Stress and Cortisol in the Athlete

Cortisol is often called the “stress hormone” and treated as an enemy of muscle. In reality it is a vital hormone, without which neither training nor adaptation to it would be possible. The problem arises not when cortisol rises, but when the body loses the ability to bring it back to normal. The editorial team looked at how stress and cortisol affect an athlete and what is worth keeping an eye on.

What cortisol and the stress axis are

Cortisol is a glucocorticoid hormone synthesized by the adrenal cortex. Its production is regulated by the so-called hypothalamic–pituitary–adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone, the pituitary responds with adrenocorticotropic hormone (ACTH), and that in turn stimulates the adrenal glands. When cortisol reaches a sufficient level, it inhibits the upper links of the axis through a negative-feedback mechanism.

The main functions of cortisol are energy mobilization and control of inflammation. It raises blood glucose through gluconeogenesis in the liver, promotes the breakdown of fats and, in part, proteins, and restrains an excessive immune response. For a body facing a threat or physical work, these are precisely the changes that help it survive and complete the task.

Cortisol has a clear circadian rhythm. The highest level is observed in the morning: in the first 30–45 minutes after waking, the so-called cortisol awakening response occurs. Over the course of the day the concentration gradually declines and reaches a minimum around midnight. This rhythm is closely tied to sleep, light exposure and eating patterns.

In parallel with the HPA axis works the sympathoadrenal system, which releases adrenaline and noradrenaline within seconds. If adrenaline is the “fast” response to stress, then cortisol is the “slow” one, unfolding over minutes and lasting for hours. For an athlete it is important that both systems respond not only to training but also to psychological factors: competitions, conflicts, lack of sleep, financial worries.

The classic review by Sapolsky and co-authors (2000) emphasizes that glucocorticoids simultaneously allow the stress response to unfold and restrain it, preventing it from becoming destructive. In other words, cortisol is not only the “gas” but also the “brake” of stress.

How training affects cortisol

Physical exercise is one of the most powerful physiological stimuli for the HPA axis. However, not every workout raises cortisol. A study by Hill and co-authors (2008) demonstrated an intensity-threshold effect: moderate work at around 40% of maximal oxygen uptake did not cause the hormone to rise, whereas loads at 60% and 80% of VO2max were accompanied by its increase.

In strength training cortisol responds most to protocols with high volume, moderate weight and short rest pauses — that is, to those that produce pronounced metabolic stress. The review by Kraemer and Ratamess (2005) notes that heavy sessions with long rest and few repetitions raise the hormone considerably less.

An acute rise in cortisol after a session is a normal part of adaptation. It helps to mobilize fuel and regulates the inflammatory response needed for tissue repair. Attempts to “block” cortisol after every workout have no scientific basis and may even interfere with adaptation processes.

Duration also matters. Endurance loads lasting more than an hour, especially against a background of low glycogen stores, produce a higher and more prolonged cortisol response. That is why marathon runners and triathletes with a chronic carbohydrate deficit more often have an elevated basal level of the hormone.

When stress becomes a problem

The key question is not the height of the peak but the system's ability to return to normal. If training stress alternates with adequate rest, the HPA axis works flexibly. But if loads accumulate together with stress from study, work or personal life, while sleep and nutrition are insufficient, the system loses this flexibility.

The ECSS and ACSM consensus on overtraining syndrome (Meeusen et al., 2013) describes a continuum: functional overreaching, non-functional overreaching and overtraining syndrome proper. In the later stages, changes in the hormonal response to stress may be observed, including a blunted ACTH and cortisol reaction to exercise. The authors stress, however, that no single hormonal marker exists for diagnosis.

Chronically elevated cortisol has well-known consequences: impaired sleep, immune suppression, catabolism of muscle protein, a tendency to accumulate visceral fat, reduced bone mineral density. Some of these effects overlap with relative energy deficiency in sport (RED-S), described in the IOC consensus.

It is important not to confuse physiological stress with pathology. Persistently high cortisol may be a sign of endocrine diseases, such as Cushing's syndrome, while low cortisol may indicate adrenal insufficiency. Such conditions are diagnosed only by an endocrinologist, and “overtraining” should not become the explanation for symptoms that require examination.

• Warning signs:sleep disturbance despite fatigue, a persistent decline in results, frequent colds, loss of appetite or cravings for sweets, irritability.
• Risk factors:a sharp increase in volume, the competitive period, a deficit of calories and carbohydrates, psychological pressure, air travel and time-zone changes.

Tests: what can be measured

Cortisol can be measured in blood, saliva and urine, and each method answers its own question. Morning serum cortisol shows the level at a specific moment, but it is highly sensitive to the time of collection, the stress of the procedure itself and sleep the night before. A single result rarely provides useful information for an athlete.

Salivary cortisol reflects the free, biologically active fraction of the hormone and allows several samples to be taken over the day at home. This makes it possible to assess the circadian rhythm and the awakening response. Daily excretion of free cortisol in urine is used mainly in endocrinology to screen for hypercortisolism.

In sports science the testosterone-to-cortisol ratio was popular as a marker of anabolic–catabolic balance. However, modern reviews consider it unreliable for individual decisions: it fluctuates strongly, depends on the time of day and does not always match the athlete's condition.

Practical conclusion: hormonal tests make sense as part of comprehensive monitoring, not as a standalone “stress meter”. A systematic review by Saw and co-authors (2016) showed that subjective well-being questionnaires are often more sensitive to changes in training load than conventional objective markers.

How to manage the stress load

The most effective tool for regulating the HPA axis is sleep. Lack of sleep disrupts the circadian rhythm of cortisol and raises its evening values. A regular bedtime, a dark, cool bedroom and limiting bright light in the evening are basic but the most effective measures.

Second is energy adequacy. A prolonged deficit of calories and especially carbohydrates against a background of intense training amplifies the cortisol response. Carbohydrate intake during long loads reduces the hormonal reaction to them, which is confirmed by the ACSM position on athlete nutrition.

Third is periodization. Planned deload weeks, alternating hard and easy days, and a gradual build-up of volume give the system time to adapt. It is also useful to account for non-training stress: during an exam period or a rush at work it is sensible to reduce the training load.

Fourth is psychological techniques: breathing exercises, meditation, work with a sports psychologist. They influence sympathetic activity and the perception of stress. Supplements advertised as “cortisol blockers” have a limited evidence base, and the editorial team does not recommend relying on them instead of the basic measures.

Editorial conclusions

Cortisol is not the athlete's enemy but a necessary regulator of energy and inflammation. Its acute rise during intense training is normal and part of adaptation.

The problem arises with chronic strain, when training and life stress overlap while recovery cannot keep up. That is when sleep, immunity and progress suffer.

A single cortisol test says little about an athlete's condition. It is better to combine subjective well-being scales, the trend in results and, if needed, laboratory examination under a doctor's supervision.

We also recommend reading our materials on meditation and breathing practices for athletes, on the signs that you are not recovering, and on the role of sleep in athletic progress.

References

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7. Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med. 2018;52(11):687–697.
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