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Aspartame’s New Status as a “Possible Carcinogen”: What Does It Mean?

A woman puts a sweetener into a cup.
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Last week, conclusions from an upcoming report by the International Agency for Research on Cancer (IARC), a branch of the World Health Organization, were leaked by the Reuters news agency. Reuters reported that the IARC was set to list the common sweetener aspartame – present in drinks and foods like diet sodas, sugar-free chewing gum and low-calorie yogurt – as a “possible carcinogen”.

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The announcement follows several studies that have explored the physiological effects of the sweetener. What does aspartame’s new designation mean, and should it alter your food choices? Let’s take a deep dive.

Risks and hazards

The IARC classifies carcinogens by evidence of hazard rather than by level of risk. What this means in practice is that top-level class one compounds, which show high evidence of being cancer-causing, include both neutron radiation – emitted by nuclear reactors – and processed meats, like salami. That doesn’t mean, unsurprisingly, that eating a deli sandwich should be considered as risky as hopping into a nuclear cooling tower.

What is a “possible carcinogen”?

Carcinogens are organisms, substances or agents that can cause cancer. The IARC classifies agents that it has assessed for cancer risk into four groups. Aspartame is in group 2b.

  • Group 1: Carcinogenic to humans.
  • Group 2A: Probably carcinogenic to humans
  • Group 2B: Possibly carcinogenic to humans
  • Group 3: Not classifiable as to its carcinogenicity to humans

In the same sense, the IARC’s new designation for aspartame simply reflects the body’s recognition of a growing evidence base around the compound’s cancer hazard. Other “possible cancer risks” in the same category include aloe vera extract, chemical exposure from working as a dry cleaner, pickled Asian vegetables, weapons-grade tungsten alloys and the chemotherapy medicine Bleomycin. This is not an easily comparable grouping.

What people drinking or eating aspartame beverages really want to know is will they materially increase their cancer risk? The IARC’s simplistic classification system doesn’t answer that question. Reaction from the research community has reflected that reality, with many experts first calling for the full report to be released so that its findings can be assessed in depth.

They also highlight that risk is a question of dose. “We know UV light in sunlight causes cancer, that’s why we put on sunscreen at the beach – but we don’t put on sunscreen when we go outside in winter even though we are still exposed to sunlight, why?  Because the dose is lower in winter,” said Professor Oliver Jones of RMIT University in Melbourne.

Hitting the sweet spot of aspartame consumption

Kevin McConway, an emeritus professor of applied statistics at the Open University, emphasized the difference between hazard and risk in his comments on the leak. Many activities and substances on Earth are hazardous to our health but pose only a tiny risk. The Australian box jellyfish, armed with venom-laced stingers and hard-to-see translucent bodies, is certainly hazardous. But, unless you happen to spend a significant amount of your time drifting in the South China Sea, you probably are never going to encounter one, meaning the overall risk of a jellyfish-related death is low.

Aspartame is a very common sweetener, but the cancer hazard it poses may only become a meaningful risk in rare scenarios. While it is as yet unclear what evidence has factored into the IARC’s decision-making, recent studies in mice fed the sweetener sucralose only found negative physiological impacts at very high daily doses.

McConway points out that the Joint FAO/WHO Expert Committee on Food Additives (JECFA) ruled over 40 years ago on a “safe” daily consumption limit for aspartame that would require a 150-lb person to drink more than a dozen 12-ounce cans of diet soda to hit. That limit is also currently under review and the findings are set to be released alongside the IARC report on 14 July.

The findings are part of a series of studies that have explored the potentially negative side effects of artificial sweeteners. A study published in the Journal of Toxicology and Environmental Health, Part B found that the sucralose metabolite sucralose-6-acetate may cause direct or indirect damage to DNA molecules. The WHO announced in May in what it called a “low certainty” recommendation that using artificial sweeteners does not reduce body weight, and may even be linked to an increased risk of type 2 diabetes and cardiovascular disease.