If you want to be mentally fit in middle age, make sure you are physically fit when young.

A study of almost 3,000 Americans has found that those who had the most endurance at the age of 25 performed best in a suite of cognitive tests 20 years later.

Participants were asked to stay on a treadmill as long as possible, while the speed and incline slowly increased. The first time round, they averaged ten minutes. In middle age that had dropped to just over seven.

At the same time, they were assessed for skills such as memorising a list of 15 words, the ability to substitute numbers for symbols and the speed of identifying the colour of words when the words themselves are colours – so if “yellow” was written in green ink, the correct answer would be “green”.

For every minute extra people completed on the initial test, people memorised 0.12 words and replaced 0.92 numbers for symbol. Those who had a smaller drop in treadmill time between the age of 25 and 45, an indicator that they had kept fit in the interim, performed better in the test of identifying colours.

“This is one more important study that should remind young adults of the brain health benefits of cardio fitness activities such as running, swimming, biking or cardio fitness classes,” said Dr David Jacobs, from the University of Minnesota.

It is not just about keeping a sharp mind in middle age. Previous studies have shown that poor performance in these cognitive tests at middle age was correlated with dementia later in life, the scientists. One study found that for every additional word memorised there was an 18 per cent drop in the risk of dementia after ten years.

However, while the correlations are relatively well understood, the precise mechanism is more of a mystery. “One possible mechanism is that low cardiorespiratory fitness leads to morphologic brain changes, including white matter lesions and brain atrophy in certain regions in grey matter,” the authors write, in the journal Neurology. It is also possible that having a stronger heart could stave off dementia by improving blood flow to the brain.

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