On a UK label
E951 appears as "aspartame", sometimes alongside brand names like NutraSweet or Canderel. Any UK product containing aspartame must carry the warning "contains a source of phenylalanine" — that warning IS the easiest way to spot it. In July 2023 the WHO's International Agency for Research on Cancer classified aspartame as Group 2B "possibly carcinogenic to humans", while JECFA reaffirmed the existing acceptable daily intake.
What it is
E951 is aspartame, a synthetic sweetener about 200 times sweeter than sucrose. It's a dipeptide methyl ester — when digested it breaks down into three components the body already handles in food: the amino acids aspartic acid and phenylalanine, plus a small amount of methanol. Discovered in 1965; approved in the UK and EU through the 1980s.
- ~200 times sweeter than sugar
- Effectively zero calories at use levels (it contains 4kcal/g like protein, but you use milligrams)
- Not heat-stable — loses sweetness in baking, which is why it's rare in cooked products
- The dominant sweetener in UK diet colas, often blended with acesulfame K
Where you'll see it on a UK label
- Diet drinks: Diet Coke, Coke Zero (with acesulfame K), Pepsi Max, sugar-free squash
- Chewing gum and mints: most sugar-free gum
- Reduced-calorie desserts: sugar-free jelly, low-fat yoghurt, reduced-sugar ice cream
- Protein bars and shakes
- Table-top sweeteners: Canderel, NutraSweet
- Sugar-free sweets and lozenges
Phenylketonuria (PKU) — mandatory UK warning
People with phenylketonuria must avoid aspartame. PKU is a rare inherited disorder (screened in the UK heel-prick test) where the body cannot break down phenylalanine. Build-up causes neurological damage if uncontrolled. UK food law mandates the warning "contains a source of phenylalanine" on every product containing aspartame — that warning exists specifically so people with PKU can identify and avoid these products.
What the science shows
Cancer: the 2023 IARC Group 2B classification
In July 2023 the WHO's International Agency for Research on Cancer (IARC) classified aspartame as Group 2B — "possibly carcinogenic to humans". Group 2B means there is limited evidence in humans and limited or sufficient evidence in experimental animals; other things in the same group include aloe vera whole-leaf extract and working night shifts. On the same day the Joint FAO/WHO Expert Committee on Food Additives (JECFA) reviewed the same evidence and reaffirmed the existing acceptable daily intake of 40mg/kg body weight per day. EFSA's 2013 opinion, which set the EU ADI, remains in force.
The headline number that drove media coverage: at the ADI, a 70kg adult could drink roughly 9–14 standard cans of diet cola per day before exceeding the limit. The IARC signal is at intakes above this, and the human evidence underpinning the 2B call came largely from observational studies of heavy long-term consumption. Heavy daily intake of diet drinks is the bracket the classification flags.
Methanol and formaldehyde
Aspartame releases a small amount of methanol when metabolised, and methanol metabolises further to formaldehyde — a fact frequently weaponised online. The dose matters: a 330ml diet cola releases less methanol than a glass of fruit juice or a serving of tomato juice, both of which contain methanol naturally. Published clinical and toxicology reviews place dietary aspartame methanol below physiological thresholds. The chemistry is real; the dose-equivalence framing isn't.
Headaches and individual reactions
Aspartame is one of the most commonly self-reported triggers for headache and migraine. Controlled trials are inconsistent — some show a signal in sensitive individuals, others find none. If you notice a consistent reaction, eliminate it for 2–4 weeks and reintroduce to test. That's a sensible personal experiment, not a population claim.
Gut bacteria and glucose tolerance
Animal and short human studies have reported microbiome shifts with aspartame consumption, with some signals on glucose tolerance. The clinical relevance at typical dietary intakes is unsettled. The data sit alongside similar findings for other non-nutritive sweeteners — see our sucralose page for the same picture.
How much is in everyday products
- 330ml can of Diet Coke: ~180mg aspartame
- Stick of sugar-free gum: 5–10mg
- Sachet of Canderel: ~18mg
- Sugar-free jelly pot: 30–50mg
The 40mg/kg ADI for a 70kg adult is 2,800mg per day — about 15 cans of diet cola. Most people don't approach this. The IARC 2B call relates to heavy long-term intake, not the occasional can.
Regulatory status
UK/EU: Approved (E951). ADI 40mg/kg body weight per day (EFSA 2013, reaffirmed by JECFA July 2023). Mandatory PKU warning on every product.
US: FDA approved. ADI 50mg/kg body weight per day.
IARC 2023: Group 2B — "possibly carcinogenic to humans". The IARC classification and the JECFA ADI sit alongside each other; they're answering different questions (hazard identification vs intake risk assessment).
Reading a UK label
Look for "aspartame", E951, NutraSweet or Canderel in the ingredient list. The faster signal is the phrase "contains a source of phenylalanine" — that warning is mandated specifically because the product contains aspartame. In diet colas you'll often see aspartame paired with acesulfame K (E950). Cooked or baked products rarely contain aspartame — it loses sweetness on heating, which is one reason sucralose dominates that category.