Funded by Diabetes UK and published in Diabetes Care, researchers spent 12 months giving half the sample of 93 women flavonoid-enriched chocolate. Half the group of women were given placebo chocolate bars, by the group at University of East Anglia.
The results show those who ate chocolate with enhanced flavonoids reduced their risk of heart attack in the next decade by 3.4%. Their risk of stroke, resistance to insulin and cholesterol levels were also significantly reduced, the trial shows. The researchers stressed that this did not mean women with type 2 diabetes should eat more chocolate.
Commercially available chocolate does not contain "nearly as much of it as the women consumed" and eating too much chocolate would adversely affect weight unless the diet was balanced for energy intake.
The chocolate bars used in the trial were specially formulated to provide a high dose of two flavonoid sub-classes - flavan-3-ols which are found in cocoa and tea, and isoflavones which are found in soy.
"These results are significant from a public health perspective because they provide further concrete evidence that diet has a beneficial clinical effect over and above conventional drug treatment," said lead researcher Prof Aedin Cassidy, from Norwich Medical School.
The women who took part in the research were part of the "high risk" age group between 51 and 74. Dr Iain Frame, director of research at Diabetes UK, said: "This trial assessed the effects of flavonoids on the risk of heart disease in post-menopausal women with type 2 diabetes over a period of one year.
"Although it involved quite a small number of women already at high risk of heart disease, these compounds appeared to offer them better protection against heart problems than conventional drugs when administered under very carefully controlled circumstances."