The research, published in the American Journal of Clinical Nutrition, found elderly patients with low levels of B12 suffer twice as much shrinkage of the brain as those with higher levels of the substance in their bodies.

They are now conducting a clinical trial that aims to show it may be possible to treat memory problems in the elderly with vitamin supplements.

Professor David Smith, from the Oxford Project to Investigate Memory and Ageing, said: "There are 550 people who come down with dementia, mainly Alzheimer's every day in the UK - it is a major epidemic."

He added: "Our study shows that consuming around half a litre of milk or more per day, and it can be skimmed milk, could take someone who has marginal levels of B12 into the safe range. But even drinking just two glasses a day can protect against having low levels."

Previous studies by the group have indicated chocolate and wine may have a similar effect.