"You can’t ignore that chocolate is a relatively calorie-dense food and large amounts of habitual consumption is going to raise your risks for weight gain," said lead researcher Dr Murrray Mittleman, director of the Cardiovascular Epidemiology Research Unit at Harvard Medical School’s Beth Israel Deaconess Medical Center in Boston.

"But if you’re going to have a treat, dark chocolate is probably a good choice, as long as it’s in moderation."

High concentration of compounds called 'flavonoids' in chocolate may lower blood pressure, among other benefits, according to mostly short-term studies.

But this is the first study to show long-term outcomes related specifically to heart failure, which can result from ongoing untreated high blood pressure.

Dr Mittleman said differences in chocolate quality affect the study’s implications for Americans.

Higher cocoa content is associated with greater heart benefits.

In Sweden, even milk chocolate has a higher cocoa concentration than dark chocolate sold in the United States.

Also, the average serving size for Swedish women in the study ranged from 19 grams among those 62 and older, to 30 grams among those 61 and younger.

In contrast, the standard American portion size is 20 grams.

"Those tempted to use these data as their rationale for eating large amounts of chocolate or engaging in more frequent chocolate consumption are not interpreting this study appropriately," said Professor Dr Linda Van Horn of Northwestern University’s Feinberg School of Medicine in Chicago.

"This is not an ‘eat all you want’ take-home message, rather it’s that eating a little dark chocolate can be healthful, as long as other adverse behaviours do not occur, such as weight gain or excessive intake of non-nutrient dense ‘empty’ calories."