Staying fit is a Sisyphean endeavor—as soon as you stop, the boulder starts rolling down the hill. Lie in bed for 20 days, one classic bed-rest study found, and your aerobic fitness will drop by 28 percent. Other effects like impaired glucose tolerance start after as little as 10 days.
But what about the much-vaunted effects of exercise on the brain? Do they fade away too? That’s what a recent study in Frontiers in Aging Neuroscience, from researchers at the University of Maryland, set out to discover.
The study involved a dozen hardcore masters runners (though only nine completed the full protocol and were included in the analysis), with an average age of 61. They’d been running for 29 years, on average, and were doing 37 miles per week on average. For the study, they agreed to (gasp!) stop running completely for 10 days.
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Before and after the 10-day rest, they underwent a specialized MRI scan to measure blood flow to various regions of the brain. They also completed a brief cognitive test called a verbal fluency test, in which they named as many animals (or fruits) as they could in 60 seconds.
The blood flow results were clear, showing decreased blood flow to several regions of gray matter in the brain, as well as to the hippocampus. The hippocampus is particularly important, lead researcher J. Carson Smith pointed out in a press release, because it plays a key role in learning and memory, and is one of the first brain regions to shrink when Alzheimer’s strikes.
Here’s a look at the individual changes in blood brain flow in the right hippocampus (the left was similar):
What about cognitive performance? Despite the decreased blood flow, the researchers didn’t find any obvious differences. Before the rest period, the subjects came up with an average of 19.9 words in the verbal fluency test; after the rest period, they came up with 17.4, a difference that was not statistically significant (though that doesn’t rule out the possibility of a significant difference in a study with more subjects).
In reality, the consequences of decreased blood flow are more likely to be something that builds up over a long period of time, rather than an acute problem.
The study can’t tell us exactly what’s happening in the brain during the rest period, but the researchers point out that increases in blood flow to the brain when people get fit seem to be linked to a denser web of tiny blood vessels to distribute blood throughout the brain. It’s possible, then, that decreased blood flow would correspond to the disappearance of some of this network of vessels.
Anyway, this shouldn’t be construed as a warning against ever taking time off running! Go ahead and take a week or two off—but let this study be a reminder to start again after your break.
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