Aspartame: What the WHO IARC Classification Actually Means

What is aspartame?

Aspartame is an artificial sweetener approximately 200 times sweeter than sucrose (table sugar). Discovered in 1965 and approved by the FDA in 1981, it is used in thousands of products including diet sodas, sugar-free chewing gum, tabletop sweeteners (Equal, NutraSweet), flavored yogurts, and some medications.

Chemically, aspartame is a dipeptide methyl ester — a compound made from two amino acids (aspartic acid and phenylalanine) joined together. When digested, it breaks down into these amino acids plus methanol, all of which occur naturally in common foods. Aspartame contains no calories because the quantities used to sweeten food are so small that their caloric contribution is negligible.

One firm constraint: aspartame is unsafe for people with phenylketonuria (PKU), a genetic metabolic disorder. All products containing aspartame carry a mandatory warning: "Phenylketonurics: Contains Phenylalanine."

Two WHO bodies, two different questions

The July 2023 announcements confused many people because they appeared contradictory. They were not — they came from two different WHO bodies asking fundamentally different questions.

The International Agency for Research on Cancer (IARC) classifies hazards — it asks: "Is there evidence that this substance can cause cancer under any conditions?" IARC does not consider dose, exposure level, or typical human consumption patterns. It is essentially a hazard identification exercise.

The Joint FAO/WHO Expert Committee on Food Additives (JECFA) conducts risk assessments — it asks: "At the levels people actually consume this substance, is there a meaningful risk?" JECFA considers dose, metabolism, and real-world exposure. It sets Acceptable Daily Intakes.

IARC found limited evidence and classified aspartame as Group 2B ("possibly carcinogenic"). JECFA found that the evidence did not warrant changing the ADI of 40 mg/kg body weight, which has stood since 1981. Both assessments are scientifically valid — they just answer different questions.

What studies did IARC cite?

IARC's Group 2B classification for aspartame was based primarily on three types of evidence:

• Observational epidemiology: Several large cohort studies, including the French NutriNet-Santé cohort, found associations between high artificial sweetener consumption and increased cancer risk — particularly hepatocellular carcinoma (liver cancer). These are observational studies; they show correlation, not causation, and confounding is a significant issue.
• Animal studies: Some rodent studies reported increased lymphomas and leukemias at very high doses of aspartame. Methodological concerns were raised about these studies, including dose levels far exceeding any plausible human exposure.
• Mechanistic evidence: In vitro studies showed aspartame metabolites may cause oxidative stress in cells. Whether this occurs at relevant concentrations in living humans consuming normal dietary amounts is not established.

The FDA's disagreement

The FDA responded to the IARC classification with a rare direct rebuttal, stating that it disagreed with IARC's conclusion. The FDA's position, maintained since aspartame's original approval, is that the preponderance of evidence does not support a cancer risk at typical exposure levels.

The FDA noted methodological concerns with the observational studies IARC cited, including the difficulty of isolating aspartame consumption from other dietary and lifestyle factors in population studies. The FDA's ADI for aspartame is 50 mg/kg body weight — slightly higher than JECFA's 40 mg/kg — and has not been revised since 1981.

EFSA's most recent re-evaluation of aspartame (2013) also found no safety concern at current intake levels. EFSA has flagged some studies for follow-up but has not changed its position that aspartame is safe within the established ADI.

Putting the ADI in context

The JECFA ADI of 40 mg/kg body weight means a 70 kg adult could consume 2,800 mg of aspartame per day without exceeding the safety threshold. A 330ml can of diet cola contains approximately 180-200 mg of aspartame. That same adult would need to drink roughly 14 cans of diet soda every day to reach the ADI.

Survey data consistently shows that the vast majority of aspartame consumers — including regular diet soda drinkers — consume well under 10% of the ADI. The people closest to exceeding ADI thresholds are those with very high diet beverage intake combined with multiple other aspartame-containing products daily.

Frequently asked questions