Cortisol and the stress response

Cortisol, a steroid hormone produced by the adrenal gland, plays a key role in mobilizing the body’s resources to cope with stressful challenges, including the challenge of running. Among its many roles is the regulation of blood glucose. When demands are high, cortisol acts to conserve glucose for the brain by minimizing uptake of glucose into other tissues and by promoting the production of glucose in the liver.   Because healing is not a priority when dealing with an acute challenge, cortisol suppresses inflammation and the immune system. In a healthy person, cortisol levels return to normal over a time scale of 30-60 minutes after the stress resolves. However if the transient surge of cortisol required to deal with acute stress is not switched off, cortisol inhibits healing by suppressing the formation of collagen while promoting breakdown of protein, thereby damaging many tissues of the body.

Recent evidence from a study by Skoluda and colleagues indicates that endurance athletes tend to have persistently high levels of cortisol. This increases in proportion to training volume. Thus the regulation of cortisol is potentially of great importance not only for ensuring that an athlete obtains benefits from training, but also for long term health.

The relationship between cortisol and inflammation is complex. In the short term cortisol suppresses inflammation, but sustained elevation of cortisol can lead to a suppression of the receptors that mediate the effects of cortisol on body tissues, and consequently, sustained elevation of cortisol can actually promote chronic inflammation which in turn damages tissues by laying down non-functional fibrous tissue as described in my recent post.

Although excessive cortisol is harmful, reduced ability to generate cortisol when required can be even more harmful. Addison’s disease, a rare condition in which the adrenal gland is damaged by autoimmune attack, is characterised by non-specific symptoms such as weakness and fatigue, and can be result in fatal inability to respond to stress. There is some evidence that sustained stress can reduce the capacity of the adrenal glands to produce cortisol when required, though the concept of adrenal fatigue, popularized by some alternative-medicine practitioners, remains an ill-defined entity.

Cortisol production is regulated by a feedback mechanism that takes account of information about the overall metabolic state of the body. This feedback system acts via the hypothalamo-pituitary-adrenal axis (HPA). The release of cortisol from the adrenal glands is stimulated by a hormone, ACTH, that is produced in the pituitary gland. The release of ACTH is in turn stimulated by a hormone, corticotrophin releasing factor, that is secreted by the hypothalamus. Information about the state of the body is funnelled via the amygdala and hippocampus in the temporal lobe of the brain, to the hypothalamus. This complex feedback system allows a diverse array of neural and hormonal signals to control cortisol release in a way that balances the catabolic effects of cortisol, promoting tissues breakdown, with the anabolic effects of other hormones, such as DHEA (a steroid hormone produced in the adrenal glands) and growth hormone, produced in the pituitary gland, that play a role in promoting the repair and strengthening of damaged tissues.  Thus many complex, interacting processes are involved in ensuring the optimal balance between mobilising body resources to deal with acute challenge and subsequent healing. Factors such as levels of ongoing stress from life circumstances and age contribute to the balance.

 Strategies for optimising the stress response

In summary, an athlete requires healthy adrenal glands which can generate enough cortisol to meet the challenge of stress but then to switch off cortisol production to promote recovery. The simple principle is that for optimum training benefit and long term health, we need to avoid excessive stress. However, the best way of achieving this is likely to determined by individual’s genes and life circumstances. While each individual has to find what works for him or her, there are several issues likely to be relevant to most athletes.

1)      Avoiding over-training. As demonstrated by Skoluda, the sustained excess of cortisol is greater in those who train more. Both volume and intensity matter though it is noteworthy that prolonged duration of exercise promotes increase in cortisol, whereas high intensity promotes hormones such as growth hormone and anabolic steroid hormones that promote strengthening of tissues. Consistent with this, some evidence indicates that the over-training syndrome is more strongly linked to high volume training than to high intensity training.

2)      Recovery from training and racing is crucial.   Not only does inadequate recovery increase the risk of persisting inflammation (as discussed in my previous post) but it impedes the transition from the cortisol induced catabolic state to the anabolic state required to rebuild and strengthen body tissues. This raises the major question of how best to determine if recovery is adequate. Subjective indices such as the Profile of Mood States, and autonomic measures such as resting heart rate and heart rate variability provide a guide, but no single test provides the full answer.  This is an issue I will return to again in the near future.

3)      Resistance training promotes the release of anabolic hormones and has many other beneficial effects on metabolism including increased sensitively to insulin. The major metabolic benefits of resistance training can be achieved by two 15 minute sessions per week.

4)      Life-stress and relaxation. Many of us have relatively limited control over the pressures of work and other responsibilities. However the way we react to these pressures is largely under our own control. Sleep plays a cardinal role. During sleep, cortisol levels fall while release of growth hormone is promoted. During our waking hours we can do a great deal to minimise stress. In recent years, the practice of Mindfulness has been proven to be effective in treating clinical disorders including anxiety and depression. It is a technique derived from Eastern meditative practices in which the aim is cultivation of a calm, non-judgmental awareness of one’s present physical and mental state.   Accumulating evidence indicates that this mental state is the optimum state for individuals such as US Navy Seals for whom remaining calm and focussed under intense pressure is crucial. Some studies show that Mindfulness lowers cortisol levels, while other studies have found evidence of beneficial reduction in stress and improved sleep but did not observe significant reduction of cortisol levels. Mindfulness is a knack that can be acquired by practice. Although the evidence for its effectiveness is still preliminary, my own experience is that it is effective in lowering mental and physical tension. I practice it at any time when I feel pressure is building, and also experiment with employing it while running to promote a constructive focussed mental state.

5)      Fuelling before and during training is a debateable topic. Some evidence indicates that training in a fasting state leads to improved endurance performance, perhaps mediated by the development of increased capacity to utilise fat as fuel, but overall the studies have yielded mixed results, as I have discussed in a previous post. I suspect this is because training in a fasted state also promotes increased cortisol levels that might be harmful. I have made appreciable gains in fitness in the past following training in a fasted state, but suffered one of the few serious muscle strains I have ever experienced after three weeks of high volume training predominantly in a fasted state.   This is mere anecdote, but when combined with the mixed evidence from scientific studies, leads me to conclude that training in a fasted state should be done cautiously, ensuring that overall stress levels are not excessive.

6)      Long term nutrition.  In light of the mechanism by which the hypothalamo-pituitary axis (HPA) adjusts cortisol levels in order to maintain metabolic homeostasis, it would be expected that a diet that promotes healthy energy metabolism would also be expected to promote healthy regulation of cortisol. As discussed in several of my recent posts, there is growing evidence that a Mediterranean diet promotes healthy metabolism. In accord with this, the available evidence indicates that a Mediterranean diet does promote healthy regulation of cortisol. For example a study of Spanish women found that those who chose a dietary pattern closer to the Mediterranean diet, with high mono-unsaturated fatty acid intake, showed more stable regulation of cortisol by the HPA.

 Conclusions

The evidence obtaind by Skoluda indicating that endurance athletes suffer sustained elevation of cortisol suggests that taking steps to maintain healthy regulation of cortisol is likely to result not only in a better response to endurance training but also in better long term health. This might be achieved by avoidance of over-training, ensuing good recovery, incorporation of some resistance training into the schedule and a number of life-style adaptations including adequate sleep, stress reduction via strategies such as Mindfulness, and a healthy diet, such as the Mediterranean diet.