Dark chocolate may not have as many health benefits as people think, an editorial in a leading medical journal claims.
The treat naturally contains chemicals called flavanols that are believed to protect the heart.
But The Lancet editorial says many manufacturers take these out of chocolate because they have a bitter taste.
And the chocolate will still contain high levels of fat and sugar, both of which are harmful to the heart and arteries, it added.
It comes after recent studies suggested that flavanol-rich chocolate could help blood vessels dilate and improve heart function.
The journal said: "Dark chocolate can be deceptive. When chocolate manufacturers make confectionery, the natural cocoa solids can be darkened and the flavanols, which are bitter, removed, so even a dark-looking chocolate can have no flavanol.
"Consumers are also kept in the dark about the flavanol content of chocolate because manufacturers rarely label their products with this information.
"To gain any health benefit, those who eat a moderate amount of flavanol-rich dark chocolate will have to balance the calories by reducing their intake of other foods - a tricky job for even the most ardent calorie-counter.
"So, with the holiday season upon us, it might be worth getting familiar with the calories in a bar of dark chocolate versus a mince pie and having a calculator at hand."