An occasional chocolate treat can help prevent heart failure in older women, research suggests.
One or two servings of good-quality chocolate a week reduced the risk of middle-aged and elderly women developing the condition by almost a third, a study found.
Scientists looked at the association between chocolate and heart failure in almost 32,000 Swedish women aged 48 to 83.
Moderate chocolate consumption significantly reduced heart failure risk, but the protective effect lessened as more or less was eaten.
One or two 19 to 30 gram servings a week led to a 32% risk reduction. This fell to 26% when one to three servings a month were eaten, while one serving a day or more showed no benefit.
A typical chocolate bar weighs around 100 grams, but the amount of healthy cocoa solids it contains varies greatly.
Dark chocolate can contain as much as 75% cocoa while standard milk chocolate may have 20% or less.
Antioxidant plant compounds called flavonoids in cocoa are believed to protect against heart disease and high blood pressure.
The study authors pointed out that chocolate eaten in Sweden tends to have a high cocoa content.
Although 90% of chocolate consumed in the country is milk chocolate, it consists of around 30% cocoa solids.
The lack of benefit from eating chocolate every day was probably due to the extra number of calories consumed, said the researchers.
This in turn could lead to increased weight and higher blood pressure, which is a risk factor for heart failure.
"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 study leader Dr Murray Mittleman, director of the Cardiovascular Epidemiology Research Unit at the Beth Israel Deaconess Medical Centre in Boston, US.
"But if you're going to have a treat, dark chocolate is probably a good choice, as long as it's in moderation."
The findings are reported in the American Heart Association journal Circulation: Heart Failure.
Heart failure occurs when damage to the heart means it can no longer pump blood around the body efficiently. Sufferers may feel tired, breathless and find themselves unable to climb hills or stairs.
Around 68,000 new cases of heart failure are diagnosed in the UK each year.
Commenting on the research, US expert Professor Linda Van Horn, from Northwestern University in Chicago, said: "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.
"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 extensive intake of non-nutrient dense 'empty' calories."