A new study finds that dark chocolate products sold nationwide may contain excessive amounts of heavy metals. 

The research, led by scientists at George Washington University and published Wednesday in the peer-reviewed Frontiers in Nutrition, examined over 70 dark chocolate products from retailers such as Whole Foods Market, Amazon and GNC. The products were tested to see whether the heavy metals lead, cadmium or arsenic were in them. 

Overall, 43% of the products studied exceeded acceptable levels of lead and 35% exceeded cadmium levels, according to the study, which was based on a California law that sets maximum allowable dose levels for heavy metals in food. Food researchers often use the 1986 regulations, known as Prop 65, as a safety standard because the Food and Drug Administration doesn’t set limits on heavy metals in most foods, said Leigh Frame, director of integrative medicine at George Washington University School of Medicine and Health Sciences and lead author of the study.

The FDA does have suggested limits for chocolate and sugar-based candy but only for children.

According to the California guidelines, the threshold for heavy metals in foods is 0.5 micrograms a day. For the study, the scientists estimated the number of micrograms a day people would be exposed to if they ate the suggested serving amounts on the chocolate product labels. They found that the chocolate samples ranged from 0 to as high as 3.316 mcg per daily serving. Levels of cadmium, a carcinogen at high levels, ranged from 0.29 to 14.12 mcg, with the limit being 4.1 mcg per day.

None of the products exceeded the maximum level for arsenic. 

Frame said that because the products had varying amounts of lead in them, limiting consumption is the only sure way to reduce exposure.

“Don’t have large amounts of chocolate every single day,” she said. “One ounce serving size is what we recommend, or maybe you have 2 ounces every other day.” 

How to avoid lead in foods

Researchers agree that avoiding heavy metals entirely in our diet is nearly impossible. Foods such as rice, fish, fruits and vegetables have been known to contain varying amounts of metals. While heavy metals can be naturally excreted by the body through sweat and urine, if they are consumed in high amounts they can accumulate in the body and damage major organs. 

“You actually cannot avoid exposure to heavy metals in the diet,” Frame said. “It’s really not about avoiding them; it’s about making sure you’re not getting too much.” 

She emphasized a diversity in diet to avoid excessive exposure to any particular substance.

“Not eating the same thing day after day is going to help protect you from many different things, including heavy metals,” she said.

The authors intentionally left out which brands had what concentrations of metals given that levels could vary even within the same company. Interestingly, the study found that organic cocoa products were more likely to have higher levels of cadmium and lead. 

“Organic food doesn’t necessarily mean that it’s been checked for exposure to toxic metals like lead, cadmium and arsenic,” said Manish Arora, vice chairman of the department of environmental medicine and climate science at the Icahn School of Medicine at Mount Sinai in New York City.   

“I think to most of the public ‘organic’ just means cleaner, and in this case it’s counterintuitive,” he said. 

Arora, who was not involved with the study, said that while the new research was strong, a big unknown with the paper is how the heavy metals got into the chocolate products in the first place. 

“Is it the processing, the farming or the type of soil or the fertilizer or any other farming process that they’re using?” he asked. “We are left not being sure where the metal actually entered the food chain.”

Previous research has found that lead and cadmium can enter dark chocolate through different ways. Cadmium comes primarily through the cacao plant’s taking it up from the soil, while lead can be introduced at various points in the manufacturing process, including the harvesting, drying and fermenting of the cocoa bean. 

There is no safe level of lead. While the FDA does not set limits for cadmium or arsenic, almost all of the chocolate bars in the study were below its recommended level for lead: 2.2 mcg a day for children under age 7 and 8.8 mcg a day for women of child-bearing age.

That contrasts with the California guidance because levels set by Prop 65 tend to be more conservative, Frame said.

Tewodros Godebo, an assistant professor of environmental health sciences at Tulane University School of Public Health and Tropical Medicine, said that in his opinion the Prop 65 standards are too conservative and may cause unnecessary panic. He has published his own study this year, in which his team tested over 100 chocolate products. 

Instead of the Prop 65 limits, Godebo’s research used a method proposed by the Environmental Protection Agency that found levels of heavy metals in chocolate were not enough to be concerning to adults. The EPA commonly uses the formula, called the hazard quotient, to determine a substance’s toxicity. 

Still, he recommended consuming no more than an ounce of dark chocolate per day and limiting consumption for children and pregnant women. 

The new study did not examine milk chocolate, but theoretically it should have a lower risk of heavy metal contamination, Frame said. That is because the metals are believed to come from the cocoa powder itself, which is present in higher levels in dark chocolate.