Those attempting to guard against dementia should put down the crossword and go for a jog, British researchers have advised.
For years, people have assumed that puzzles kept the brain active. But Professor Clive Ballard, professor of age-related disease at King's College London, said those who wanted the best chance of staving off dementia should opt for a run or a brisk walk.
It has long been known that those who exercise regularly are less likely to develop the condition, but it was only last month that a study at the University of Pittsburgh showed that instead of the brain shrinking - as it does normally at a rate of about 1 per cent a year - it grew by about 2 per cent in those who took a brisk 40-minute walk three times a week.
Among those aged between 55 and 80, exercise increased the size of the hippocampus - the brain's memory hub - knocking almost two years off its biological age.
Ballard, who was until recently the director of research for the Alzheimer's Society, is leading a study investigating the long-term impact of brain training on older people.
The project has already found that puzzles appear to have little short-term impact on the brain.
Ballard said some exercises seemed to have some impact in protecting against general cognitive decline, yet had no effect on Alzheimer's disease.
"If people enjoy a crossword that's great, and it's possible it does some good. But if people want the best chance of protecting themselves from dementia the answer is to go for a run or a brisk walk, the evidence is clear," he said.
Studies have also suggested social interaction can help to protect against the disease.