Marathon running causes demyelination, a known cause of conditions such as Multiple Sclerosis (MS)
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I am sure this study will earn me quite a bit of hate-mail, but the evidence against “endurance” exercise has been accumulating for years and is becoming very hard to ignore or dispute. Medicine even has a name for this controversy. They call it the “exercise-cortisol paradox“. Namely, they admit that exhaustive exercise raises cortisol and as such should be causing many health problems associated with elevated cortisol. However, medicine argues that the benefits of such exercise outweigh the risks so much that the elevation of cortisol is apparently not something to worry about, hence the “paradox” benefits of cortisol-inducing activities such as running long-distance. Of course, there is no actual “paradox” and, as has become customary for medicine, the word is simply a euphemism for lies/fraud, similarly to the word “controversial” (aka inconvenient truth). One of the most well-known effects of cortisol is its opposition to the effects of myelinating steroids such as pregnenolone, progesterone, DHEA, testosterone, DHT, etc as this catabolic hormone is just as good at shredding muscle and connective tissue, as well as directly killing neurons, as it is shredding lipid-based structures in the nervous system such as myelin. In view of cortisol’s de-myelinating effects, it is mighty ironic (and utterly stupid) to “treat” de-myelination conditions such as MS with this steroid. Yet this is exactly what medicine does to this day, as cortisol is a routine add-on treatment for MS, especially the more severe types such as PPMS and SPMS. As such, it is little wonder that MS cases are not only rising, but the severity of newly diagnosed cases is also increasing together with disability and death from the condition. Marathon running, which is what the study looked at, can trigger cortisol levels rivaling the increases seen after glucocorticoid administration for MS and other “autoimmune” conditions. Thus, the findings of the study are not at all surprising. What is somewhat surprising is that the study found that the de-myelination triggered by exhaustive running was reversible after the exercise ended. However, that recovery occurred only after two (2) full months had passed since the running event, and no other strenuous physical activity occurred. As such, it is not clear whether such recovery would occur in people who run regularly (e.g. 3-4 times a week), though for shorter distances/duration. In addition, the study did not look at changed in brain volume/mass as a result of marathon running, and there is plenty of evidence that exhaustive exercise can shrink the brain and directly destroy brain cells (likely through the elevated cortisol). The fact that the study admits regular exercise is a risk factor for developing the deadly neurological condition ALS, as well as kidney/GI/heart damage, should be enough of a warning sign for everybody that undergoing self-induced periods of extreme stress is highly unlikely to positively impact health, especially the nervous system.
https://www.nature.com/articles/s42255-025-01244-7
“…By contrast, strenuous physical activity is a risk factor for amyotrophic lateral sclerosis (ALS) in individuals with a genetic predisposition to the disease51,52. Therefore, endurance exercise may pose an additional danger to heavily myelinated disease-vulnerable motor areas (for example, corticospinal tract) in individuals with ALS genetic risk, as myelin itself and myelin-producing oligodendrocytes are also vulnerable to glutamate excitotoxicity53.”
“…Marathon running may seem like the ultimate sport for staying in shape. That said, it can take a heavy toll on the body, potentially damaging the kidneys, upsetting the stomach, increasing the risk of heart failure, and triggering musculoskeletal injuries. Let’s not forget that the original marathon runner, the ancient Greek soldier Pheidippides, is said to have dropped dead immediately after reaching his destination. Now, a new study suggests that marathons also affect the brain — causing the organ to eat itself to make up for fuel lost during the run. In the research, published March 24 in the journal Nature Metabolism, scientists observed declines in a key biomarker of myelin, a fatty tissue that insulates the connections between brain cells, in marathon runners just after a race. The study authors say this suggests the brain consumes myelin as an energy source under these extreme conditions.”
“…The team scanned 10 runners’ brains 48 hours before they ran a marathon and then again two days, two weeks and two months afterward. They used MRI, which can reveal the presence of water molecules trapped between myelin layers. The brain scans showed that, two days post-run, the MRI signals in 12 brain areas were depleted compared with before the race — in some cases, by up to 28%. Changes to the brain’s overall myelin levels were not statistically significant, however, suggesting that any changes were very isolated to specific regions. “The areas that we saw have more significant changes were those related to motor circuits and the center of emotional control of the brain,” Cabrer said. He suggested that this reflects the mental and physical effort the brain must maintain to get through a marathon. For any runners concerned about this cerebral cannibalism, there’s good news: After two months, all of the runners’ myelin measures had recovered to their baseline levels.”