Laboratory vs. The Long-Haul: Cortisol, You Sneaky Bast*#d!
Training progress takes time. Progress takes repeated brutal efforts with consistent nutrition and adequate recovery over the course of months and years. The temporal window through which researchers peer when using small muscle mass acute exercise tests (often using untrained subjects—which I’ll address in Part 2) may not predict long haul gains (for various, fairly obvious reasons such as uncontrolled dietary changes, level of training effort, general variability among trainees (25), etc.). One dastardly culprit clouding the picture may also be cortisol (the “sneaky bastard”), a catabolic/proteolytic stress hormone (26) antagonistic to the anabolic effects of insulin and growth hormone (27-29) that also gets released during exercise simply for fuel handling (carb, fat, and protein oxidation).
The harder you push yourself, the heavier the load (30). Also, the more work you do in a given amount of time (31-33), the greater the cortisol release. This means that a brutal 90-minute training session while blasting Iron Maiden in your garage gym just might elicit a bit more cortisol release than a 45- to 60-minute workout guided by an anonymous, lab coat-donning graduate student counting your reps in tempo with a metronome. On the other hand, research (and common sense) suggests that keeping cortisol levels under control, both post-exercise—which actually occurs during the course of positive training adaptation (34)—and in general (“resting levels”), may be vitally associated with training progress (35, 36). In other words, living with high stress doesn’t help you grow.
Unfortunately, using acute post-workout cortisol release to gauge how an RS impacts recovery may be complicated by the fact that the macronutrient composition of the RS itself can cause cortisol variation. Consuming an extremely high protein meal or a protein-only meal raises cortisol (37, 38), whereas consuming carbs will reduce blood cortisol levels (38, 39). Not surprisingly, post-resistance exercise RS blends elevate (31) or have no effect (33, 40) on cortisol, while intra-RS also had no effect(41) and reduced cortisol post-workout (42). In the context of this mish-mash of results, it seems that the uncertainty about whether carbohydrates are a worthwhile addition to peri-workout RS (8, 9) rests primarily with a single acute study that lacked cortisol measurements. However, it demonstrated that adding 50 grams of carbohydrates to 25 grams of protein to a post-exercise RS had no effect on muscle protein breakdown or synthesis during the first three hours post-exercise(43).
More revealing to me, however, is a training study showing that repeated consumption of an intra-workout RS that reduces cortisol during and after exercise (42) also translates into less muscle breakdown (44). Most importantly, it translates into greater muscle growth, week by week. In fact, the muscle gains in this study correlated with the attenuation of the cortisol response over the course of training (45). In other words, the less cortisol was elevated, the better the subjects grew. The difficulty in connecting cortisol elevations with an immediate impact on protein metabolism (i.e., when the experimental design only includes short-term measurements) may lie in cortisol’s relatively slow proteolytic effects, which can take hours to manifest (46) and be imperceptible when using the protein tracer methodology typically employed to measure metabolism in acute exercise studies (27, 47, 48).
Still, it is true that other research has shown that resting cortisol levels many hours after your last training session predict muscle fiber growth (49) and strength performance (50) over the course of long periods of training. In one study where a carb-based RS essentially eliminated exercise-induced cortisol release (34, 51), most of the variability in muscle fiber growth among subjects could be explained simply by “cortisol control.”
This is not to say that training should be easy just so that you can keep “the sneaky bastard” at bay. On the contrary, in a recent resistance training experiment (no RS was given), generating greater post-workout cortisol (testosterone and IGF-1 responses were statistically irrelevant) meant better muscle growth (52). This was likely because, as I mentioned above, the harder you train, the greater the cortisol release. On the other hand, the subjects who grew best were obviously also able to recover between training session.
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