A swathe of research on cognitive function and exercise suggests that the benefits of physical activity can have profound advantages for your brain. Research published in Comparative Physiology, for instance, has noted that exercise, in conjunction with a healthy diet, may help to prevent neurological and cognitive disorders. What’s more, the Centers for Disease Control and Prevention reports that physical activity can be a boon for problem solving, emotional balance, learning and memory.
However, a new study published in Nature Metabolism has results that seem to contradict those prior findings. According to researchers, marathon running can deplete a crucial brain substance for motor coordination, as well as sensory and emotional regulation.
Although that might sound alarming, here’s the good news – this process may actually benefit your long-term brain health.
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What did the research involve?
Researchers did magnetic resonance imaging (MRI) on 10 runners aged 45 to 73, both 24 and 48 hours before and 24 and 48 hours after they completed a marathon. They also did an MRI on two of these participants two weeks after the marathon and on six of these participants two months after the marathon. (Keep in mind that this was a small study!)
The researchers found a substantial reduction in myelin water fraction, which, according to Carlos Matute, the study’s co-author and a researcher in the department of neurosciences at the University of the Basque Country in Spain, is an indication of overall myelin content.
Myelin is a fatty substance that insulates nerve fibres in the brain and spinal cord. It also makes up about 40% of the brain’s white matter, which is the type of tissue in the brain responsible for nerve signalling, enabling motor control, cognition, learning, memory and sensory perception.
In addition to prompting faster transmission of nerve impulses, which can help you to move more quickly or learn with better focus, myelin plays a role in helping to convert glucose into energy for the brain. This is incredibly important, considering the brain needs a high amount of energy for all those nerve signals – especially when you’re a running marathon.
But why does the brain turn to myelin during extreme exercise like marathon running? For energy. As explained in the research, marathon runners rely on carbohydrate, which is stored in the body as glycogen, as their main energy source during a race. When these glycogen stores run out, the body turns to fat as fuel. Similarly, when your brain runs out of glucose, the researchers hypothesise that it turns to myelin lipids for energy.
Should marathon runners be concerned about the findings?
The new findings may sound scary in light of previous studies, which have shown that lower myelin content in the brain is linked to cognitive decline – especially in areas related to verbal fluency and executive function.
However, runners should not worry about the depletion of myelin content. In fact, Matute says that we should instead consider myelin breakdown a perk of endurance exercise that optimises how your myelin functions. Plus, as shown by the new study’s results, myelin regenerates within a couple months anyway – and sometimes, within two weeks.
‘This should not at all be seen as a negative result,’ explains Matute. ‘That’s because it’s likely that using and replenishing these myelin lipids is beneficial, because it exercises the brain’s metabolic machinery. Also, any alterations that do occur are very subtle and, in our research, we found that they’re reversible.’
In other words, the brief myelin reduction caused by marathon running could resemble how muscles react to strength training – muscle fibres break down and get depleted of muscle glycogen, which then optimises function in the long run. You can also mitigate the extent of that depletion – since overdoing it in strength training can lead to longer recovery times and, occasionally, non-reversible effects – through effective pre- and post-workout fuelling. As Matute says, that may be the case with your myelin as well.
‘In principle, taking on carbohydrate and fuelling well help to sustain the effort may possibly reduce the amount of myelin lipids used,’ adds Matute. ‘In our research, some participants took carbohydrate during the marathon and some didn’t fuel [at all] – and there didn’t seem to be any differences between them. But we need to examine this in the future.’ For example, eating more carbohydrate during or after a marathon might shorten the duration of myelin reduction, but that’s a question for future research.
Another variable that wasn’t investigated in this study is running speed. So, to flesh out their findings, the researchers are now exploring whether running faster – particularly if you’ve fuelled inadequately – might exacerbate the reduction in brain myelin.
However, even if factors like this are shown to change the timeframe of myelin reduction, Matute says that they’re still only likely to be minor considerations.
‘The fact is that brain myelin reduction is rapidly reversed,’ he explains. ‘Also, the biggest takeaway here is that this is just one more way that an activity like marathon running can help your brain function. All the researchers are runners – I’ve done 18 marathons myself – and none of us plan on stopping.’
