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Neuroscientist Dr Gary Small, director of the centre for ageing at the University of California, says adopting a healthy lifestyle is an important way to minimise the risk of developing neurodegenerative diseases.

There is a lot we can do to keep our brains young

Dr Gary Small

This includes keeping physically active, eating a low fat diet with plenty of anti-oxidant foods such as fruit and vegetables and avoiding long-term chronic stress.

But he also advocates exercising the brain by doing what he terms "mental aerobics".

This can include doing crosswords, learning a new language, or writing with your left hand if you are normally right handed.

Dr Small said: "We have done studies repeatedly looking at the brain activity of people at every age, and we are finding that we can see the subtle signs of Alzheimer's of people even in their thirties and forties.

"But genetics is only one third of what predicts brain ageing, the other two-thirds is environment and the lifestyle choices we make, and there is a lot we can do to keep our brains young."

Dr Small has written a book called "Memory Bible".