Scientists keep finding heavy metals in dark chocolate. Should you worry?

The researchers tested 72 dark chocolate bars, cocoa powders and nibs to see if they were contaminated with heavy metals in concentrations higher than those deemed safe by California’s Proposition 65, one of the nation’s strictest chemical regulations.

Among the products tested, 43 percent contained higher levels of lead than the law considers safe, and 35 percent had higher concentrations of cadmium. Both metals are considered toxic and have been associated with a range of health issues. The study did not name specific brands, but found that organic products were more likely to have higher concentrations. Products certified as “fair trade” did not have lower levels of heavy metals.

But on the whole, the levels were not so high that the average consumer should be concerned about eating dark chocolate in moderation, said Jacob Hands, the lead author on the paper and a medical student at George Washington University School of Medicine and Health Sciences.

Nearly all of the chocolates contained less than the Food and Drug Administration’s reference limits for lead, which are less stringent than the California requirement. And while both cadmium and lead can carry significant health risks, it’s not clear at this point that eating a few squares of dark chocolate poses a risk to most healthy adults.

“Just the fact that it exists doesn’t necessarily mean immediately there’s going to be some terrible health consequence,” said Laura Corlin, an associate professor of public health and community medicine at Tufts University School of Medicine who was not involved in the study.

But eating a lot of dark chocolate alongside other products known to contain heavy metals, like some seafoods, teas and spices, might add up to concerning levels.

“I wouldn’t be worried about someone who goes on a little bit of a dark chocolate craze and for a couple of weeks they’re adding dark chocolate to their shopping cart and eating it every night,” said Melissa Melough, an assistant professor of nutrition at the University of Delaware. “But if it’s truly a core part of your routine or you eat it in larger quantities, that’s what I’d worry about.”

Cocoa can become contaminated with cadmium and lead depending on where it’s grown and how it is handled and processed. (Source: Freepik)

Pregnant women and very young children, who are particularly vulnerable to the health effects of heavy metals, should limit how much dark chocolate they consume, said Tewodros Rango Godebo, an assistant professor at the Tulane School of Public Health and Tropical Medicine who also published a study on heavy metals in chocolates this month. His research found that eating an ounce of dark chocolate each day poses no risks to healthy adults who are not pregnant.

The process of producing dark chocolate makes it particularly susceptible to contamination with these metals, because it contains more cocoa than milk chocolate does. Cocoa can become contaminated with cadmium and lead depending on where it’s grown and how it is handled and processed. The new study found that concentrations of heavy metals can vary widely from brand to brand, and even from bar to bar, Mr. Hands said.

While most of the bars had relatively low levels of lead, he said, two surpassed the F.D.A.’s limits for the metal.

Dr. Melough added that it’s possible that there are more products on the market with these high levels than the data suggest.

If you are concerned about the levels in a specific brand, As You Sow, a corporate responsibility nonprofit, maintains a list of cadmium and lead levels in many chocolate products. People who want to avoid heavy metals as much as possible may want to think about which bar they consume, Mr. Hands said, but they do not need to cut out the sweet altogether.

“It would be tragic for me if anybody stopped consuming dark chocolate on the basis of this,” he said.