Endurance Athletes Bath in Cortisol: Dose-Dependent Elevations in Hair Cortisol of Triathletes and Runners.

Image 1: There are two types of endurance exercise - the one that is healthy for obese diabetics and the one that chronically stresses and, in the worst case, eventually kills healthy but overambitious hobby athletes; the photo shows a collapsed runner at the 2007 London Marathon, where 22 of his comrades died
(source thisislondon.co.uk)

In view of the fact, that the series of blogposts on High Intensity Interval Training (HIIT) I made in the course of the last weeks were pretty popular, I thought it may also be of interest that there are interesting new results on the effects of resistance training which goes beyond the common "we took 12 obese sedentary post-menopausal women and had them ride a stationary bike for 45min three times a week" approaches, as well. One of these studies happens to come from a group of German scientists from the Universities of Dresden, Marburg and Hamburg (Kirschbaum. 2011); and the results underline my previous comments on the difference between endurance exercise as it is understood by the medical orthodoxy (and as I described it in the previous sentence) and endurance exercise as many hobby athletes and fitness enthusiasts define it. While the former is unquestionably beneficial for obese diabetics or anyone else who would otherwise sit on the couch watch TV and stuff himself with potato chips and ice-cream, 3x45 min of riding a bike probably won't improve the physical condition of a reasonable conditioned hobby athlete. Now, the Kirschbaum study shows quite convincingly that the opposite extreme, i.e. jogging / running and training loads way beyond 40km per week, comes with another certainly more annoying sting in the tail: chronic stress.

In their study, Clemens Kirschbaum et al. analyzed the hair samples of 304 amateur endurance athletes (long-distance runners, triathletes and cyclists; 190 females, 114 males,  mean age ~38 years) and 70  active control subjects (all recruited at local sport events or from friends and family of the authors ;-) and found on average +42% higher hair cortisol concentrations in the endurance athletes.

Figure 1: Relative increase in hair cortisol levels in endurance athletes compared to controls (Kirschbaum. 2011)

As figure 1 goes to show the increase and, more specifically, it's statistical significance largely depended on the type and the duration of exercise. While both cycling and 10k runs increased cortisol levels by about +36%, in this group there were too many "outlayers", i.e. persons with either much higher or much lower cortisol levels, for this increase to reach statistical significance (as defined by a p-value of p<0.05, meaning that chances that this observation happens to be mere coincidence are <5%). In the study participants who stated that they were running half-marathons, triathlons and marathons, on the other hand, the increases in cortisol (+36%, +48% and +66%) were statistically significant, and in case of the marathon runners even dead (consider this a "forerunner" of what may befall marathon junkies) certain.

Figure 2: Relative increase in hair cortisol levels of endurance runners in relation to average weekly training load in kilometers (calculation based on a regression with r=0.32, indicating a below average precision; Kirschbaum. 2011))

If we disregard any reservations concerning the general validity of hair analysis as long-term marker of cortisol levels (recent studies like Manenschijn. 2011 a.o. would suggest that they are valid), Kirschbaum et al. are thusly right to conclude that their data suggests ...

that repeated physical stress of intensive training and competitive races among endurance athletes is associated with elevated cortisol exposure over prolonged periods of time.

Even more important is their advice that, due to the possibly important implications of these findings, it would be necessary to study potentially detrimental effects on the somatic and mental health of the athletes in the future! Well, I probably don't have to tell you that the SuppVersity is going to be the place, where you will read about those studies first ;-)