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The Role of Cortisol in Concurrent Training
Rob Robergs, Ph.D. and Len Kravitz, Ph.D.

Many exercise enthusiasts and personal training clients strive to optimize muscular fitness gains with their fitness program. However, a number of clients refrain from doing cardiorespiratory training because they have been informed that the increases in blood cortisol during aerobic exercise will increase protein catabolism, resulting in the break down, or loss, of muscle. At fitness conferences, this very topic is regularly discussed and debated by fitness professionals and personal trainers.

This evidence-based article will aim to resolve many of the confusing issues with cortisol and muscular growth by discussing the newest concepts on how hormones work, the many functions of cortisol, the association of cortisol with aerobic exercise, and the possible catabolic affect of cortisol on muscle mass and muscular strength. From this review, practical applications for the fitness professional will be imparted to share with students and clients.

How do Hormones Work In the Body?
There are two main types of hormones; amine and peptide versus steroid. The biological differences between these hormone types are caused by a combination of their chemical structure and cellular mechanism of action.

Amine and peptide hormones are comprised of one or more amino acids (building blocks of proteins). As many amino acids have small electrical charges on some of their atoms, the molecules interact well with the small charges found on water molecules, and as such, can be dissolved in water. However, since cell membranes are mainly comprised of lipids, the amine and peptide hormones cannot cross cell membranes to get inside cells. Consequently, amine and peptide hormones bind to specialized protein receptors on the outside of cell membranes. Such binding causes changes in structure and/or charge distribution within the receptor, leading to the production of specific molecules that alter cell metabolism. For example, insulin released from the pancreas binds to an insulin receptor on target cells to increase the movement of glucose transport proteins to the cell membrane, thereby increasing glucose uptake into cells.

Steroid hormones are derivatives of cholesterol that are soluble in lipid and repelled by water. This means that to be transported in blood, which is largely water, they must be connected to proteins. Steroid hormones do not need a protein to bind to on the cell membrane to influence cell function. The steroid hormones pass into and through cell membranes where intracellular proteins aid in their transport to the nucleus, where they exert their functions. Steroid hormone functions are often related to increasing or decreasing protein synthesis.

Few hormones just do one thing, and often there is a need for hormones to interact with other molecules to exert their function. This is especially true for steroid hormones. For example, testosterone stimulates increased protein synthesis, but to do this requires the presence of molecules called somatomedins (also called insulin-like growth factors), produced in working skeletal muscle. This interdependence enhances muscle hypertrophy (growth) from strength training exercises.

What are the Functions of Cortisol in the Body?
Cortisol has multiple functions, one of which is to increase amino acid supply to the liver, and thus stimulating increased protein catabolism. However, the functions of cortisol are complex and simple interpretations can be misleading. For example, cortisol, like many steroid hormones, is released in a complex manner throughout the 24-hour daily cycle, with clear alterations in release caused by eating, sleeping and exercise. As the bulk of cortisol release and presence in the blood occurs during the sleeping hours, it could be argued that the daily profile of cortisol release may be more important for overall daily hormone regulation of body metabolism rather than its response to exercise. This makes research of hormone responses to exercise very difficult, as the acute response of cortisol may be totally different to the over-night release profile, which could be when cortisol has its most marked impact on the balance between muscle protein synthesis and catabolism. This will be discussed more in the next section.

What Function Does Cortisol Play with Aerobic Exercise?
When we exercise, regardless of whether it is strength training or aerobic exercise, cortisol is released in proportion to the intensity of the effort. Thus, even when you strength train, cortisol is released, and far more so than compared to aerobic exercise! With increases in exercise intensity, other hormones also increase, such as glucagan, adrenaline, noradrenaline and growth hormone. The concentration of other hormones in the blood, such as insulin, decrease during exercise. To accurately interpret the influence of hormones on body metabolism during and in response to exercise, you need to know how all hormones that influence a given metabolic function (eg. muscle catabolism) are responding. As previously explained, for some hormones, this may also require blood samples taken throughout the 24-hour cycle.

When concerned with prolonged aerobic exercise, cortisol clearly functions to preserve body carbohydrate stores. Cortisol increases alternate fuels for muscle, such as fatty acids and amino acids (from muscle amino acid stores and protein catabolism), impairs glucose entry into skeletal muscle, and supplies the fuels (amino acids) for the liver to increase glucose production (see Figure 1). All these functions are increased during times of low body carbohydrate nutrition, such as when blood glucose falls. Thus, when doing prolonged aerobic exercise, the muscle catabolic effects of cortisol can be diminished simply by maintaining blood glucose, which in turn is best done through the ingestion of carbohydrate (liquid and/or solid). Alternatively, when aerobic exercise is performed for durations short enough to not critically lower muscle or liver glycogen (e.g. < 45 min), then the exercise-induced cortisol release will most likely be irrelevant to muscle protein balance. Also, the theoretical metabolic effects of these increases in cortisol may be overcome by simultaneous increases in growth hormone, testosterone, and muscle specific somatomedins. The net result is the preservation of muscle mass.

What Else Could Explain The Difficulty to Increase Muscle Mass When Aerobic Exercise Is Added To Resistance Training?
This is the crucial question that needs answering. The additional explanations are two-fold. First of all, aerobic exercise adds a caloric expenditure to your training that needs to be matched with proper fuel intake. If insufficient carbohydrate is being ingested, which many individuals who strength train may experience due to diets higher in protein and low in carbohydrate, then these situations are ripe for generating the low carbohydrate conditions conducive to the action of cortisol as a carbohydrate sparing, protein catabolizing hormone. The problem here is not the aerobic exercise, but the inadequate carbohydrate nutrition!

The additional explanation lies in the counterproductive effects of aerobic training on the cell stimuli for muscle strength and hypertrophy. Although scientists do not know what these stimuli are as yet, muscle strength (and hypertrophy) gains may be inhibited if strength training occurs too soon after aerobic exercise (e.g. the same day). This has nothing to do with a hormone response mechanism, although it is easy to blame cortisol. Rather, we believe that the signals given to working muscle that induce either strength vs. endurance adaptations are diluted when combining strength and endurance (aerobic) training. This is like giving the muscle mixed messages. On the one hand we are telling the muscle to build more proteins for increasing mitochondrial mass (increasing the cell’s organelle involved in energy production) during aerobic exercise, and then we tell the muscle not to focus on mitochondria, but to now increase muscle contractile protein synthesis during resistance exercise. No matter what the order of the training, both exercise conditions may end up with a somewhat lessened training response.

Summary and Practical Applications
Here are some “take-home” messages from this article. First, do not view a single hormone response in the body to be interpreted to cause a hormone-related effect. Body metabolism is the balance of multiple hormone regulation in response to varying metabolic states. Second, adding aerobic exercise to a workout routine will not appreciably affect muscle strength and/or hypertrophy as long as carbohydrate intake is adequate. However, if a client is a competitive (or aspiring) body builder or athlete trying to maximize muscle growth, then careful thought should be given on how much and when aerobic exercise is done. Finally, for so many of our clients who wish to attain changes in body composition and body weight, adding aerobic exercise to their training programs will be an essential exercise program design component. Aerobic exercise can help these clients reach their goals while at the same time experiencing the improved health benefits of a balanced (strength, endurance, flexibility, etc.) fitness status.

Robergs R.A., Roberts S.O. Exercise Physiology: Exercise, Performance and Clinical Application. Mosby, St. Louis, 1997.
Griffin J.E., Ojeda S.R. Textbook of Endocrine Physiology. Oxford University Press, New York, 1988.
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