Why a cancer scare around aspartame is mostly unfounded

KATE MOSS, a British model, once quipped that “nothing tastes as good as skinny feels”. Drinkers of Diet Coke, the sugar-free version of the stuff in red cans, and of which Ms Moss is the current face, may agree. Why else would they drink it, when everyone knows (or at least this correspondent does) that Diet Coke isn’t a patch on the real thing? An announcement from the International Agency for Research on Cancer (IARC), an arm of the World Health Organisation (WHO), may give them pause. On July 14th it categorised aspartame, the artificial sweetener used in Diet Coke, as “possibly carcinogenic”. Aspartame is in more than 5,000 products, including cough drops and toothpaste. Should you be worried?

The agency put the sweetener in the third of four tiers of hazard. The first—and most serious—is for things that are deemed definitely carcinogenic, such as tobacco and sunlight; the second is for “probable” cancer hazards. For products in the third tier, which are “possible” carcinogens, the evidence of the potential to cause cancer in humans is “limited”. Aspartame ranks alongside bracken ferns, some pickled vegetables and aloe vera. There is stronger evidence that very hot drinks and working in hairdressing are hazardous.

Although the IARC report flagged aspartame as a possible hazard, that does not mean it poses a risk when consumed in the amounts typically found in daily life. “The IARC assesses whether [a substance] would be capable of presenting a risk, under any circumstances, even if the only harmful circumstances are really, really unlikely to occur,” said Professor Kevin McConway of the Open University in a statement. The agency that evaluates the actual risk of food additives, a joint committee convened by the WHO and the UN, released an independent but complementary report at the same time as the IARC. It found “no convincing evidence from experimental animal or human data that aspartame had adverse effects after ingestion.” Its guidelines suggest that it is safe to consume 40mg of aspartame for each kilogram of body weight. That means a person weighing 60kg would need to drink more than 12 cans of Diet Coke every day for their aspartame intake to possibly pose a risk.

The studies of aspartame on which the IARC based its conclusions have limitations. Among the three suitable cancer studies in humans that were assessed by the IARC, artificially sweetened drinks were used as a proxy for aspartame. They showed a positive association between the drinks and the risk of liver cancer, but could not rule out “chance, bias or confounding” as the explanation. Three published studies also found increased incidence of tumours in rodents, but the IARC questioned the reliability of their design, interpretation and reporting. Separately the committee found no evidence of genotoxicity—ie, no evidence that aspartame might damage DNA or chromosomes in a way that could provoke cell mutation, which might in turn induce cancer. It concluded that “it is not possible to establish a link between aspartame exposure in animals and the appearance of cancer”.

Some companies have substituted aspartame for other artificial sweeteners in recent years. PepsiCo removed it from its low-calorie fizzy drinks in America in 2015, before reinstating it and then removing it again in 2020. But not all sweeteners are alike: companies have to consider price, how easy they are to process and, perhaps most important, taste. When Coca Cola changed the recipe of its original drink in 1985, it received over 40,000 calls and letters of complaint; a psychiatrist hired to listen in to the company hotline told executives that some people sounded as if they were discussing the death of a family member. Many brands may feel that sticking with aspartame is their best bet. So far there has been nothing to suggest that this need worry consumers. ■