Cortisol: Catabolic Muscle Killer Or Misunderstood Hormone Written By Layne Norton

A significant portion of the column has been dedicated to myth busting and challenging the status quo of what we ‘know’ about bodybuilding.


One of the accepted ‘truths’ that I’ve always had a problem with is the notion that if you train for more than an hour, you will become catabolic due to increases in cortisol.

Cortisol is a catabolic hormone secreted by the adrenal glands. During periods of stress, cortisol is secreted to help the body cope with stress and mobilize fuel stores.

Obviously, bodybuilders want to avoid cortisol at all costs since it is catabolic right? While this seems very simple, the truth of the matter is actually much more complex.

What is cortisol?

Cortisol is a catabolic steroid hormone that acts as a transcription factor and affects muscle tissue balance by increasing protein breakdown and decreasing protein synthesis (1). In the 1990’s muscle magazines made quite a stink about not training more than an hour due to increases in cortisol.

However, I have thought of this as an overblown concern for a long time for a few reasons:

  1. Protocols much shorter than 1 hour have also been shown to increase cortisol if they are intense enough (2,3).
  2. While cortisol increases in response to an intense exercise bout, so do testosterone, IGF-1, and Growth Hormone
  3. I followed the mantra of ‘do not train for more than an hour to limit cortisol release’ and had limited success. When I later switched to a higher volume routine which lasted well past an hour I saw much better gains.
  4. There is nothing magical about the 60 minute time point. It’s not as if cortisol sits at baseline and then suddenly spikes and maxes out immediately at 60 minutes. It’s likely that cortisol release is much more so related to the stress applied to the body from the intensity of the workout than it is duration of the workout. I’m quite sure you’d see a much greater cortisol release from doing a single set of squats to absolute failure than you would from doing 5 lb dumbbell curls for an hour.
  5. Cortisol is not like insulin which is a hormone that binds to receptors on the cell surface and almost immediately causes changes in signaling that produce a rapid response. Cortisol operates at the DNA level as a transcription factor which binds to specific sequences of DNA to either increase or decrease their rate of transcription to mRNA. It takes much longer for those mRNA transcripts to build up and cause some sort of effect than it would for a hormone that functions more acutely like insulin. Furthermore, just increasing or decreasing mRNA accumulation is not assured to produce an outcome as the translation from mRNA to their constitutive proteins are also highly regulated (1).

Breaking it down

Now I realize that last point has you scratching your head if you aren’t a science major, however the major point to be made is that cortisol needs a much longer time course of action in order to exert an influence than many other hormones. It is because of this that I have always thought transient increases in cortisol produced by exercise were mostly inconsequential and not catabolic.

Rather, it is the long term, sustained elevations in cortisol produced by stress and disease that are likely to result in catabolism.


The Science

Not only did I believe that short term exercise induced increases in cortisol weren’t problematic, as I examined exercise data more and more over my time in graduate school, I noticed an interesting trend. Many of the exercise protocols that produced the greatest amount of muscular growth, also seemed to increase cortisol the most! So when Dr. Stuart Phillips, an outstanding researcher from McMaster University in Canada published his latest paper on this very topic, I was extremely interested. Dr. Phillips examined a large group of resistance trainers and correlated the increases in lean body mass, muscle hypertrophy (assessed by changes in muscle fiber cross sectional area), and strength (assessed by leg press) to hormones such as testosterone, IGF-1, growth hormone, and cortisol (3). What his lab discovered will likely shock you.

They found that increases in lean body mass and hypertrophy of Type II muscle fibers were most closely associated with cortisol! That’s right, muscle growth was mostly associated with cortisol, not testosterone, not growth hormone, not IGF-1, but cortisol!

Summing it up

I want to be clear, I am NOT saying that cortisol is anabolic. This was only a correlation study and correlation does not equal causation. More likely what this data represents is that the type of resistance training protocols that produce the greatest hypertrophy also cause the greatest cortisol release, most likely due to the increased acute stress response. Furthermore, the authors of this paper also suggested that cortisol was secreted in order to help with fuel mobilization as it can increase gluconeogenesis and fatty acid mobilization (1,3). At the very least this data confirms that it is time for bodybuilders to stop focusing on short term increases in cortisol when designing programs.

Train past the 60 minute mark with no fear my friends.

Author: Layne Norton, BS Biochemistry, PhD Nutritional Sciences


  1. Garrett RH, Grisham CM. Biochemistry, 2nd Ed. Orlando, FL: Saunders College Publishing; 1999.
  2. Kraemer WJ, Fleck SJ, Dziados JE, Harman EA, Marchitelli LJ, Gordon SE, Mello R, Frykman PN, Koziris LP, Triplett NT. Changes in hormonal concentrations after different heavy-resistance exercise protocols in women. J Appl Physiol. 1993; 75:594-604.
  3. Kraemer WJ, Dziados JE, Harman EA, Marchitelli LJ, Gordon SE, Mello R, Frykman PN, Koziris LP, Triplett NT. Effects of different heavy-resistance exercise protocols on plasma beta-endorphin concentrations. J Appl Physiol. 1993; 74:450-9.
  4. West DWD, Phillips SM. Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training. Eur J Appl Physiol. 2012; 112:2693-2702.