training is Stress and Here’s Why That’s Not a Bad Thing Until It Is
- 3 days ago
- 12 min read
Shaun Provost is a women’s health and performance expert, endurance athlete, and founder of Live Unbreakable. With more than 16 years of coaching experience and 34 certifications, she helps women transform their health through strength training, hormone education, and lifestyle coaching.
There is a strange contradiction built into endurance sports. We spend enormous amounts of time deliberately teaching the body to tolerate stress while simultaneously being told that stress is something we should avoid. For runners and triathletes, stress is quite literally part of the training plan. A twenty-mile run is stress. Five hours on a bike is stress. Threshold intervals are stress. An Ironman build that includes long rides, brick workouts, strength training, early mornings, and months of progressively increasing volume places a significant demand on nearly every physiological system in the body.
That is not necessarily a bad thing. In fact, it is the entire reason training works. Endurance exercise creates temporary disruptions in normal physiology that force the body to adapt. Cardiovascular efficiency improves. Mitochondrial density increases. Plasma volume expands. Muscles become more efficient at using available fuel, and the body becomes increasingly capable of maintaining work under fatigue. The remarkable thing about endurance training is not that it stresses us. It is that, given adequate recovery, the human body can become extraordinarily good at adapting to that stress.
The better question, then, is not whether endurance training causes stress. It does. The real question is why that stress makes one athlete fitter while eventually leaving another slower, exhausted, injured, unable to sleep, hormonally disrupted, or sick.
The answer begins with a distinction that endurance athletes are particularly good at forgetting: training and adaptation are not the same thing.
Training provides the stimulus. Adaptation is what the body does afterward. Completing more miles does not automatically create more fitness simply because those miles appear on Strava. They create a physiological demand. When the body has sufficient energy, sleep, nutrition, and recovery to respond to that demand, the result can be greater fitness. When it does not, adding more training can eventually create the opposite.
This is where the conversation about cortisol and endurance sports becomes considerably more complicated than social media would have us believe.
Cortisol is not ruining your life
Cortisol may be one of the most misunderstood hormones in modern health and fitness. It has become an all-purpose explanation for fatigue, weight gain, burnout, poor sleep, anxiety, declining athletic performance, and just about anything else that cannot be neatly explained in a thirty-second video. The problem is that cortisol is not a physiological mistake. It is essential to both survival and athletic performance.
During prolonged exercise, working muscles require a continuous supply of energy. As exercise duration and intensity increase, the hypothalamic pituitary adrenal, or HPA, axis helps coordinate the body’s response to that demand. Cortisol concentrations rise alongside changes in catecholamines and other metabolic hormones, helping maintain blood glucose, mobilize available energy, regulate inflammation, and support cardiovascular function.
If you are three hours into a long ride, this is exactly what you want your body to be capable of doing. That does not mean chronically elevated or dysregulated stress physiology is irrelevant. It means that an acute rise in cortisol during exercise should not automatically be interpreted as harmful. There is an important difference between the normal hormonal response required to complete a demanding training session and the cumulative physiological burden created when stress repeatedly exceeds recovery.
Exercise physiology is full of these distinctions. Glycogen depletion is not inherently bad. Inflammation is not inherently bad. Muscle damage is not inherently bad. Even fatigue is not inherently bad. In the appropriate dose, these responses provide signals that contribute to adaptation. The problem is the dose, the context, and what happens next.
Your training plan does not exist in a vacuum
A twelve-hour training week may be completely manageable for one athlete and entirely inappropriate for another. Even for the same athlete, it may be productive during one season of life and unsustainable during another. The reasoning here is simple: the body does not experience training stress independently of life stress.
A training plan may prescribe a ninety-minute Zone 2 run, but the physiological environment surrounding those ninety minutes matters. An athlete might begin that run after eight hours of sleep, adequate carbohydrate intake, appropriate hydration, and a low-stress day. The same athlete might attempt the identical workout after sleeping five hours, working through lunch, handling a stressful deadline, relying heavily on caffeine, and carrying residual fatigue from the previous session. The workout is identical. The physiological cost is not.
This becomes particularly important during marathon and Ironman preparation because training stress accumulates over months. One hard run or long ride is rarely the issue. More often, the problem is the combination of high training volume with insufficient sleep, underfueling, occupational stress, travel, family responsibilities, illness, heat exposure, and the psychological pressure athletes place on themselves to complete every session exactly as written.
In physiology, allostasis describes the body’s ability to maintain stability by continuously adjusting to changing demands. The cardiovascular, metabolic, immune, neural, and endocrine systems are not fixed. They constantly adapt to what is happening around us. That flexibility is one of the reasons humans are capable of adapting so well to endurance training. But adaptation has a cost.
When multiple stressors remain elevated for long periods, the cumulative burden is often described as allostatic load. This is one reason performance can appear to decline “out of nowhere.” Training volume looks reasonable. Work is busy but manageable. Sleep has only been slightly worse. Nutrition has slipped a little because appetite is low after long sessions. None of those variables seems disastrous alone. The body experiences the sum.
The body is constantly deciding where to spend energy
One of the most useful ways to understand hormonal changes in endurance athletes is to stop thinking about hormones as isolated problems and start thinking about resource allocation.
The human body has priorities. Staying alive sits considerably higher on that list than running a personal best. When energy and recovery are abundant, the body can afford to invest resources broadly. It can support training adaptation, reproductive function, bone remodeling, immune defense, tissue repair, cardiovascular health, thermoregulation, and normal endocrine function. When resources become limited, the tradeoffs begin.
This becomes particularly relevant when large training loads are paired with low energy availability. Energy availability refers to the dietary energy remaining to support normal physiological function after the energetic cost of exercise is accounted for. When that amount becomes insufficient for long enough, the body adapts to the shortage. That adaptation may be very different from the one the athlete was hoping to create.
This is the framework behind Relative Energy Deficiency in Sport, or RED S. Importantly, RED S is not exclusive to women and does not require the presence of an eating disorder. The International Olympic Committee describes RED S as a syndrome of impaired physiological and psychological functioning associated with problematic low energy availability in both female and male athletes. For endurance athletes, this matters because underfueling is not always intentional.
A recreational runner may increase from four hours of weekly training to eight while maintaining approximately the same eating habits. A triathlete may suddenly add a four-hour Saturday ride, a Sunday long run, two swims, strength sessions, and higher-intensity bike work without proportionally increasing energy intake. Appetite does not always perfectly track energy expenditure, and the logistics of work, parenting, training, and life can make eating enough surprisingly difficult. Sometimes the math quite simply gets away from us.
The resulting changes are not limited to hunger or fatigue. Prolonged low energy availability can affect reproductive function, thyroid-related metabolism, bone health, immunity, cardiovascular health, protein synthesis, gastrointestinal function, mood, recovery, and ultimately athletic performance. In women, problematic low energy availability can contribute to menstrual dysfunction and reduced estrogen exposure, which carries implications well beyond reproduction, particularly for bone health. Losing a menstrual cycle should never be treated as evidence that someone has simply become “athletic enough.”
Men are not immune to the same basic problem. Low energy availability in male endurance athletes has also been associated with changes in metabolic and reproductive hormones, including testosterone in some populations, as well as effects on bone health and performance. The exact endocrine response is not identical in every athlete, which is why symptoms and trends matter more than assuming one hormone will always move in one predictable direction.
This is where trying to “lower cortisol” can become a distraction. If an athlete is training twelve hours per week, sleeping six hours, chronically underfueling, and attempting to maintain a calorie deficit because they believe becoming lighter will automatically make them faster, the problem is probably not that they have failed to find the correct cortisol supplement. The body may be responding entirely appropriately to an environment that is asking too much while providing too little.
Carbohydrates matter before race morning
Endurance athletes generally understand that carbohydrates are important during races. We calculate grams per hour, practice race nutrition, experiment with gels, and spend entire long sessions training the gut to tolerate fuel. Yet some of those same athletes routinely underfuel the training sessions responsible for creating race day fitness.
Carbohydrate availability influences more than whether an athlete can maintain pace late in a workout. During prolonged exercise, inadequate carbohydrate availability can increase physiological stress, while carbohydrate ingestion can attenuate aspects of the stress hormone and immune response under certain conditions. That does not mean every easy run requires aggressive fueling, nor does it mean there is never a place for carefully planned training with lower carbohydrate availability. Nutrition should reflect the duration, intensity, and purpose of the session, as well as overall energy intake and the individual athlete.
But strategically manipulating carbohydrate availability and simply failing to eat enough are not the same thing. For an Ironman athlete completing a four-hour ride followed by a transition run, carbohydrate is not a character flaw. It is fuel. If the body repeatedly lacks the resources necessary to recover, no amount of compression boots, cold plunges, red light, supplements, or expensive recovery technology can completely compensate for that.
Sleep is part of the program
Sleep creates a similar problem because endurance athletes often sacrifice it in order to train. The reasoning makes sense on paper. If the only way to fit a long workout around work and family responsibilities is to wake up earlier, sleep becomes the easiest place from which to borrow time. But repeatedly reducing sleep while increasing training volume creates a fairly obvious physiological contradiction: the athlete is increasing the stimulus for adaptation while decreasing one of the resources required to adapt.
Sleep is deeply involved in endocrine, metabolic, immune, and nervous system regulation. Insufficient sleep can influence cortisol rhythms, glucose regulation, inflammatory signaling, appetite, muscle protein synthesis, and the hormonal environment involved in recovery. That does not necessarily mean one terrible night of sleep destroys your hormones or makes the next workout worthless. As with menstrual cycle research, this conversation requires much more nuance than that. Human physiology is remarkably resilient, and short-term fluctuations are normal. The issue is pattern and persistence.
A 4:30 a.m. alarm may occasionally be necessary. Building an entire Ironman training block around five hours of sleep per night and assuming discipline will somehow override physiology is a different conversation. At some point, the recovery run is no longer recovery. It is simply another stressor.
Why endurance athletes are particularly good at missing the warning signs
One of the reasons underrecovery is difficult to identify in endurance sports is that fatigue is expected. During a productive marathon or Ironman build, there will be periods of heavy legs, reduced motivation, temporary performance decreases, soreness, and increased perceived effort.
Functional overreaching describes exactly this situation: training load temporarily exceeds the athlete’s current capacity, performance declines briefly, recovery occurs, and the athlete ideally emerges fitter. The important part is that recovery occurs.
Nonfunctional overreaching and overtraining syndrome exist further along the spectrum, where performance impairment becomes more prolonged and physiological and psychological disturbances can begin to accumulate. Researchers have not identified one perfect hormonal biomarker that tells us when an athlete has crossed that line. Cortisol alone cannot diagnose it. Neither can testosterone, HRV, resting heart rate, or a few ugly Garmin recovery scores. What matters is the overarching pattern.
If training that once felt manageable begins to require disproportionate effort, performance continues declining despite additional work, sleep becomes less restorative, motivation disappears, illness becomes more frequent, recovery takes longer, injuries accumulate, menstrual function changes, or libido declines, those pieces deserve attention together.
None of them proves that an athlete has “high cortisol.” They may indicate something more useful: the athlete is no longer successfully adapting to the total load being imposed. That changes the intervention.
Many athletes do not have a hormone problem
This is perhaps the part of the conversation I feel most strongly about. Hormones communicate information. Cortisol participates in the stress response. Insulin regulates nutrient utilization and storage. Thyroid hormones influence metabolic regulation. Testosterone and estrogen affect bone, muscle, cardiovascular function, reproduction, and recovery. When several of these systems begin changing, immediately trying to “balance hormones” without asking why they changed can miss the entire point.
Before assuming the endocrine system has mysteriously stopped working, look at the environment. How much are you training? How much are you actually eating? Are you fueling long and demanding sessions appropriately? How much are you sleeping? Is performance improving, or are you simply completing more training? Are injuries becoming more frequent? Has menstrual function changed? Has libido changed? How much psychological stress exists outside the training plan?
There are absolutely circumstances in which persistent fatigue, menstrual dysfunction, unexplained performance decline, recurrent stress injuries, libido changes, or symptoms of endocrine dysfunction warrant medical evaluation. Not every problem in an endurance athlete is caused by training, and coaches should not attempt to diagnose medical conditions. Many athletes do not need another wellness product. They need more food, more sleep, less accumulated fatigue, or a training plan flexible enough to acknowledge that human beings are not always predictable.
Train hard enough to adapt, not hard enough to prove a point
I’m also speaking to myself in this one. None of this is an argument for training less simply because endurance exercise is stressful. High-level endurance performance requires substantial training. Progressive overload matters. Hard workouts matter. Long runs, long rides, threshold work, intervals, and periods of accumulated fatigue all have a legitimate place in well-designed endurance programs.
Women and men do not need to be treated as though their endocrine systems make them fragile. Inherently, they do not. But there is a major difference between productive training stress and simply accumulating more stress.
Sometimes the most appropriate intervention is reducing training volume. Sometimes it is replacing intensity with easy aerobic work. Sometimes it is increasing carbohydrate intake, abandoning a calorie deficit, or sleeping another ninety minutes instead of adding another session. Sometimes the answer is medical evaluation. Those are not failures of discipline. They are performance decisions.
Endurance athletes are exceptionally good at doing difficult things over and over and over. We will rearrange entire weekends around long rides, wake up before sunrise to run, and voluntarily finish workouts that most people would happily abandon halfway through, if they even started them at all.
The hardest skill any endurance athlete can master has nothing to do with strength or power. It is recognizing when doing more is no longer creating more.
The goal of endurance training has never been to avoid stress. Stress is precisely what gives the body a reason to adapt. The goal is to apply enough of it to stimulate change, provide enough recovery and energy for the body to respond, and repeat that process consistently enough for fitness to accumulate without the body breaking down.
We already understand this concept when we taper. We reduce training stress before a race so the fitness we have built can finally show up without being hidden beneath accumulated fatigue. Perhaps we should respect that same physiology during the months leading up to race day.
Endurance training is supposed to be uncomfortable and stressful. That is why it works and why not everyone does it. The skill is knowing when that stress is still making you better and when it is simply making you tired.
Read more from Shaun Provost
Shaun Provost, Women’s Health and Performance Expert
Shaun Provost is the founder of Live Unbreakable and a women’s health and performance coach with more than 16 years of experience in fitness, nutrition, and hormone health. She is passionate about helping women build strength, improve confidence, and create sustainable habits that support long-term wellness. As an endurance athlete and educator, Shaun combines science-backed coaching with a relatable, empowering approach that makes health feel achievable in real life. Through her writing, she aims to educate, inspire, and help women feel truly unbreakable.
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