The research, presented this weekend at the European Society of Cardiology's annual meeting (abstract P638), is not enough to support a health claim for the food but will add to the perception of chocolate and cocoa-products as 'functional foods'.
Dr Charalambos Vlachopoulos of the Hippokratian Hospital at the University of Athens, told the conference that eating 100 grams of dark chocolate improved blood vessel function in healthy young adults for at least three hours.
The research backs the findings of a US team, published in the June issue of the Journal of the American College of Nutrition, who also reported increased dilation of blood vessels, reducing the chances of clots forming. Blood clots can cause thrombosis, stroke and heart attack.
The study involved 17 volunteers who ate either 100 grams of dark chocolate or a non-chocolate substitute. On another day the groups were swapped over.
The results showed that functioning of the endothelium, a thin layer covering the innermost surface of blood vessels, was improved in the dark chocolate group but not in the group that ate the chocolate substitute.
"Ingestion of dark chocolate rich in cocoa flavonoids is associated with an acute beneficial effect on endothelial performance of healthy subjects. This finding provides further insights into the favourable effects of chocolate into the cardiovascular system," said the researchers.
Penelope Alexandre of Caobisco, the association of the EU's chocolate, biscuit and confectionery industries, told NutraIngredients.com: "Obviously we would like to have health claims for chocolate but our current line remains the same as that for the whole food industry - all claims, including nutritional ones, must be supported by science."
She said that nutritional claims - underlining a high content of a particular nutrient without reference to disease- are the only ones currently being used by chocolate producers.
Mars manufacturer Masterfoods has carried out research on its CocoaVia chocolate milk drink and claims on its website that the antioxidant capacity of one 85ml bottle of CocoaVia is higher than that of a portion of many fruit and vegetables.
"CocoaVia drink cannot replace any fruit or vegetables in your diet - although it can help top up your body's antioxidant defences and help keep your heart healthy," claims the company.
Chocolate makers trying to promote the health benefits of their products face counter arguments from nutritionists who say that the weight gain from eating a lot of chocolate would cancel out much of the benefit.