The Dark Side of Dark Chocolate: Why Even Organic Chocolate Can Contain Heavy Metals

Dark chocolate has acquired an almost medicinal reputation. It contains flavanols and other compounds associated with potential cardiovascular benefits, has less sugar than milk chocolate, and is routinely presented as the chocolate you can eat without feeling quite so guilty.

Buy organic dark chocolate and surely you have improved the equation even further? Unfortunately, nature has thrown a rather unpleasant complication into the story. Tests of dark chocolate have repeatedly detected the heavy metals cadmium and lead, sometimes at levels high enough to raise concerns about frequent consumption. Even more surprisingly, putting the word "organic" on the wrapper does not necessarily solve the problem.

Why not? Because you can grow something organically in soil that contains heavy metals. This is the basic misconception behind much of the surprise over chocolate contamination. Organic certification concerns how a crop is produced. It does not magically purify the geology beneath the farm. Cadmium occurs naturally in soils, with concentrations varying considerably between regions. Soil can also acquire additional contamination from mining, industrial activity and some fertilisers.

Cacao trees have the unfortunate ability to take cadmium from the soil through their roots. The metal can then accumulate in the cacao beans. By the time somebody buys an expensive organic chocolate bar from a health-food shop, the cadmium may have begun its journey years earlier beneath a cacao tree thousands of kilometres away.

The US Food and Drug Administration confirms the general mechanism. Cadmium can enter foods from the environment in which crops are grown, and levels vary according to the natural geological composition of soils as well as human sources of contamination. In cocoa specifically, cadmium concentrations depend partly upon the region where the cacao was grown and the proportion of cocoa solids in the finished chocolate.

That last point helps explain why dark chocolate receives particular attention. The compounds that make dark chocolate attractive to health-conscious consumers are concentrated in its cocoa solids. Unfortunately, the heavy metals are associated with those cocoa solids as well. Generally speaking, the more cocoa solids a chocolate contains, the greater the opportunity for cadmium exposure compared with products containing much less cocoa.

There is an additional twist. Cadmium and lead appear to reach chocolate by somewhat different routes. Research indicates that cadmium is predominantly an agricultural problem. The cacao plant absorbs it from the soil and it is already present in the beans when they are harvested. Lead contamination appears more likely to increase after harvesting. Cacao beans are fermented and commonly dried outdoors, where lead-containing soil and dust can settle upon them. Processing and handling therefore become important parts of the lead story.

This distinction matters because it means there is no single solution. Reducing cadmium may require identifying growing regions and individual farms with lower-cadmium soils, changing agricultural practices, selecting cacao varieties that accumulate less cadmium, treating soils where practicable, or blending beans from different regions to reduce concentrations in the finished product.

Reducing lead may be more immediately achievable through better harvesting, drying, cleaning, transport and manufacturing practices that prevent contaminated dust from reaching the beans.

Then we come to organic chocolate. A multi-year study published in Frontiers in Nutrition examined 72 dark chocolate and cocoa products tested between 2014 and 2022. It found that 43 percent exceeded the researchers' chosen California Proposition 65 benchmark for lead and 35 percent exceeded the corresponding benchmark for cadmium. Most products nevertheless fell below the US FDA interim reference level used in the study for lead, and the median metal concentrations were below the much more conservative Proposition 65 benchmarks.

Most intriguing was the organic result. Organic designation did not produce lower heavy-metal concentrations. Organic products were actually associated with significantly higher concentrations of cadmium and lead in the analysis, although the researchers cautioned about interpreting that association too strongly.

That finding should not be converted into the equally simplistic conclusion that organic farming causes heavy-metal contamination. The study did not establish that.

Rather, it demonstrates why the word "organic" tells us surprisingly little about this particular problem. If a cacao tree is growing naturally in cadmium-rich volcanic or mineralised soil, the absence of synthetic pesticides does not remove the cadmium. Indeed, the finding that organic products were not protected against contamination tends to undermine the intuitive assumption that pesticides must be the principal explanation.

Consumer Reports has similarly found considerable variation between individual chocolate products. Some organic chocolates performed relatively well while others did not. The sensible conclusion is therefore not "organic chocolate is dangerous" but that organic certification is simply not a reliable proxy for low heavy-metal content.

This is also a useful lesson in the ambiguity of the word "natural." Cadmium is natural. Lead is natural. Arsenic is natural. Nature contains plenty of substances that human beings would be better off not consuming in significant quantities. A farming system can minimise synthetic chemicals while still producing food containing undesirable substances derived from the earth itself.

None of this means that people should panic and throw every block of dark chocolate into the rubbish bin. The FDA notes that internationally cocoa products generally represent a relatively minor contribution to total dietary cadmium exposure. The problem is principally one of cumulative exposure. Heavy metals can come from numerous foods and environmental sources, so repeatedly eating a chocolate product that happens to contain comparatively high concentrations can unnecessarily increase total exposure.

There is also enormous variation between products. Consumer Reports has found dark chocolates with relatively low levels of both lead and cadmium alongside products containing substantially higher amounts. That suggests the problem is not inherent at the same level in every piece of dark chocolate. Where the cacao is grown and how the beans are handled matter enormously.

Manufacturers therefore have considerable responsibility here. If companies can test finished chocolate for heavy metals, they can also test beans and soils, select suppliers accordingly and improve post-harvest practices. A premium chocolate manufacturer charging premium prices should be capable of knowing something about the contamination profile of its raw material.

Consumers should also abandon the assumption that the most expensive organic 85 percent cacao bar must automatically be the healthiest choice. Higher cocoa content provides more cocoa compounds, but it can also provide more of the contaminants concentrated in cocoa solids.

Modern food marketing encourages us to divide food into reassuring categories: organic or conventional, natural or artificial, clean or processed. Chemistry is not nearly so accommodating. A cacao tree does not read the certification label that will eventually appear on its chocolate bar. It sends roots into the earth and absorbs what is available there.

Sometimes that includes nutrients. Sometimes it includes cadmium. And no amount of green lettering saying "organic" on the wrapper can change the geology of the soil in which the tree grew.

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