The coloring is not necessary for the production of soft drinks and is included purely for esthetics.
Matching laboratory tests conducted by Consumer Reports on 11 different soft drinks, first reported last year, with an analysis of average consumption by Americans, the researchers found that one can a day could be enough to expose them to potentially cancer-causing levels of the chemical known as 4-MEI (short for 4-methylimidazole).
The potential carcinogen is formed during the manufacture of the familiar caramel color that is added to many widely-consumed beverages.
A law in California requires that drinks must carry warning labels if they contain enough 4-MEI to pose an excess cancer risk of more than 1 case in every 100,000 exposed people (an exposure of 29 mcg of 4-MEI every day).
Testing on 110 samples of soda brands carried out by the Consumer Reports researchers, led by a team at the Johns Hopkins Center for a Livable Future in Baltimore, MD, found that drinks contained levels ranging from 9.5 mcg per liter (mcg/L) to 963 mcg/L.
Concentrations of 4-MEI varied considerably by soda brand and state of purchase, the researchers concluded, "but were generally consistent across lots of the same beverage purchased in the same state/area."
They add: "Routine consumption of certain beverages can result in 4-MEI exposures greater than 29 mcg a day" - the level that triggers a new case of cancer in every 100,000 people consuming the drink, toxicity that was established by previous studies in mice and rats conducted by the US National Toxicology Program.
The researchers say there was not enough data from individual drinks samples to recommend one brand over another in terms of carcinogen exposure, but suggest: "State regulatory standards appear to have been effective in reducing exposure to carcinogens in some beverages."
Lead author of the study Tyler Smith, a program officer with the Johns Hopkins Center for a Livable Future, says 4-MEI levels can "vary substantially across samples, even for the same type of beverage." Smith explains:
"For example, for diet colas, certain samples had higher or more variable levels of the compound, while other samples had very low concentrations."
In the lab sampling, Malta Goya had the highest 4-MEI concentration while Coca-Cola produced the lowest value.
California listed 4-MEI as a carcinogen in 2011, under the Safe Drinking Water and Toxic Enforcement Act of 1986 - better known as Proposition 65. The authors say their results suggest that "federal regulation of 4-MEI in caramel color may be appropriate."
Variety of cola-drinking habits
To estimate consumers' exposure to the potential carcinogen, the researchers took the laboratory readings and analyzed soft drink consumption using data from the National Health and Nutrition Examination Survey (NHANES).
The data from this survey covered overall health and nutrition patterns between 2003 and 2010 for tens of thousands of US children and adults aged between 3 and 70 years.
The authors say: "We analyzed consumption of all sodas, and further categorized soda into five mutually exclusive categories: 1) cola, 2) diet-cola, 3) root beer, 4) pepper cola and 5) other (non-diet) cola."
They found the proportion of the population consuming each type of soft drink varied, with "colas being the most popular and root beer and pepper colas being the least popular."
Adolescents and young adults consumed the most of any soft drink compared with young children and older adults. Average consumptions of any soda were:
- Between 550 and 1,070 milliliters drunk each day by 16- to 20-year-olds
- Between 457 and 864 milliliters drunk each day by 45- to 64-year-olds.
Lifetime cancer risks
"This study sought to answer a critical question," says Urvashi Rangan, PhD, executive director of the food safety and sustainability center of Consumer Reports: "How much soda do American consumers drink on average?"
"This new analysis underscores our belief that people consume significant amounts of soda that unnecessarily elevate their risk of cancer over the course of a lifetime," Dr. Rangan says.
She adds: "We believe beverage makers and the government should take the steps needed to protect public health."
"California has already taken an important step by setting a threshold," Dr. Rangan says.
Keeve Nachman, PhD, is a senior author of the study and director of the food production and public health program at the Johns Hopkins Center for a Livable Future.
Also an assistant professor at the Johns Hopkins Bloomberg School of Public Health, Dr. Nachman says:
"Soft drink consumers are being exposed to an avoidable and unnecessary cancer risk from an ingredient that is being added to these beverages simply for esthetic purposes.
This unnecessary exposure poses a threat to public health and raises questions about the continued use of caramel coloring in soda."
ConsumerReports.org has produced a video giving information about 4-MEI in soda drinks.
Daily consumption of sugar-sweetened drinks is widely linked to diabetes and obesity, but less well-known include a link to girls starting their periods earlier and being put at a higher risk of breast cancer.