The scientific evidence demonstrating that moderate exercise brings numerous health benefits is very solid. However, the changes and consequences of extreme exercise, the practice of which has been increasing in recent years, are largely unknown. Extreme exercise generates much greater and longer-lasting haemodynamic and energy overload and stress on the body, even generating a transient pro-inflammatory period after exercise.

To study its impact on the brain, in this study a team of researchers performed a brain MRI on 10 marathon runners (eight men and two women) a few hours before, a few hours after and a few weeks after a race, focusing on determining a parameter that estimates the amount of myelin. Myelin is the coating that covers neurons and allows them to function properly. The findings suggest that during the race the amount of myelin is selectively reduced in certain areas of the brain, only to progressively return to normal between two weeks and two months after the race. This work adds to other previous studies that demonstrated acute and transient changes in the structure of the brain after exercise, generally a reduction in its volume.

This is a very striking and interesting piece of work that opens the door to future studies, but it should be considered preliminary. The causes of the reduction in myelin are unknown and, in particular, whether its quantity is reduced as a cellular strategy to generate energy and maintain its viability, as the authors hypothesise. But perhaps the most important thing would be to identify whether this reduction has a short- and long-term clinical impact. In the long term, the consequences should be studied, especially in highly trained athletes who manage to complete several marathons a year, potentially without recovering myelin levels to those prior to the race. At a cardiovascular level, for example, it has been suggested that after each episode of very high intensity exercise, microdamage could be inflicted on the heart and that its accumulation over the years could be the substrate for the development of arrhythmias in a few athletes.

The authors recognise the limitations of the study, perhaps the most important of which is the very small number of athletes included. Physical adaptation and the performance of strenuous exercise vary greatly between individuals and the small sample size of this study does not allow for this interpersonal heterogeneity to be taken into account. How variable is this reduction? Is it much greater in some athletes than in others? What impact does it have on those athletes with more significant reductions? Nor does it allow for the study of whether there are differences between the adaptation of men and women. Finally, training for and completing a marathon is a milestone that few ever achieve and it is worth studying whether similar changes occur, at lower intensities, in untrained people. In order to address these uncertainties, but also to confirm the conclusions, especially necessary when we talk about small studies, new studies with larger sample sizes are needed.