Cortisol is the primary member of a family of glucocorticoids, and is considered the main catabolic hormone. Corticosterone is the other glucocorticoid, but is thought to be much less potent than cortisol (accounting for approximately 4-5% of total glucocorticoid activity). Cortisol is made and secreted from the adrenal cortex, via the hypothalamic-pituitary-adrenal (HPA) axis, with a small amount also derived from the conversion of cortisone. The catabolic effects of cortisol can result in decreased protein synthesis and increased protein degradation. Chronically high cortisol levels have been linked to various stressors (trauma, depression, overtraining); thus, this glucocorticoid is also considered one of the primary stress hormones. In blood, greater than 90% of this hormone is bound with plasma proteins, mainly with cortisol binding globulin and the rest with albumin, with the remaining fraction circulating freely.
The amount of cortisol present in the serum undergoes diurnal (having a daily cycle or occuring everyday) variation, with the highest levels present in the early morning, and the lowest levels present late in the evening (generally 3-5 hours after going to sleep). It has been shown that stress or disease-induced increases in plasma cortisol and corticosterone results in lower testosterone secretion from the testes. It has also been shown that cortisol downregulates IGF-1 mRNA levels, implying that some of the catabolic effects of glucocorticoids is due to reduced autocrine /paracrine expression of IGF-1. Cortisol levels within normal ranges cause numerous actions which help restore homeostasis after stress. When cortisol levels are chronically high problems can occur.
In general cortisol increases with increased duration of intense exercise. Hypertrophy lifting schemes cause a greater cortisol response than neuronal lifting schemes. A hypertrophy scheme performed among trained females revealed a much greater stress response than a neuronal scheme. Dynamic power schemes have also resulted in hyper cortisol levels, although not to the same extent as hypertrophy schemes. On average, dynamic power schemes have resulted in a 20% increase in blood cortisol whereas hypertrophy schemes have resulted in a 45% increase. Neuronal schemes have not produced any change in cortisol levels across exercise. Some people suggest measuring testosterone/cortisol ratios can be used as an indicator for overtraining. According to this suggestion an elevated cortisol level in relation to testosterone suggests overtraining (overtraining is a word used too often in this industry. What is overtraining to one individual may not be to another. Different people define overtraining differently.). A study looking at endurance trained cyclists showed that decreased testosterone levels, increased cortisol levels, and a decreased testosterone to cortisol ratio does not automatically lead to a decrease in performance or overtraining. There are probably many high level athletes who have elevated cortisol and depressed testosterone that function fine. In my opinion the importance of the cortisol to testosterone ratio is overemphasized and is only one factor that should be considered when designing or evaluating athletes’ programs.
One study found that a greater cortisol response was observed among untrained males, compared with trained males, after a lower body workout. On average trained males exhibit a 35% increase in cortisol in response to a hypertrophy training scheme, while untrained males exhibit a 48% increase. In contrast, many studies have found no differences in cortisol response between the two groups. Studies have also found little to no change in exercise-induced cortisol response when comparing previously untrained males with recreationally trained males.
Once your body becomes familiar with a training regimen the cortisol response is lessened. A popular statement often made by the mainstream fitness media is You shouldn’t train over an hour because cortisol levels will be so high you will lose muscle, strength and performance will suffer. Let’s put that myth to sleep and never have to discuss it again. Many world-class athletes train multiple times per day in 2-3 hour blocks. My current training regimen consists of multiple sessions per day, 5-7 days per week (drug free). I am in the best condition of my life. I have had a few of the most conducive training sessions of my life after 37 hour fasts (more than likely my cortisol levels were relatively high). It would be very hard for athletes to fit in appropriate amounts of skill and conditioning work if they were limited to one hour training blocks.
Thyfault and colleagues looked at two nutritional interventions (carbohydrate or placebo), taken 10 minutes prior and immediately after a bout of resistance exercise. A similar increase in cortisol occurred in both groups. Numerous other studies have also found little or no differences in the cortisol response to resistance exercise, performed with (carbohydrate and/or protein) or without (placebo) supplementation. However a study by Tarpenning and colleagues found that a carbohydrate solution lowered the exercise induced cortisol response. The study investigated the influence of carbohydrate consumption on the acute hormonal response, and chronic adaptation to weight lifting exercise. The authors concluded that the modification of the cortisol response associated with carbohydrate ingestion can positively impact the skeletal muscle hypertrophic adaptation to weigh training. A study conducted by Bird and colleagues investigated the influence of nutritive interventions on acute hormonal responses to a single bout of resistance exercise in untrained young men. The main goal was to determine whether the acute hormonal milieu conducive to enhancing skeletal muscle hypertrophic adaptation to resistance training could be created. The potential role of cortisol in inhibiting training-induced muscle growth was of particular interest, as was whether exercise-induced cortisol release could be attenuated by nutritive interventions. After a 4 hour fast, 32 subjects performed a single bout of resistance exercise (approximately 60 min), during which they consumed a 6% carbohydrate (CHO) solution, a 6-g essential amino acid (EAA) mixture, a combined CHO+EAA supplement, or a placebo beverage. The researchers concluded that CHO and/or EAA ingestion during a single bout of resistance exercise suppresses the exercise-induced cortisol response. The primary research data concerning nutrition and exercise induced cortisol response seems to be conflicting. In my opinion if you are performing well and reaching your goals there is no reason to be overly concerned about the cortisol response induced by exercise. In my next article we will address cortisol’s effects on bodyfat.