Soft drinks have never been the most nutritious dietary choice, but new research suggests that an unnecessary colouring ingredient may pose a cancer risk, even to people who consume an average amount of soda.

The chemical in question is a byproduct of caramel colour, a common ingredient that gives colas, root beers and iced teas their dark hue. During production of caramel colour type III and IV, a chemical known as 4-methylimidazole (4-MEI) can be produced. 

And here's where the problem lies, because 4-MEI has been classified as possibly carcinogenic to humans by the International Agency for Research on Cancer, as a result of "equivocal evidence of carcinogenic activity in female rats", but no human studies.

Of course, that doesn't mean the chemical definitely causes cancer, but the evidence was strong enough that, in 2011, California ordered that all drinks containing more than a safe level of 4-MEI, set as 29 μg/day, carry a warning label. The safe level of a chemical is determined by the amount that would cause less than one cancer case per 100,000 people exposed to it.

Now a team of researchers, led by the John Hopkins University Centre for a Liveable Future in the US, has assessed how much soft drink the average American drinks, and compared this to the levels of 4-MEI in cans of drink to assess whether the chemical really poses a risk to the average American consumer.

Publishing in PLOS ONE, they found that between 44 and 58 percent of people over the age of six in the US drink at least one can of soft drink every day on average. And given this rate of 'average consumption', they conclude that average soda drinkers could potentially be exposed to unsafe levels of 4-MEI over a lifetime.

"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 aesthetic purposes," said Keeve Nachman, senior author of the study, in a press release. "This unnecessary exposure poses a threat to public health and raises questions about the continued use of caramel colouring in soda."

Their research built on a previous study they'd done with Consumer Reports in 2013 and 2014. Over those two years, the team analysed 110 soft drinks bought in either California (where a warning label must be used above certain 4-MEI levels) or New York. 

Altogether they looked at 11 different types of soft drinks, all in can form, except for Goya Malta, a carbonated and, according to the brand's website, "nutritious", malt drink, which is only sold in glass bottles.

The team admits that it didn't look at enough soft drink samples to make any recommendations about which particular brands of soft drink have lower levels than others (you can see the full table in their paper, but keep in mind that these results definitely need to be replicated), but it did show that 4-MEI levels can vary greatly from can to can, even in the same type of soft drink.

"For example, for diet colas, certain samples had higher or more variable levels of the compound, while other samples had very low concentrations," said Tyler Smith, another lead researcher, in the release.

The team has now submitted their results to the US Food and Drug Administration, in the hopes it will set some national limits for how much 4-MEI can be present in drinks.

"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," said Urvashi Rangan, the executive director for Consumer Reports' Food Safety and Sustainability Centre, in a press release. "We believe beverage makers and the government should take the steps needed to protect public health."