Dark chocolate has long been considered a healthy alternative to milk or white chocolate. According to the Cleveland Clinic, dark chocolate has lower added sugar and fat, is rich in antioxidants that are beneficial to heart health, is high in fiber, protects skin from sun damage, and may enhance your mood.
The Consumer Reports article presents quite a scary picture. After testing 28 dark chocolate candy bars for lead and cadmium, they concluded that
“For 23 of the bars, eating just an ounce a day would put an adult over a level that public health authorities and Consumer Report’s experts say may be harmful for at least one of those heavy metals.”
A little context
Cadmium and lead are both naturally occurring metals in soil widely distributed throughout the environment. Cacao (it is cacao in its raw form and cocoa once processed) plants take up cadmium from the soil, with cadmium accumulating in cacao beans as the trees grow. Lead gets into cacao beans after the beans are harvested when the beans are exposed to soil and dust. Dark chocolate tends to have higher levels of metals than milk chocolate because of its higher cacao content.
How did Consumer Reports Reach Their Conclusions?
To determine the risk posed by the chocolate, Consumer Reports compared the levels found in the dark chocolate with the State of California’s maximum allowable dose level (MADL) for lead, 0.5 micrograms (mcg), and cadmium, 4.1 micrograms (mcg). A microgram is a millionth of a gram.
Their article did not provide lead or cadmium levels in the chocolate bars but instead provided the percentages of the MADL supplied in an ounce of each chocolate. A typical chocolate candy bar is 1.55 ounces.
The chart below shows the percentages of the MADL for lead and chocolate (from Consumers Reports) and the levels of lead and cadmium that I calculated based on a candy bar’s average weight.
“We used those [California MADL] levels because there are no federal limits for the amount of lead and cadmium most foods can contain, and CR’s scientists believe that California’s levels are the most protective available.”
Although it is true that there are no legally enforceable limits for the amount of lead and cadmium allowed in food in the US, there are several guidance levels set by the EPA, FDA, and international organizations that could have been used instead of the California MADLs. Just because the numbers are the lowest does not mean they are based on sound science.
A Lead MADL of 0.5 micrograms was set in 1989, based on the Occupational Safety and Health Administration’s (OSHA) permissible exposure limit (PEL) for workers. The PEL, 50 micrograms per cubic meter of air, multiplied by ten cubic meters (the amount OSHA determined workers breathed over 8 hours), yields 500 micrograms. When divided by 1000, an arbitrary number designed to be exceedingly protective, it generates a MADL of 0.5 micrograms per day.
The scientific basis for this value is flawed. It is NOT standard procedure to convert OSHA numbers, intended for worker exposure through air, into safe levels for contaminants in food. (I assume this is why California’s basis for lead MADLs lacks transparency. The only way to find this information was to dig through summaries of old court cases).
A Cadmium MADL of 4.1 micrograms was set in 2001 with a more acceptable scientific basis. It was based on a study that reported adverse effects (reduced weight gain and locomotor activity) in the offspring of pregnant rats exposed to cadmium in drinking water. The level at which no adverse effects were seen was again divided by 1,000 to determine the MADL.
A More Reasonable Approach
A recent outstanding paper demonstrated a more reasonable approach for evaluating heavy metals in the diet. This paper compared levels of metals, including lead and cadmium, from food and water consumption with reference values (“safe” levels) set by federal or international agencies. The purpose was to see if levels in food and water were higher than reference values, which would demonstrate a potential public health concern.
The levels of lead and cadmium in food were derived from population-based national surveys, such as the National Health and Nutrition Examination Survey (NHANES), and water concentrations were from the EPA and FDA. The reference values were daily maximum safe exposure levels set by EPA, FDA, or international agencies; they did not use the California MADLs.
Running the Numbers
To re-evaluate the potential health concern from the dark chocolate bars from the Consumer Reports article, I used the data provided in this more recent, publicly available study. I compared the amount of lead and cadmium supplied in food and water (background exposure levels) with the reference values selected in the paper and determined how many candy bars would need to be eaten to exceed the reference value.
It is essential to understand that the reference values are set with a large margin of safety and do not represent a bright line between unsafe and safe.
Lead
The highest level of lead found in a candy bar was 0.905 mcg
- Adults - Since an average adult gets 8.4 micrograms/day from food and water, an adult could eat two average size candy bars a day containing the highest lead levels measured and still be under the reference value of 12.5 micrograms per day.
- Children – As for toddlers, no candy bars for you! In 2018, the FDA revised its interim Reference Level for lead from 6 micrograms to 3 micrograms, based on CDC guidance lowering blood lead reference value from 10 micrograms per deciliter of blood to 5 micrograms per deciliter of blood. With this change, children (2-3 years old) with food and water consumption at an average lead level of 3.3 micrograms per day are already slightly above the reference value of 3 micrograms per day
As I’ve previously written, lead is a concern for children’s development; even small amounts can cause damage to the brain and nervous system. However, lead from other sources, such as paint and soil, is of much greater concern than lead in dark chocolate, which due to its bitter taste, is rarely a treat of choice for children.
Cadmium
The highest level of cadmium found in a candy bar was 10.37 micrograms. With a background exposure level of 20.8 micrograms, an adult could eat three candy bars daily and still be under the reference value of 80 micrograms per day.
It is disappointing that the Consumer Reports article went for the headlines instead of delving into the actual risk. It is also concerning that California has not updated its MADL for lead since it was set in 1989 using a fundamentally flawed approach and that Consumer Reports chose to use this reference value. A comparison with reference values, other than those from California, shows minimal risks for consuming dark chocolate, so chocoholics can breathe easily.
[1] There are two reference values, one for women of childbearing age and one for children, because these are the populations most at risk from lead exposure. Food and water levels are provided for all adults. The background exposure levels are standardized to EPA values of 80 kg for an adult and 13.8 kg for a 2–3-year-old child
