A recent study of CrossFit training and inflammation by Tibana et al., 2015 was summarized by the media and the takehome message was two consecutive days of CrossFit training will suppress your immune system. This is a serious claim. We therefore have to ask if the actual study supports this phenomenon.
The Takehome: Even if we put the media’s interpretation of this study aside, the authors themselves do not interpret their data quite right. First, they look at IL-6 levels as a pro-inflammatory molecule. But it can also be anti-inflammatory (the anti-inflammatory action of IL-6 is in part due to inhibition of TNF-alpha and IL-1). The authors can’t tell in which capacity it is acting, so any changes they see are hard to interpret. Regardless, IL-6 levels return to baseline 24 hours after 1 WOD and also 24 hours after the 2nd day’s WOD. So, there is no elevation of IL-6 unique to doing 2 WODs on consecutive days. Similarly IL-10 which is pro-inflammatory did not even increase significantly after the 2nd day’s WOD. So, this study does not show an increase in inflammation that is unique to CrossFit WODs done on consecutive days. I should also note that a study like this, looking at only a few markers of inflammation, could never show “immune system suppression.” Beyond this, the immune system works in such a way that both reduced and increase activity can be beneficial depending on the specific location in the body. For example, we have a lot of evidence which supports a need for increased inflammation at sites of tissue growth and repair. Thus, even if a study were to show cumulative increases inflammation in CrossFitters during training (which this one does not), those increases may necessary in order to become more fit.
- 2 training days were administered. Each began with a Olympic lifting segment, followed by a Gymnastics segment, and finished with a metabolic conditioning piece in the form of an AMRAP (as many rounds and reps as possible).
- 9 CrossFitters were recruited (mean age of 26). They each had a minimum of 6 months of CrossFit Training.
- A variety of markers were measured in blood including lactate, glucose, IL-10, IL-6.
- Muscular power output during a back squat (5 reps at 50% of 1 repetition maximum) was also measured during each session using a linear position transducer.
Note: IL-6 is a pro-inflammatory cytokine (promotes inflammation) when secreted by immune cells, but it can also be anti-inflammatory (reduce inflammation) when secreted by muscle cells. IL-10 is a pro-inflammatory cytokine. The authors of the study do not clarify what specific effect IL-6 is having during training, so meaningfully interpreting the results below is not easily done:
- Immediately after WOD 1 and WOD 2, IL-6 increased significantly from baseline.
- 24 hours after both WOD 1 and WOD 2 IL-6 levels were no longer significantly elevated.
- Immediately after WOD 1, IL-10 increased significantly from baseline. IL-10 did not increase significantly after WOD 2.
- 24 hours after WOD 1, IL-10 levels were not significantly different from baseline.
- IL-10/IL-6 ratio decreased significantly after WOD 2 when compared to WOD 1, but again what does this mean when IL-6 can be pro- or anti-inflammatory?
- OPG concentration 48 hours after WOD 2 was significantly lower than before WOD 1, but no different from levels post-WOD 1 or immediately post-WOD 2.
- Mean change from baseline data was reported, but see limitations below on this.
- Peak power was significantly higher after WOD 2 than before WOD 1.
- A minimum of 6 months isn’t very specific. A participant with 6 months experience versus 6 years of experience will likely respond very differently to the WODs.
- Similarly, we don’t know how hard each participant executed the WODs. A score for the AMRAP and/or an RPE (rating of perceived exertion) would help confirm uniformity of the results.
- Although many CrossFit classes are composed of multiple pieces before the metabolic conditioning, this is not always the case. In fact, optimal CrossFit programming as espoused by the Founder, would, on days where there is metabolic conditioning, only have skill work beforehand, or nothing at all. The training sessions in this study are of higher volume.
- The authors are not able to distinguish if IL-6 is being produced by immune cells (pro-inflammatory) or muscle cells (anti-inflammatory).
- OPG is not a pure inflammatory molecule. It is also a signaling molecule involved in bone formation and repair and both low and high levels can be beneficial depending on the circumstance. The authors do not show how OPG is actually acting in this study.
- Statistically analyzing “mean change from baseline” data for cytokines when the individual values for those post-WOD data points were not different from the baseline is not a meaningful analysis.
- There were no female participants in this study.