The study shows that a naturally occurring substance in chocolate may improve the function of a part of the brain related to natural memory decline.

As we age, we tend to become more forgetful - struggling to remember people's names or where we left our wallet. This sort of age-related memory loss affects a part of the brain that is different to the area affected in Alzheimer's disease.

The Columbia University scientists had previously identified the age-related memory loss area in mice. They had also found that a compound within cocoa, called flavanols, boosted this part of the brain.

But they weren't sure whether it was the same with humans.

To test the theory, the scientists took a group of 37 healthy volunteers and randomly gave them a chocolate drink rich in flavanols or low in flavanols.

Brain imaging and memory tests were conducted on the volunteers, aged between 50 and 69, before and after the three-month study.

"When we imaged our research subjects' brains, we found noticeable improvements in the function of the dentate gyrus in those who consumed the high-cocoa-flavanol drink," said lead author Adam M. Brickman, PhD, associate professor of neuropsychology at the Taub Institute for Research on Alzheimer's Disease and the Ageing Brain.

The group on the high-flavanol drink also performed better in the memory test.

"If a participant had the memory of a typical 60-year-old at the beginning of the study, after three months that person on average had the memory of a typical 30 or 40-year-old," said senior author Dr Scott Small, a professor of neurology, in a statement.

Other experts have called the study's findings "promising" but pointed out that it was a "very small" study so needs to be conducted on a larger scale.

"Given a globally ageing population, by isolating a particular area of the brain that is weakening in functioning as we grow older, and demonstrating that a non-pharmacological intervention can improve learning of new information, the authors have made a significant contribution to helping us improve our cognitive health," said Dr Ashok Jansari, a cognitive neuropsychologist at the University of East London.

Liz Coulthard, a senior lecturer in dementia neurology from Bristol University, said further research needed to be done to check that it was in fact the flavanols and not the increased levels of caffeine or theobromine (another substance found in chocolate) that enhanced cognitive performance in the cocoa-rich group.

She also said further tests needed to explore accuracy of performance, not just improved reaction times.

"It would be very exciting if such a cognitive benefit of flavanols were shown in a larger study that probed several aspects of cognition," she said.

And before we race out to buy the family sized bag of Snickers, it's worth remembering that most methods of processing cocoa - and certainly the processing that takes place before it reaches the supermarket aisle - removes many of the flavanols.

"The supplement used in this study was specially formulated from cocoa beans, so people shouldn't take this as a sign to stock up on chocolate bars," said Dr Simon Ridley, Head of Research at Alzheimer's Research UK.

The best dietary way to reduce our risk of memory loss, Alheimer's or dementia, is to stick with eating your greens and other good stuff.

"Continued investment in research is crucial to find ways to protect the brain and prevent the diseases that cause dementia," Ridley said.

"Although there's currently no certain way to prevent dementia, research shows that a healthy lifestyle can help reduce the risk of the condition.

"A healthy diet, regular exercise, not smoking, and keeping blood pressure and weight in check can all help lower the risk of dementia."