Cortisol is a steroid hormone that's produced by the adrenal gland and released in response to stress. Cortisol levels tend to be higher in athletes than in non-athletes.

Cortisol plays an important role in how the body responds to training and competitive load.

When under stress, the body releases cortisol into the bloodstream to help deal with the situation at hand. 

Anticipatory competitive stress may also spike cortisol in correlation with the level of competition and its social relevance, with veteran athletes typically handling the stress better.

There are three major ways that athletes’ performance is acutely influenced by increased cortisol levels; 

• higher values increase muscle breakdown by suppressing protein synthesis, 
• release stored fat for utilization, and 
• help muscles use stored glucose more efficiently so they don't tire as easily during exercise. 

When we're talking about cortisol levels in athletes, there are two factors that we need to consider: too much and too little. 

Constantly high cortisol for extended periods will lead to fatigue, and it can cause weight gain through visceral fat accumulation, decreased muscle mass and insomnia. 

Cortisol suppresses insulin sensitivity—meaning that if thereäs too much of it in circulation in the body, the body won't be able to process sugars properly, and will become resistant to insulin over time. This leads to more food ending up being stored as fat instead of being used as energy.

Not enough cortisol can cause its own set of problems: low energy levels and difficulty recovering from workouts or competitions. 

Athletes who are serious about their training should get their cortisol levels regularly tested in order to analyze their body’s response to their training regime and to form a comprehensive understanding of what the body needs to perform optimally.

Cortisol monitoring helps validate that the immediate cortisol spike post-exercise is sufficiently high to trigger positive adaptation, and that over long-term, cortisol levels are low enough as not to signify overtraining syndrome, or other adverse effects of long-term high cortisol.