More than a million people of nearly all ages lace up their trainers and hit the pavement in a marathon every year. And while running any distance has numerous health benefits, researchers warn that going the distance may not be so good for the brain – with the negative effects lingering for a month following a race.
Scientists at Spain's University of the Basque Country (UPV/EHU) have identified a link between marathon running and a sharp reduction in the protective myelin that surrounds nerve fibers – axons – in the brain. This insulating layer is critical for facilitating the transmission of electrical signals in the brain and spinal cord – and a loss of myelin is one biomarker of many neurological conditions, including stroke and multiple sclerosis.
So what does running long distance have to do with this? Well, when the body's go-to energy sources – like glycogen, which is stored in muscles and the liver – are all used up, it'll switch to burning fat – lipids – for fuel. And myelin is 70-80% lipids. Essentially, marathon runners are burning through some very important fat in their brains as they race to the finish line.
In this observational study, the researchers examined MRI brain scans of runners before the race and then again within 48 hours of completing the grueling 26.2-mile (42.195-km) route. They found that in a dozen areas of the brain, in regions associated with motor coordination, senses and emotions, myelin had been noticeably depleted following a race.
But there's some good news – further scans showed that myelin had naturally increased two weeks after the run, and then had returned to normal levels after two months. And, interestingly, it was only specific areas of the brain where myelin dramatically thinned; elsewhere, the axon sheaths were untouched.
And even though this myelin known as myelin water fraction (MWF) has "water" in its name, the depletion in these 12 areas had nothing to do with a runner's hydration levels.
"Our findings show that running a marathon reduces runners’ MWF levels in white matter areas, with a similar impact in both hemispheres," the researchers noted. "MWF values recover thereafter and reach pre-run values two months after the event. This reversible reduction in MWF upon prolonged exercise and recovery after lowering physical activity strongly suggests changes in myelin structure and content that may open up a new view of myelin as an energy store ready to use when common brain nutrients are in short supply. We define this process as metabolic myelin plasticity."
Scientists are unsure as to what – if any – issues this change in the brain has in the two months following a marathon, but it provides exciting new avenues to understand just how such endurance sports and cognitive function are potentially linked. And more work is needed – as this study only looked at the brain scans of 10 runners (eight male), aged 45–73 years – and it offers an association, but not causation.
"This pilot observational imaging study has limitations, including a relatively small number of participants that will require validation in a larger cohort, difficulty in assessing grey matter MWF changes due to relatively low myelin content that compromises the signal-to-noise ratio and limited spatial resolution due to long scanning times and the inherent slight movement of subjects," they wrote about their study. "Nonetheless, the potential relevance of the findings described here to brain energy metabolism warrant additional studies."
Rest assured, there is no evidence that running a marathon is harmful to either short- or long-term cognitive function. Though some people (not us!) might joke that runners couldn't possibly be thinking straight to want to enter such a challenging race to start with.
The study was published in the journal Nature Metabolism.
Source: University of the Basque Country via Scimex
