A new Nature paper has confirmed a finding first hinted at in a 2024 pilot study: when distance runners cross the marathon finish line, the levels of myelin in several regions of their brain drop measurably, and they recover those levels over the following two months. Myelin is the fatty sheath that insulates the long-distance nerve fibres responsible for fast signalling, and the implication is that the brain, like muscle, is willing to tap into its own structural stores once carbohydrate and lipid reserves run low at the back end of a hard race.
The study, led by the Achucarro Basque Center for Neuroscience and the University of the Basque Country, ran high-resolution myelin water imaging on ten amateur marathoners 48 hours before their event, again within 48 hours of the finish, and at two weeks, eight weeks and six months afterwards. The largest reductions appeared in white matter tracts that connect the motor cortex to the brainstem, with smaller but measurable drops in regions associated with emotional processing. The researchers stress that no runner showed any clinical change, and that levels returned to baseline well within the six-month window.
For the running audience, the result reframes a debate that has run for two decades about whether prolonged endurance work is a stressor or a reward for the brain. The Nature paper sits alongside a decade-long heart-imaging cohort from Madrid that found marathon-induced reductions in right-ventricular function recover within days with no cumulative scarring across ten years of racing. Both lines of work converge on the same message: the marathon imposes transient stress on tissues you would not expect, and healthy adults appear to be remarkably good at repairing it.
The likely metabolic story is that the brain begins to break down myelin lipids for ATP production once glycogen and free fatty acids dip below a usable threshold, mirroring what muscle tissue does during the same period. The Basque team is now collaborating with a Norwegian Olympic-Committee group on a follow-up study that scans ultra-runners across 100-kilometre races and compares the magnitude of myelin loss with race-duration and bonk severity. Preliminary results from a sub-set of UTMB finishers point to larger and slower-recovering reductions in 100-mile runners than in marathon finishers.
Practical guidance for runners from the paper is intentionally limited. The authors stop short of recommending nutritional changes, and they note that the recovery window of two months sits comfortably inside the gap most amateur marathoners take between their two annual races. The clearer training implication is for back-to-back hard events: the paper suggests that runners who race a spring and an autumn marathon are probably operating with a fully restored myelin profile by the time the second start line arrives, but those running a marathon-then-ultra block inside eight weeks may want to lean more cautiously into the recovery period than the muscle-soreness clock alone implies.