People who are not physically fit in their 40s may experience accelerated ageing of the brain by the time they reach 60, a new study has found.

"Many people don't start worrying about their brain health until later in life, but this study provides more evidence that certain behaviors and risk factors in midlife may have consequences for brain ageing later on," commented the study's lead author, Dr Nicole Spartano, of the Boston University School of Medicine in the US.

She and her colleagues followed the progress of over 1,200 people who had participated in an exercise test in the 1970s, when their average age was 41. Then in 1999, when their average age was 60, they started having MRIs (magnetic resonance imaging) of their brains. They also underwent cognitive testing.

None of the participants had cognitive or heart problems at the start of the study.

The researchers noted that when a person has poor physical fitness and then undertakes a low level of exercise, their blood pressure and heart rate responses tend to be much higher than people with good physical fitness.

The study found that those with low fitness levels in their 40s, or who had a bigger increase in blood pressure or heart rate measurements a few minutes after starting a low-intensity fitness test on a treadmill, had lower brain tissue volume later in life. Lower brain volumes are an indicator of accelerated ageing.

Furthermore, those who recorded a large increase in blood pressure during low-intensity exercise, performed worse later in life on a cognitive test designed to assess the ability to make decisions.

The study also noted that aside from exercise, people who had higher blood pressure at the age of 40 were more likely to display brain changes in their 60s that indicate a loss of blood flow normally associated with ageing.

The researchers said that the promotion of physical fitness in midlife could play an important role in ensuring that the brain does not age prematurely.

"It will be interesting to follow up with these participants in another 10 years to determine how many developed dementia, and if that may be related to their fitness or exercise blood pressure or exercise heart rate in midlife," they added.

Details of these findings were presented at the American Heart Association EPI/Lifestyle 2015 meeting in Maryland in the US.