Aspartame (E951)

What is Aspartame?

Aspartame is made from two amino acids — aspartic acid and phenylalanine — joined together. It is about 200 times sweeter than sugar, so tiny amounts replace a lot of sugar and add almost no calories. In the body it breaks down into those two amino acids and a small amount of methanol — substances that are also produced when you digest many ordinary foods.

It is sold as a tabletop sweetener under brand names like NutraSweet and Equal.

Why is Aspartame used in food?

Aspartame lets manufacturers cut sugar and calories while keeping a sweet taste. It is common in diet and "zero sugar" soft drinks, sugar-free chewing gum, light yogurts, flavored waters and tabletop sweeteners.

It is not heat-stable, so it is rarely used in baked goods. It is often blended with acesulfame potassium to give a more sugar-like taste.

Is Aspartame safe? What the research says

The 2023 WHO review. In July 2023, the WHO's International Agency for Research on Cancer (IARC) classified aspartame as "possibly carcinogenic to humans" (Group 2B), based on limited evidence for liver cancer in people and limited evidence in animals. Group 2B is a statement about the strength of evidence, not about how dangerous something is at normal intakes. At the same time, the WHO/FAO expert committee (JECFA), which assesses risk, kept the acceptable daily intake at 40 mg/kg of body weight.

What that means in practice. JECFA calculated that a 70 kg (154 lb) adult would need to drink more than 9–14 cans of diet soda a day, every day, to exceed the ADI — assuming no other sources of aspartame.

What the FDA says. The FDA disagreed with IARC's classification, saying the studies it relied on had significant shortcomings, and that FDA scientists have no safety concerns when aspartame is used under approved conditions. The FDA's ADI is 50 mg/kg; EFSA reached 40 mg/kg in its 2013 re-evaluation.

The studies behind the concern. The best-known is the French NutriNet-Santé cohort (Debras et al., 2022), which followed about 103,000 adults: people with the highest intake of aspartame had a modestly higher cancer risk (about 15% overall, 22% for breast cancer) than non-consumers. Observational studies like this cannot prove cause and effect — people who drink a lot of diet soda may differ in other ways.

Bottom line: within normal amounts, regulators consider aspartame safe. The evidence is not strong enough to call it harmful, and not settled enough to call the question closed.

Aspartame regulation: US vs EU

Aspartame is approved in the US, the EU, the UK, Canada and most other countries. The FDA first approved it in 1974 for limited uses and as a general-purpose sweetener in 1996. In every major market, products containing it must warn people with phenylketonuria that it is a source of phenylalanine.

Who should limit or avoid Aspartame?

• People with phenylketonuria (PKU) must avoid it. PKU is a rare inherited condition in which the body cannot break down phenylalanine, one of aspartame's building blocks. That is why US labels say "Phenylketonurics: contains phenylalanine."
• People who drink diet soda in large amounts every day — they are the group closest to the acceptable daily intake.
• Anyone who prefers to limit sweeteners while the research continues. Water, unsweetened drinks, or products sweetened with small amounts of sugar are alternatives.