Until now, however, no one knew it could signify underlying brain health so much earlier in life.

The correlation was so stark, however, that the US scientists now say walking tests could be used to provide an early indication of dementia.

Slower walkers were shown to have 'accelerated aging' and their lungs, teeth and immune systems tended to be in worse shape.

Published in the Journal of the American Medical Association (JAMA), the study revealed an average difference of 16 IQ points between the slowest and the fastest walkers at the age of 45.

This reflected both the participants' natural walking speed and the pace they achieved when asked to walk as fast as they could. Those with a slower gait also scored less well in physical exercises as well as biological markers of poor health.

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Slower walkers were shown to have "accelerated ageing" and their lungs, teeth and immune systems tended to be in worse shape than the people who walked faster.

The team at Duke University in North Carolina said genetic factors may explain the link between walking speed, brain capacity and physiological health, or that better brain health might promote physical activity, leading to higher walking speed.

"The thing that's really striking is that this is in 45-year-old people, not the geriatric patients who are usually assessed with such measures," said Dr Line Rasmussen, who led the research.

The 904 New Zealand men and women who were tested at 45 were tracked from the age of three, each undergoing multiple tests over the years.

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The long-term data collection meant researchers established that toddlers with lower IQ scores, linguistic ability, capacity to tolerate frustration, motor skills and emotional control tended to have slower gait-speeds by middle age.

MRI exams during their final assessment at 45 showed the slower walkers tended to have lower total brain volume, lower mean cortical thickness and less brain surface area.

The researchers said some of the differences in health and cognition may be the result of lifestyle choice.

Telegraph UK