Concentrations of heavy metals found in single servings of some chocolates and cocoa-based products are too low in most cases to pose a health risk to consumers, research published on Tuesday in the journal Frontiers in Nutrition shows.

This is true when products are consumed as a single serving, but consumption of more than one serving per day or in combination with other sources of heavy metals – such as seafood and unwashed brown rice – may cumulatively add up to exposure that exceeds recommendations, the researchers said.

Some consumer groups and independent test agencies have previously reported heavy metal contamination in cocoa products such as dark chocolate, with possible causes being the type of soil where cocoa is grown and industrial processing.

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Researchers at the George Washington University School of Medicine and ConsumerLab.com found 70 of the 72 cocoa-containing products they analysed fell below limits set by the Food and Drug Administration (FDA) for lead contamination.

Using stricter limits set by the state of California, however, 31 of the 72 exceeded limits for lead, while 13 of 37 exceeded limits for cadmium.

The researchers said the findings of their study showed the products may not pose a health risk when consumed as single servings, though larger portions could exceed the strict California limits set in a law known as Prop 65.

The recommended single serving for chocolate is about 30gm to 60gm.

“If contaminated products as a whole are consumed in small amounts and infrequently by most, these contaminants may not be a public health concern,” read the paper, which concluded with a call for more testing of consumer products.

“In contrast, if many such products are consumed fairly regularly by the average consumer, the additive exposure may be a public health concern.”

US chocolate industry group the National Confectioners Association said the research confirms that “chocolate and cocoa are safe to eat and can be enjoyed as treats as they have been for centuries”.

The long-running research analysed 72 products for potential contamination with heavy metals such as lead, cadmium and arsenic in four different cohorts in 2014, 2016, 2019 and 2022.

“Median concentrations of each metal tested were lower than even the conservative Prop 65,” they said.

“However, consuming some of the products tested, or more than one serving per day in combination with non-cocoa derived sources … may add up to exposure that would exceed the Prop 65 (limits).”