Epicatechin is found in grapes

The Salk Institute study could lead to further tests to see if epicatechin also works on humans.

The study, published in the Journal of Neuroscience, suggests it improves blood flow in the brain - especially in combination with extra exercise.

However, nutritionists warn chocolate is high in fat and sugar, which may undo any potential benefits.

We'd obviously recommend a diet rich in fruit and vegetables, with perhaps a small amount of chocolate - but eating five bars a day is likely to just make you fatter, without any noticeable improvement in memory

Spokesman, British Nutrition Foundation

This is not the first study to find a link between 'flavanol' chemicals in certain foods and health benefits - other studies claim that cardiovascular health can be improved by including them in the diet.

The researchers, led by Dr Henriette van Praag, working with chocolate firm Mars, compared mice fed a typical diet with those fed a diet supplemented with epicatechin.

Half the mice in each group were allowed to run on a wheel for two hours each day and then, a month later, were trained to find a platform hidden in a pool of water.

Those that both exercised and ate the epicatechin diet remembered the location of the platform longer than the other mice.

The epicatechin-fed mice who did not exercise also showed enhanced memory, but to a lesser degree.

Human studies

The mice on the special diet appeared to have greater blood vessel growth in certain parts of their brain, alongside more mature brain nerve cells.

Chocolate is another source

Dr van Praag said: "A logical next step will be to study the effects of epicatechin on memory and brain blood flow in aged animals - and then humans, combined with mild exercise."

Dr Mark Mattson, from the US National Institute on Ageing, said: "This is an important advance because it identifies a single natural chemical with memory-enhancing effects, suggesting it may be possible to optimise brain function by combining exercise and dietary supplementation."

However, a spokesman for the British Nutrition Foundation said that while there was some evidence that diets rich in flavanols could be beneficial in humans, often when flavanols such as epicatechin were given on their own, no benefits could be spotted.

She added: "We'd obviously recommend a diet rich in fruit and vegetables, with perhaps a small amount of chocolate - but eating five bars a day is likely to just make you fatter, without any noticeable improvement in memory."