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Aspartame is not, repeat not, a dangerous carcinogen

Recent media reports on the widely used artificial sweetener have created fear and confusion

The announcement last week by the International Agency of Research on Cancer (IARC) that artificial sweetener aspartame was to be designated a group 2B carcinogen made headlines across the world. The articles expounded on the potential dangers of the ubiquitous substance, which is found in everything from diet drinks to chewing gum.

But beneath the media glare, the fact that aspartame is one of the most studied foodstuffs in history with no solid evidence for carcinogenicity was conspicuously muted. The same week the Joint Expert Committee on Food Additives (JECFA) ruled that there was no “convincing evidence from experimental animal or human data that aspartame has adverse effects after ingestion”.

These seemingly conflicting statements from complementary reports is a microcosm of the persistent problem of communicating risk about our health and quantifying uncertainty.

To untangle this, we need to place the new designation in context. The IARC’s classification places aspartame in the “possibly” causes category, alongside an ocean of other things, including magnets, aloe vera, pickled vegetables and wifi. This terminology creates confusion, much of which stems from the fact that IARC’s classification system is not remotely intuitive.


Its primary purpose is to classify substances not by the degree of hazard those substances might pose, but by the strength of evidence for that substance having an inherent cancer risk. Substances and activities designated group 1 are those with strong evidence of cancer risk in humans. Smoking, alcohol, obesity and sunlight are all group 1, with overwhelming evidence they can cause cancer. But crucially IARC classification tells us nothing about how likely any of these things is to cause cancer.

By rough analogy, one could light one’s fireplace either with matches or lob a Molotov cocktail into it instead; both have some intrinsic risk of causing a house fire, but these risks are in no way equal. This nuance with IARC classification can and does confound the public.

In 2015 the designation of processed meats as a group 1 carcinogen led to a clutch of dubious headlines worldwide asserting bacon was as bad as smoking. This was a fundamental and glaring misconception; over a lifetime, those consuming the highest level of processed meat had a 1 per cent higher incidence of colorectal cancer relative to those who never ate such products. This extremely modest risk is not remotely comparable to the stark rise in the risk of lung cancer intrinsic to smoking.

Commenting on that controversy, writer Ed Yong noted that IARC is “notable for two things. First, they’re meant to carefully assess whether things cause cancer, from pesticides to sunlight, and to provide the definitive word on those possible risks. Second, they are terrible at communicating their findings”.

Confounding issues is the inherent difficulty of proving a negative in science. To date, only one substance in existence has been classified by IARC as group 4, “not causing cancer” – caprolactam, a substance found in yoga pants. Group 3 agents are not classifiable, while group 2 is further subdivided into group 2A (“probable carcinogens”) and group 2B (“possible carcinogens”).

This in practice frequently makes group 2B an epistemic and epidemiological dumping ground. Anything without copious evidence behind it is confined to this wilderness, and even agents with much experimental work reaffirming their safety can remain in this state for decades. Scientific results often conflict due to differing methodological quality of the underlying research, with agents remaining in limbo even without clear evidence of harm.

While this is the first time the IARC has considered the evidence base on aspartame, it is the third time its sister body the JECFA has reaffirmed its safety, with both WHO bodies advocating that a safe dose limit set in the 1980s remains unchanged.

For aspartame, there is simply no solid evidence of harm and plenty of evidence to the contrary

The furore over aspartame has an additional confounding factor – a history of suspicion. That aspartame is much sweeter than sugar without the calorific density has long made it an object of scorn by those who project moral values on to food. It has variously been deemed unnatural and viewed with contempt, with periodic controversies swelling over it since the 1970s.

In the mid-1990s it was the subject of one of the first internet hoaxes. A chain email was circulated, purporting to be from a scientific conference, linking aspartame to lupus, cancer and the then topical Gulf-war syndrome. While quickly exposed as a fraud by a notorious crank, its canards persisted on the nascent web, resurfacing even today.

In the early 2000s an Italian group’s claim to have found evidence that aspartame caused cancer in rats captured worldwide headlines, before subsequent analysis by both the European Food and Safety Authority and the US Food and Drug Administration (FDA) found the study was riddled with severe methodological flaws, rendering their ostensible result illusory.

Curiously, the current IARC designation has seen an echo of such criticism from other regulatory bodies. The FDA publicly disagreed with IARC’s conclusion, finding “significant shortcomings in the studies on which IARC relied”, reiterating that “aspartame is one of the most studied food additives in the human food supply. FDA scientists do not have safety concerns when aspartame is used under the approved conditions”. Such disagreements on borderline findings are not uncommon in science, where degrees of uncertainty are sadly unavoidable, despite our craving for simple answers. For aspartame, there is simply no solid evidence of harm and plenty of evidence to the contrary.

In the sound and fury it is easy to lose sight of the reality that our recommendations and risks remain unaltered. While the IARC has an important mission, its classification system is often fundamentally misunderstood, remaining obtuse to many, including media outlets. The net result is cyclical panics that can do lasting harm to public understanding, to our collective detriment.

Dr David Robert Grimes is a scientist, CFI fellow, and author of The Irrational Ape – Why we Fall for Disinformation, Conspiracy Theory and Propaganda