WHO called aspartame a possible carcinogen and left the safe intake unchanged

In July 2023, the International Agency for Research on Cancer classified aspartame as possibly carcinogenic to humans, IARC Group 2B, on the basis of limited evidence for cancer in humans — specifically for hepatocellular carcinoma, a liver cancer — plus limited evidence in animals and limited mechanistic evidence [s1]. On the same day, the Joint FAO/WHO Expert Committee on Food Additives reaffirmed the acceptable daily intake of 0–40 mg per kilogram of body weight, concluding there was no sufficient reason to change it [s1]. Both statements were released together, and the apparent contradiction between them is the whole story of how food-safety science is reported.

Hazard is not risk

IARC identifies hazard: whether an agent has the capacity to cause cancer at all, considering every route of exposure — dietary, occupational and otherwise [s1]. Its classifications reflect the strength of the scientific evidence, not the risk of developing cancer at a given exposure level [s1]. Group 2B is the third of four levels, used when there is limited but not convincing evidence in humans, or convincing evidence in animals but not both [s1].

JECFA does the other job: it estimates the probability of harm at actual exposure levels [s1]. Its arithmetic on aspartame is concrete. With a can of diet soft drink containing 200 or 300 mg of aspartame, an adult weighing 70 kg would need to drink more than 9 to 14 cans a day to exceed the acceptable daily intake, assuming no aspartame from other food sources [s1].

WHO's nutrition director summarised the joint position as safety not being a major concern at commonly used doses, while potential effects have been described that need investigation by more and better studies [s1]. IARC's own monographs representative framed the finding as underscoring the need for more research on whether aspartame poses a carcinogenic hazard at all [s1]. JECFA concluded the evidence of an association between aspartame consumption and cancer in humans is not convincing, and called for better studies with longer follow-up, repeated dietary questionnaires in existing cohorts, and randomised trials [s1].

The separate, and stronger, finding about weight

Two months earlier, WHO had issued a guideline recommending against the use of non-sugar sweeteners to control body weight or reduce the risk of non-communicable diseases [s2]. That recommendation rests on a different evidence base from the cancer question: a systematic review finding that non-sugar sweetener use confers no long-term benefit in reducing body fat in adults or children, and suggesting possible undesirable effects of long-term use including increased risk of type 2 diabetes, cardiovascular disease and mortality in adults [s2].

The guideline is explicitly conditional, not strong. WHO stated that the observed link between sweeteners and disease outcomes might be confounded by baseline characteristics of study participants and by complicated patterns of sweetener use, and that policy decisions based on it may require substantive discussion in specific country contexts [s2]. The recommendation applies to all people except those with pre-existing diabetes, and covers acesulfame K, aspartame, advantame, cyclamates, neotame, saccharin, sucralose, stevia and stevia derivatives — but not sugar alcohols, which contain calories and are not classed as non-sugar sweeteners [s2].

Where the confounding shows

The pattern WHO alludes to is visible in the underlying literature. A CMAJ systematic review pooled seven randomised trials (1,003 participants, median follow-up six months) and 30 prospective cohort studies (405,907 participants, median follow-up 10 years) [s3]. In the trials, non-nutritive sweeteners had no significant effect on body mass index (mean difference −0.37 kg/m², 95% CI −1.10 to 0.36, 242 participants) [s3]. In the cohorts, sweetener consumption was associated with a modest BMI increase (mean correlation 0.05, 0.03 to 0.06) along with higher incidence of obesity, hypertension, metabolic syndrome, type 2 diabetes and cardiovascular events [s3]. The reviewers noted publication bias for studies with diabetes as an outcome [s3].

The direction of that split is what you would expect if people who are gaining weight switch to diet drinks, rather than the reverse. It does not prove that is what happened.

A Lancet Global Health review in children and adolescents found the same tension and one further detail. Across five randomised trials (1,498 participants, median follow-up 19 weeks) comparing non-nutritive sweeteners with sugar, random allocation to sweeteners suggested less BMI gain (standardised mean difference −0.42 kg/m², −0.79 to −0.06) [s4]. Eight prospective cohorts (35,340 participants) found a non-significant association in the opposite direction (0.05 kg/m², −0.02 to 0.12 per daily 355 mL serving) [s4]. Three of the five trials were classified as having potential conflicts of interest, and removing studies with such conflicts attenuated the estimates [s4]. No randomised trial had tested sweetened beverages against plain water [s4].

What is genuinely unknown

Whether aspartame causes cancer in humans at ordinary intakes: unresolved, with the evidence described as limited on all three axes IARC assesses [s1]. Whether long-term sweetener use raises cardiometabolic risk: the cohort signal exists, the trial evidence does not support it, and reverse causation is a live explanation [s2] [s3]. And the comparison most people actually face — a sweetened drink versus water — has, remarkably, not been run as a randomised trial in children at all [s4].

This article is informational and is not medical advice. Acceptable daily intakes are regulatory exposure limits, not personal targets, and questions about sweetener intake in pregnancy, phenylketonuria or diabetes belong with a clinician.

Sources

• Aspartame hazard and risk assessment results released — WHO / IARC, 2023-07-14
• WHO advises not to use non-sugar sweeteners for weight control in newly released guideline — World Health Organization, 2023-05-15
• Nonnutritive sweeteners and cardiometabolic health: a systematic review and meta-analysis — CMAJ, 2017
• Effects of non-nutritive sweeteners on the BMI of children and adolescents — The Lancet Global Health, 2023