Eating five servings of dark chocolate, and not the milk one, a week may help reduce the risk of type 2 diabetes, according to a long-term US study.

Published in the BMJ, the researchers also found that increased consumption of milk, but not dark, chocolate was associated with long-term weight gain.

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Chocolate contains high levels of flavanols (a natural compound found in fruits and vegetables) which have been shown to promote heart health and reduce the risk of diabetes.

But the link between chocolate consumption and risk of type 2 diabetes remains controversial due to inconsistent results.

In addition, most previous studies have not looked at whether eating dark and milk chocolate – which have different cocoa, milk and sugar content – might have different impacts on the risk of type 2 diabetes.

To explore this further, researchers combined data from three long-term US observational studies of female nurses and male healthcare professionals with no history of diabetes, heart disease or cancer when they were recruited.

They analysed associations between type 2 diabetes and total chocolate consumption for 192,208 participants, and chocolate subtype (dark and milk) consumption for 111,654 participants over an average monitoring period of 25 years.

In the analyses for total chocolate, 18,862 people developed type 2 diabetes.

After adjusting for personal, lifestyle, and dietary risk factors, the authors found that people who ate at least five servings a week of any type of chocolate showed a significant 10 per cent lower rate of type 2 diabetes compared with those who rarely or never ate chocolate.

In analyses by chocolate subtypes, 4,771 people developed type 2 diabetes.

After adjusting for the same risk factors, people who ate at least five servings a week of dark chocolate showed a significant 21 per cent lower risk of type 2 diabetes, but no significant associations were found for milk chocolate intake.

The researchers also found a 3 per cent reduced risk of type 2 diabetes for each additional weekly serving of dark chocolate.

“Clinical trials needed to confirm findings and help identify reasons for dark chocolate’s protective effect,” said researchers.