Dark chocolate has long been called a “healthy candy” for its heart-healthy antioxidants and low in sugar. And that’s all true, but it doesn’t tell the full story.

A new study by Consumer Reports confirms what other groups and researchers have found: Most dark chocolate is contaminated by heavy metals lead and cadmium. Consumer Reports’ health and food safety team tested 28 popular dark chocolate bar brands and found that all of them contained these metals.

In the worst cases, they found more than 200% of California’s maximum allowable dose levels for either lead or cadmium; five of them contained more than 100% of both. So what does that mean for consumers? And how did the metals get there in the first place?

“Don’t panic about this because you can control your exposure to these heavy metals by making a couple of simple changes,” says James Rogers, director of food and safety research at Consumer Reports.

The results of Consumer Reports’ dark chocolate test. (Courtesy of Consumer Reports)

4 questions with James Rogers of Consumer Reports about the safety of consuming dark chocolate

How do metals get into dark chocolate?

“With cadmium, the research has shown that if you have soil where you grow the chocolate plants in that has cadmium in it, the roots will actually draw out the cadmium and it will end up in the cocoa seeds. And then when you process those cocoa pods into chocolate, the cadmium comes along for the ride.

“When the chocolate beans, the cocoa beans, are being processed, one of the steps is for them to be dried, usually out in the open. And if you are processing these beans close to an industrial site — a place that is manufacturing lead batteries, for instance — the soil can become contaminated. And then as you’re drying these cocoa pods, they actually have dust that blows over from the industrial site and settles onto the seeds.”

What adverse effects can these metals cause?

“Heavy metals are dangerous to humans for consumption, especially children, and especially for pregnant women. So in the case of cadmium and lead, they can cause neurological dysfunctions. They can cause a loss of IQ points in children that are exposed to these heavy metals. They can cause malformations in the neurology of a developing fetus. And there are some studies that suggest that heavy metals can be associated with cancer.”

What can manufacturers do about this?

“We suggest that the manufacturers consider testing the soil and find areas to grow the chocolate plants away from soil that’s cadmium contaminated. When you are processing your chocolate pods, why not find a location away from industrial processing places and do it where dust can not blow onto the pods?”

What can consumers do to mitigate risks?

“We’re calling on the parents to limit or eliminate dark chocolate totally from a child’s diet.

“We’re recommending, too, that pregnant women avoid eating dark chocolate during their pregnancy just to make sure that their developing fetuses are not affected by these heavy metals.

“If you’re going to eat this product, make wise choices, be a smart consumer, do the research, look for the data that’s out there with our study and other studies, and then choose the products that are safer. Not absolutely safe, but safer, if you insist on consuming this product.

“The best you can do is do your research. Write the dark chocolate manufacturers and demand lower heavy metal concentration. Write your congresspeople and have the [Food and Drug Administration] advocate for regulations that prevent high levels of heavy metals and dark chocolate.”

Karyn Miller-Medzon produced and edited this interview for broadcast with Todd Mundt. Grace Griffin adapted it for the web.

This article was originally published on WBUR.org.

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