Dark chocolate may be healthier than milk chocolate, according to a team of scientists.
Dark chocolate has anti-oxidant effects
Researchers in Scotland and Italy say dark chocolate has much better anti-oxidant properties.
This means that it can protect the heart and arteries from oxidative damage, similar to the rust that develops on metal over time.
Writing in the journal Nature, they said adding milk to chocolate may cancel out these health benefits.
Previous studies have suggested that chocolate may help to protect against heart disease and even cancer.
While some of these have distinguished between dark and milk chocolate, many have not.
In this latest study, researchers from the University of Glasgow and Italy's National Institute for Food and Nutrition Research carried out tests using both varieties.
They recruited 12 healthy volunteers, seven woman and five men, who were asked to eat dark and milk chocolate.
They found that volunteers had to consume twice as much milk chocolate as dark
chocolate to obtain the same amount of antioxidants.
To take account of this, the participants, aged 25 and 35, were offered a
double helping of milk chocolate.
The researchers also looked at what happened when the volunteers ate dark chocolate and drank milk at the same time.
Dark chocolate was found to boost blood antioxidant levels by nearly 20%.
However, there was no such effect when volunteers ate milk chocolate or drank milk with dark chocolate.
"What this tells us, is that probably the proteins in milk bind with the antioxidants in chocolate," Professor Alan Crozier of Glasgow University told BBC News Online.
"As a consequence, they are not being absorbed to the same extent as they would be with dark chocolate. Any potential protective effects are lost."
The findings raise the possibility that dairy products may interfere with the healthy properties of other foods, such as fruits, tea and red wine, which are also believed to have anti-oxidant effects.
The researchers said future studies into the health impacts of these foods should take this into account.
While lovers of dark chocolates may seize on the study findings, Professor Crozier warned against over indulging.
"Milk and dark chocolate contains high levels of saturated fats, which get into the blood stream and increase levels of cholesterol, increasing the risks of heart disease.
"I would advise moderate consumption, maybe a small bar of chocolate a day.
"It is a useful supplement to a balanced diet but it should not be a substitute for five pieces of fruit and vegetables each day."