It's a sad fact of life that our brains shrink as we get older, but there's one action you can take now to minimize how fast it happens: exercise.

New research shows that efforts made to stay in shape now now — particularly for people in their 40s — may help minimize the loss of brain tissue volume later in life. 

The study, created by researchers at Boston University School of Medicine, looked at the link between brain tissue volume in 60-year-olds and their activity levels when they were in their 40s. What they found was that the participants who were less physically active were more likely to have smaller brain tissue volume later on. 

Researchers tested the participants on a slow-speed treadmill when they were in their 40s, measuring diastolic blood pressure and heart rate to assess fitness levels. They then measured the brain tissue volume using MRI's and cognitive testing when the participants were in their 60s. 

Blood pressure and heart rate during exercise are are two good indicators of fitness. In general, the smaller the changes in the numbers, the more physically fit a person is. So a person who is not very fit will have higher spikes in diastolic blood pressure (the lower number) and heart rate throughout the day. 

For this study, the researchers did not identify exactly why people with higher fitness levels had less brain shrinkage, but it's possible that the increased oxygen delivery and better artery health that accompany higher fitness levels may also help to protect the brain from losing tissue volume. 

Unfortunately, there's not much any of us can do to stop the brain from shrinking as we get older, but the speed at which we lose brain tissue and the rate at which that atrophy is linked to cognitive decline can be slowed with greater levels of fitness. 

Need a better reason to hit the gym? 

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