A study is being launched to investigate possible links between diet and Alzheimer's disease.
Some scientists believe fruit juice could help prevent Alzheimer's
Researchers at the Alzheimer's Society will look at the effects of fruit juice, red wine and oily fish on the incidence of mental illness.
They will analyse data from new and existing studies to see if a healthy "Mediterranean diet" could reduce the risk of developing Alzheimer's.
The society says dementia will affect almost one million people by 2020.
Every five years after the age of 30, the chances of developing the disease doubles, according to the society.
And, by the time a person reaches 80, the odds are one in six.
The Mediterranean diet - rich in fruit, vegetables and cereals with some fish and alcohol and very little dairy and meat - has been generally viewed as good for health for some time.
Neil Hunt, chief executive of the Alzheimer's Society, said there seemed to be evidence that it could be "extremely beneficial".
"Nothing is going to guarantee cures or prevention of dementia, but we do think there probably are things that people can do to reduce their chances of getting these bad illnesses," he said.
"We don't want to encourage false hope, but we are hoping that this study will produce genuine advice for people."
The study's first findings are expected in July.