In Britain alone, 700,000 people now live with dementia, and the figure is expected to rise to more than a million people by 2025 and 1.7 million by 2051.

The study in the British journal said: 'Smoking, hypertension and diabetes were associated with a higher risk of the outcome.'

'Our results suggest that, for prevention of dementia, control of cardiovascular risk factors starting in midlife is likely to be more important in the prevention of dementia than control starting later on,' it said.

The researchers, based in the universities of Minnesota, North Carolina and Johns Hopkins and the University of Mississippi Medical Center, studied more than 11,000 people aged 46-70.

Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: 'Although this latest research recommends mid-life as a critical time to change our lifestyles, it's never too early, or late, to take steps towards improving heart-health.

'We should all consider stopping smoking, taking regular exercise and adopting a healthy, Mediterranean-style diet.'

On brain exercise study, she said: 'This adds to the 'use it or lose it' hypothesis that we can reap the benefits of stimulating our minds regularly, perhaps by doing crosswords, playing chess or adding up the shopping before getting to the till.'