Cortisol is a hormone that is released from the adrenal gland in response to stress or other chemical signals. Because exercise forces the body to deviate temporarily from homeostasis (natural set-point), it is perceived as a stress and causes the release of cortisol. However, regular exercise training will decrease this effect, causing the body to have a better response to stress and require less cortisol release.
Cortisol is a glucocorticoid, also known as hydrocortisone, which is released from the adrenal gland when stimulated by stress, immune regulation or to regulate the sleep-wake cycle. Because cortisol is one of the body's natural reactions to stress, the main functions of this hormone involve preparing for "fight-or-flight." The body perceives moderate to intense exercise as a stress and will release cortisol during a typical training bout. Cortisol increases available fuels by stimulating gluconeogenesis (the production of new glucose) in the liver, increasing storage of glycogen (the form in which the body stores glucose, or sugar) and by inhibiting the action of insulin, preventing glucose uptake into the muscle and increasing blood glucose. Additionally, cortisol increases protein breakdown in the muscle and fat in the adipose tissue; both processes deplete stored energy and release fuel into the bloodstream for quick use.
Cortisol is a potent anti-inflammatory agent, which is why athletes have been known to use local cortisone injections to treat persistent injuries. While the mechanism for this is complex, cortisol mainly functions by inhibiting the production and release of inflammatory molecules, the ones responsible for creating the reddening, warmth, swelling and pain at the site of injury. In large doses, cortisol may even have anti-allergy effects by acting as an antihistamine. In severe allergy attacks or anaphylactic shock, hydrocortisone may be given as a slow intravenous drip. Also, cortisol plays a role in regulating blood pressure by stimulating vessel constriction when necessary.
Unfortunately, the negative effects of cortisol outweigh the positive. Cortisol has an immunosuppressive effect, meaning that if your body constantly has high levels of cortisol, you are more susceptible to illness or infection. Also, because cortisol is a response to stress and the goal is to increase fuels in the blood, it will increase blood calcium by inhibiting bone formation and decreasing intestinal calcium absorption. This may result in a decrease in bone density over time. Cortisol also inhibits the pathway that releases sex hormones (gonadotropins), so if you are constantly stressed, you may experience a decreased libido and, in some cases, infertility or difficulty conceiving. Women who have high levels of cortisol in combination with low body weight may have amenorrhea (loss of menstrual cycle).
Because cortisol is released in response to stress, exercise training will increase the threshold of cortisol release. For example, if you begin an exercise program walking at a 20-minutes per mile pace, cortisol will be released at that intensity. However, as your training progresses and you begin walking at a 15-minutes per mile rate, the body will not perceive the 20-minutes per mile pace to be as stressful and will not release as much cortisol. Additionally, the time and intensity of exercise will dictate the level of cortisol release. If you exercise for more than 60 minutes, even at a low intensity, the body's glycogen stores (fuel) will decrease significantly and the increased stress will cause more cortisol release. The more training you do, the better your body will become at dealing with physical stresses and decrease the need to release cortisol. This effect is not limited to exercise; people who are regularly active show a decreased cortisol response to an emotional crisis when compared to sedentary controls.
Considerations for Disease
Regular intense exercise may increase total cortisol release far above the moderate training effect. If you are are at risk for certain diseases, you may want to consider limiting the time you spend participating in stressful exercise (either for longer than 60 minutes or increasing heart rate about 180 beats per minute). If you are at risk for osteoporosis, cortisol may decrease bone density. Also, if you have a weakened immune system, you may put yourself more at risk because of cortisol's immunosuppressive effects. Cortisol also increases vascular constriction and decreases insulin sensitivity, so if you are at risk for Type 2 diabetes and/or hypertension, you should consult a doctor before beginning a regular, intense exercise regime.