What Is Cortisol? Stress, Training, Immunity and What It Means for Athletes
Table of Contents
Key Takeaways: What Is Cortisol
- Cortisol is a steroid hormone made in the adrenal cortex. It helps the body respond to stress and regulate blood glucose, and it follows a daily rhythm that peaks around waking.
- A cortisol rise during moderate- to high-intensity exercise is an expected, intensity-dependent response.
- Glucocorticoids such as cortisol inhibit many of the initial events of inflammation, and periods of intensified training have been linked to lower resting lymphocyte counts.
- “Adrenal fatigue” is not a recognised medical condition; a systematic review of 58 studies found no substantiation for it.
- Resting cortisol is a poor marker of overtraining syndrome and burnout. Abnormal HPA-axis function has been implicated in depression, but that is not a reason to use cortisol as a test for it.
- In clinical practice, a cortisol test seldom helps to confirm stress, burnout or overtraining. These are clinical diagnoses, and cortisol testing belongs to situations where an endocrine cause of too much or too little cortisol, such as Cushing’s syndrome, is suspected.
- Testosterone is no better an explanation for exhaustion. Its levels vary with physiological and health-related factors, and in my clinical experience primary hypogonadism is rarely the cause in these patients.
- In my clinical experience, prolonged stress often disturbs sleep. The approach is to exclude other causes first, then offer support such as a referral to a psychologist and, where possible, address the stressor itself.
- In athletes, cortisol deserves attention mainly in context: under-fuelling, months of high training volume and a disrupted daily rhythm, rather than as one isolated number.
Introduction: What Is Cortisol and Why Is It So Misunderstood?
Few hormones have a worse reputation than cortisol. Online, it is blamed for belly fat, poor sleep, muscle loss and burnout, and products promising to “lower cortisol” are easy to find. For athletes the message is often even blunter: cortisol is catabolic, so cortisol is the enemy.
The physiology tells a different story. Exercise endocrinologists Hackney and Walz have argued that misconceptions about cortisol have taken hold in sport and exercise, mainly because its catabolic effect on protein turnover is assumed to work against training adaptation.[1]
From a sports medicine perspective, that framing misses the point twice. A post-workout cortisol rise can simply reflect that training stress happened, which is part of a normal physiological response. And the situations in which cortisol physiology genuinely deserves attention, such as under-fuelling, a disrupted daily rhythm or months of unbroken high training volume, are easy to overlook if you are only looking for a single “high cortisol” result.
This article answers the question “what is cortisol?” from the ground up: where it comes from, how its daily rhythm works, how it interacts with the immune system, what it is not, and what it means for athletes, from the acute training response and competition nerves to sleep, energy availability, overreaching and overtraining, burnout and depression.
Cortisol is central to the body’s stress response, so it is no surprise that it comes up in my practice quite often and in many different situations. As an occupational health physician, the most common setting is a patient with prolonged stress, often linked to burnout at work or a crisis in their personal life. I return to that picture in the burnout section below.
What Is Cortisol? The Basic Physiology
Cortisol is a glucocorticoid, a steroid hormone produced by the zona fasciculata of the adrenal cortex. Glucocorticoids play vital roles in the body’s defence mechanisms when it deals with stress, and they are important in blood glucose regulation.[1]
Its release is controlled by the hypothalamic–pituitary–adrenal (HPA) axis. In response to stress, corticotropin-releasing factor (CRF, also called CRH) from the hypothalamus starts a cascade of events that ends with glucocorticoid release from the adrenal cortex.[2] The middle link in that chain is adrenocorticotropic hormone (ACTH), a pituitary peptide hormone that stimulates cortisol production in the zona fasciculata.[3] The system then regulates itself: glucocorticoid feedback inhibition plays a prominent role in controlling both the magnitude and the duration of glucocorticoid release.[2]
That feedback loop is why cortisol is better understood as a regulated system than as a number to push up or down. A rise after a stressor is the axis doing its job. Cortisol measurements need to be interpreted in the context of the time of day, recent physiological stressors and the clinical question being asked. For a closer look at how cortisol behaves on a laboratory report, see Cortisol in Athletes: What Your Blood Test Is Actually Telling You.
The Daily Rhythm: Why Cortisol Is Highest in the Morning
Cortisol does not sit at a fixed level. It has a distinct circadian rhythm: concentrations are low at night, start rising in the early hours of the morning, peak on waking and then decline over the day to low levels in the evening.[4]
On top of that curve sits the cortisol awakening response (CAR), a marked increase in cortisol secretion over the first 30–45 minutes after waking. The CAR has been related to a wide range of psychosocial, physical and mental health parameters.[5] I cover its use in sport in The Cortisol Awakening Response in Athletes.
The rhythm matters because it can be disrupted. Porter and colleagues note that loss of the cortisol circadian rhythm, as seen in jet lag and shift work, is associated with fatigue in the short term and with diabetes and obesity in the medium to long term.[4] For athletes who cross time zones to compete, train very early or very late, or combine sport with shift work, this is a practical point: when cortisol is released is part of what it does. It is also why a cortisol result only makes sense alongside the time of day it was taken.
Cortisol and the Immune System
Cortisol is also an anti-inflammatory signal. In their review of glucocorticoid actions, Coutinho and Chapman explain that glucocorticoids inhibit many of the initial events in an inflammatory response and also promote the resolution of inflammation.[6] That potency is why, since their discovery in the 1940s, glucocorticoids have been among the most widely used treatments for inflammatory and autoimmune diseases. It is also why long-term treatment has a price: its metabolic effects include osteoporosis, hypertension, dyslipidaemia and insulin resistance or type 2 diabetes.[6] These effects were described for long-term glucocorticoid treatment.
For athletes, the immune connection runs through training stress. The stress hormones released in response to exercise have been reported to trigger diverse immune reactions, which is one reason researchers study ways of blunting them (see the carbohydrate data below).[7] Over weeks rather than hours, a 2022 systematic review and meta-analysis of 57 human and rodent studies reported that, in its pooled analyses, a period of intensified training significantly reduced resting total lymphocyte counts (d = −0.57) and CD8+ T cell counts (d = −0.37).[8] The authors suggest that hormonal changes during intensified training, such as a blunted cortisol response, may impair the immune response, but they also concluded that definitive immune biomarkers of overtraining remain limited.[8]
What Cortisol Is Not: The “Adrenal Fatigue” Myth
Anyone researching what cortisol is will soon run into “adrenal fatigue”, the idea that chronic stress wears out the adrenal glands until they can no longer make enough cortisol. Cadegiani and Kater, two endocrinologists, examined this idea in a systematic review. From 3,470 articles found, 58 studies fulfilled their criteria, and across most testing methods they found an almost systematic pattern of conflicting results.[3] Their conclusion was that there is no substantiation that adrenal fatigue is an actual medical condition, and they note that it has not been recognised by any endocrinology society.[3]
That does not make cortisol disorders imaginary. Adrenal insufficiency and cortisol excess are real diagnoses, but they are assessed differently. The same review states that functional tests are the only methods of assessing adrenal cortisol production endorsed by endocrinology societies, and its authors advise against using cortisol tests in clinical practice unless adrenal impairment is suspected.[3] The review also lists conditions to consider when someone presents with “adrenal fatigue”-type symptoms, including obstructive sleep apnoea, adrenal insufficiency, mental illness, overwork, night-shift work and other hormonal deficiencies.[3]
In my clinical experience, patients may ask for a cortisol test in the hope of confirming their stress level or finding out whether their stress system is dysfunctional. In this situation a laboratory cortisol measurement has little practical clinical value. A single cortisol level does not tell us much by itself. Even if the stress hormone were elevated, it would not necessarily lead to any specific action. We also do not know the person’s own baseline stress level, so measuring brings little practical benefit. In practice, prolonged stress can often be recognised from the history alone, although other causes still need to be excluded.
Cortisol measurement does have a real role in endocrine situations. It is not part of the basic laboratory panels, and as far as I understand, screening for it is generally not considered useful. Where cortisol is measured, it is usually in an endocrinology context, either in specialist care or sometimes in primary care, and the question is whether an organic cause lies behind too much or too little cortisol production. I have seen such cases in my own practice. They can be hard to detect, and in my clinical experience the diagnosis may be confirmed only after a second or third round of laboratory tests.
For athletes, the practical message is simple: persistent fatigue deserves a proper medical work-up, not a label that has not been shown to exist. If you are unsure which tests make sense in your situation, see Which Blood Tests Do Athletes Actually Need?
What Cortisol Means for Athletes: The Acute Exercise Response
Exercise is a physical stressor, so it activates the HPA axis, but not at every intensity. Hill and colleagues compared exercise at 40%, 60% and 80% of maximal oxygen uptake (VO₂max) while controlling for time of day, prior diet and activity, psychological stress and training status.[9] Moderate- and high-intensity exercise increased circulating cortisol. Exercise at 40% did not significantly raise it, and once plasma volume changes and circadian factors were accounted for, low-intensity exercise actually lowered cortisol.[9] ACTH mirrored the cortisol response, although only the 80% session produced a significant pre-to-post-exercise increase.[9]
This is cortisol doing what it is built to do. Because glucocorticoids are important in blood glucose regulation,[1] a hard session that challenges fuel supply is exactly the setting in which a cortisol rise makes physiological sense.
Fuelling changes that response. A 2024 systematic review of eleven placebo-controlled trials in experienced endurance athletes found that taking in at least 30 g of carbohydrate per hour before and during prolonged continuous exercise attenuated the rise in cortisol in the majority of included studies. Adrenaline (epinephrine) was lower with carbohydrate than with placebo in the studies that measured it, although the differences were not consistently statistically significant.[7]
The practical consequence for blood testing is that a sample drawn shortly after a hard session reflects that session. It does not, on its own, describe your baseline.
Is Cortisol Bad for Muscle Growth?
The belief that cortisol “eats muscle” is exactly the misconception Hackney and Walz describe: cortisol is often seen as counter-productive to training because of its catabolic effect on protein turnover.[1]
The training data are more interesting than that. In a cohort of 56 young men, West and Phillips measured hormone responses to an intense leg resistance session at the midpoint of a 12-week training programme and compared them with each person’s training adaptations.[10] The post-exercise cortisol response, usually assumed to reflect catabolic drive, correlated positively with gains in lean body mass (r = 0.29) and type II muscle fibre cross-sectional area (r = 0.35). None of the measured hormone responses correlated with gains in leg press strength.[10]
The authors described these correlations as weak, and correlation does not establish cause, so this study does not show that cortisol builds muscle. It is, however, hard to reconcile with the idea that a normal post-workout cortisol rise sabotages hypertrophy.
Competition Nerves: Cortisol Before the Start
Cortisol also responds to psychological stress, and competition is a reliable trigger. A 2017 meta-analysis pooled 25 studies (27 effect sizes, 348 participants) that measured salivary cortisol before real sport competition and compared it with time-matched baseline samples.[11] It found a significant anticipatory cortisol response (Hedges’ g = 0.85), and the response was greater when cortisol was sampled closer to the start of competition.[11] Female athletes and athletes competing at international level did not show a significant anticipatory response, although the direct comparisons between subgroups were not statistically significant.[11]
The authors interpret this moderate rise as part of how athletes prepare for the cognitive and physical demands of competition. That is their interpretation of the pattern, not something the meta-analysis tested directly.[11]
When I was still competing, I usually slept poorly the night before a competition, because I was nervous about the next day.
When Cortisol Deserves Attention in Athletes
Sleep loss
Sleep is often named as the main lever for “lowering cortisol”, but the evidence on acute sleep deprivation is less clear-cut. A 2024 meta-analysis of 24 studies found no significant difference in cortisol between acute sleep deprivation and normal sleep, either in 21 crossover studies (SMD = 0.18) or in 3 randomised controlled trials (SMD = 0.26).[12] Subgroups told a different story: studies that measured serum cortisol did find higher levels after sleep deprivation (SMD = 0.46), as did studies with multiple measurements (SMD = 0.28).[12] In other words, the cortisol effect of acute sleep loss appears modest and depends on how it is measured. Chronic disruption of the daily cortisol rhythm is a separate issue: as noted above, it has been associated with fatigue in the short term and with diabetes and obesity in the longer term.[4]
In my clinical experience, many patients with ongoing stress have trouble sleeping, and cortisol may be part of the reason. In some patients the stressful situation is continuous and lasts for a long time, and that can lead to difficult sleep problems. This is a different question from the effect of acute sleep deprivation on cortisol discussed above.
Menstrual disruption and energy availability
Tornberg and colleagues studied elite endurance athletes from national teams and competitive clubs: 14 with secondary functional hypothalamic amenorrhoea and 16 with regular cycles, with energy availability assessed over seven days.[13] The amenorrhoeic athletes had higher cortisol (564 ± 111 vs 400 ± 140 nmol/L) along with lower oestrogen, triiodothyronine (T3) and blood glucose.[13] Their reaction time was 7% longer, and knee muscular strength and endurance were 11% and 20% lower. The authors linked this lower neuromuscular performance to lower leg fat-free mass, glucose, oestrogen and T3, and to elevated cortisol.[13] This was a cross-sectional comparison, so it shows an association rather than cause. For how these overlapping hormone patterns are assessed in practice, see the RED-S blood work guide.
High cumulative training load
Blood and saliva capture cortisol over relatively short time periods; hair segment analysis can assess cumulative cortisol secretion over prolonged periods.[14] Skoluda and colleagues analysed hair from 304 amateur endurance athletes (long-distance runners, triathletes and cyclists) and 70 controls.[14] The athletes had higher cortisol in all three hair segments analysed, and cortisol in the segment closest to the scalp correlated positively with each indicator of training volume.[14]
That does not automatically mean harm. A review of hair cortisol in exercise and sport science cautions that elevated hair cortisol in regular exercisers is not necessarily pathological, and notes that only weak correlations exist between hair cortisol and perceived stress.[15] The context also runs the other way: a 2022 meta-analysis found that physical activity was an effective strategy for lowering cortisol levels (SMD −0.37), based on moderate-certainty evidence in the populations studied.[16] For high-volume athletes, the more relevant question is whether recovery keeps pace with load.
Overreaching and overtraining syndrome
Training is meant to be stressful, but a stress–recovery imbalance can lead to negative states of overtraining.[8] The research literature separates functional overreaching (FOR), non-functional overreaching (NFOR) and overtraining syndrome (OTS); I explain the differences in Functional Overreaching vs. Non-Functional Overreaching.
Many athletes expect a cortisol test to settle the question. At rest, it usually cannot. A systematic review of 38 studies on the hormonal aspects of OTS found that basal hormone levels were mostly normal in athletes with OTS, FOR or NFOR compared with healthy athletes.[17] The differences appeared under stimulation, mainly during maximal exercise: ACTH and growth hormone responses were blunted, while cortisol findings were conflicting.[17] The authors concluded that basal hormone levels are not a good predictor of these states, whereas blunted ACTH and growth hormone responses to stimulation tests may be.[17]
In practice, that means a normal resting cortisol does not rule out overtraining. I cover the stress-response side in more depth in Cortisol and Overtraining: Understanding the Hidden Stress-Response Failure, the hormone-ratio approach in T:C Ratio in Overtraining, and the wider laboratory question in Can Blood Tests Detect Overtraining?
Athletes may ask me for a cortisol test because they wonder whether they are overtrained. Unfortunately, I have to tell them that a cortisol measurement alone does not prove overtraining. The diagnosis of overreaching and overtraining syndrome is clinical. In the end, we do not have enough laboratory tests to establish it clearly, and it is the whole picture that counts.
The cortisol awakening response is sometimes discussed as a marker of overtraining, but in our clinics it is not measured routinely. For now it is a research curiosity rather than a tool for everyday clinical work. The cortisol-to-testosterone ratio runs into similar problems: even a change would not prove anything, because there are too many confounders. Testosterone levels also vary with physiological and health-related factors, so the ratio may not help here either. In the end, the diagnosis rests on clinical judgement.
Cortisol, Burnout and Depression
Athletes rarely carry training stress alone. Work, study, relationships and mental health load the same stress system, so it is natural to ask whether cortisol can identify burnout or depression.
For burnout, the answer so far is no. A systematic review of 31 studies covering 38 potential biomarkers found no potential biomarkers for burnout, largely because the studies were not comparable; its meta-analyses showed no differences in the cortisol awakening response.[18] Cadegiani and Kater reached a similar conclusion in their adrenal fatigue review: studies of burnout were greatly inconsistent, and they advised that HPA-axis tests should not be used by health practitioners as markers of burnout.[3]
In my clinical experience, the working idea in patients with prolonged stress is that the stress system has been under load for too long, although studies have not identified a consistent cortisol pattern or biomarker in burnout.[3][18] How this shows up varies from patient to patient. Some are mainly anxious, while others have mostly physical symptoms. Sleep often becomes harder and appetite may change. Some patients have a pounding heartbeat or pressure in the chest, and some describe a subjective sense of breathlessness. Many patients are not ready to accept that stress or another psychological factor could explain their symptoms. In practice, the first step is to rule out other causes according to the symptoms and findings, and only then, by exclusion, do I consider whether stress may be contributing. That is a clinical judgement rather than a cortisol result, which fits the findings above.
Many patients, men in particular, hope for a testosterone measurement as an explanation for their exhaustion. Testosterone levels vary with physiological and health-related factors, and in my clinical experience low values are often seen in exhausted patients. That leads to repeat measurements and to the question of whether the patient could have hypogonadism. In my experience, primary hypogonadism is an atypical explanation, and the patient may, for example, be overweight. An explanation for exhaustion is often sought. Although we measure testosterone a lot, primary hypogonadism rarely turns out to be the cause in these patients. For that reason I would not necessarily look for a clear answer to exhaustion in testosterone. If there is a clinical suspicion, it is worth excluding, but in the clinical situation its share of the explanation is usually small.
In my impression, the people who experience the most intense stress are those with a poor working climate or a sense of being under scrutiny at work. Stopping the stress is also hard in practice, because many patients find it difficult to change their working conditions. Entrepreneurs can change their circumstances more freely, but then they worry about customers and income, so I cannot say which situation is the better one.
When a patient has chronic stress, I readily refer them to support, often to an occupational health psychologist or to a psychologist more generally, for example when the stress comes from a difficult situation at work. These stressors often call for a solution-focused approach, so that the person can change how they relate to the situation or, where possible, deal with the stressor itself.
Athletes are a group of their own, because their stress hormone system is working hard. In my impression, athletes nevertheless tend to have fairly good stress tolerance on average. Burnout is still one of the ways in which some athletes end up.
Depression is a different picture. Abnormal HPA-axis function has been implicated in major depressive disorder.[19] Cadegiani and Kater even excluded studies of conditions such as depression from their review, because the disorder itself can impair cortisol status.[3] A 2021 meta-analysis of 16 randomised controlled trials and seven open-label studies, including 2,972 participants, found a statistically significant pooled effect of treatments targeting the HPA system compared with controls (SMD = 0.138).[19] That pooled effect is small, and it does not by itself establish HPA-targeting treatments as routine care for depression. HPA-axis involvement is one part of a complex condition rather than something a single cortisol value can capture. If low mood, loss of motivation or exhaustion persist, that warrants a clinical assessment.
In my clinical experience, patients who develop anxiety, panic disorder or depression often have a prolonged or difficult stress period behind them. Many may also have lived in a stressful environment or faced strong stress when they were young. This is a clinical observation rather than a cortisol finding.
Overtraining, burnout and depression can look similar from the outside, which is one reason I wrote Does Exercise Increase Your Stress Tolerance? What Happens at Work, in Burnout, and in Depression.
These patients are difficult, because exhaustion, burnout and sometimes depression and anxiety may be present at the same time, and separating them from overreaching or overtraining syndrome is hard. In my clinical experience, I see such patients from time to time. Recently, one patient was suspected of having both overreaching and exhaustion, and extensive laboratory tests were done to exclude other causes. The situation was still not entirely clear. A training break did not improve the symptoms, so work-related exhaustion, or perhaps another, rarer immunological condition, seemed the more likely explanation, although the picture remained uncertain.
Conclusion: What Is Cortisol, Really?
So, what is cortisol? It is a glucocorticoid produced by the adrenal cortex, controlled by the HPA axis and its feedback loop, and released on a daily rhythm that peaks around waking. It is central to the stress response, to blood glucose regulation and to keeping inflammation in check.
For athletes, the evidence points away from treating cortisol as the enemy. Moderate- and high-intensity exercise raises it as expected, carbohydrate during long sessions attenuated the rise in most trials, and the post-exercise response was not linked to poorer muscle gains in a 56-person training study. “Adrenal fatigue” lacks scientific support, and resting cortisol does not reliably identify overtraining syndrome or burnout. The signals worth attention are the contextual ones: under-fuelling, persistently high training volume, a disrupted daily rhythm, and changes in mood or health that do not settle with rest.
In my own practice, cortisol comes up most often in patients with prolonged stress, where sleep suffers and other causes must be excluded before stress is accepted as the explanation. A cortisol or testosterone test seldom settles the question, and overreaching and overtraining syndrome in athletes likewise remain clinical diagnoses. Cortisol testing earns its place when an endocrine cause such as Cushing’s syndrome is suspected, even though confirmation can take more than one round of tests. For the rest, the work is clinical: taking the history, ruling out other causes and, where possible, changing the stressor or the way the patient relates to it.
Understanding cortisol is less about chasing a number and more about reading the system it belongs to.
References
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5988244/
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC3181830/
[3] https://doi.org/10.1186/s12902-016-0128-4
[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC5284474/
[5] https://doi.org/10.1016/j.psyneuen.2015.10.010
[6] https://doi.org/10.1016/j.mce.2010.04.005
[7] https://doi.org/10.3389/fspor.2024.1264814
[8] https://doi.org/10.3389/fphys.2022.998925
[9] https://pubmed.ncbi.nlm.nih.gov/18787373/
[10] https://doi.org/10.1007/s00421-011-2246-z
[11] https://doi.org/10.1136/bmjsem-2017-000261
[12] https://doi.org/10.1507/endocrj.EJ23-0714
[13] https://pubmed.ncbi.nlm.nih.gov/28723842/
[14] https://pubmed.ncbi.nlm.nih.gov/21944954/
[15] https://pmc.ncbi.nlm.nih.gov/articles/PMC3763301/
[16] https://pubmed.ncbi.nlm.nih.gov/35777076/
[17] https://pmc.ncbi.nlm.nih.gov/articles/PMC5541747/
