Aspartame: Artificial sweetener to be declared possible carcinogen by WHO - but what does that mean?

One of the world's most popular artificial sweeteners, aspartame, is set to be declared a possible carcinogen by the World Health Organization (WHO), according to reports - but what does 'possibly carcinogenic' mean, exactly?

The artificial sweetener - sold under brand names such as NutraSweet, Equal, and Canderel - is commonly used as a sugar substitute in foods and beverages, including the likes of Diet Coke and Coke Zero Sugar, desserts, breakfast cereals, and sugar-free chewing gum.

It was reported in June that the WHO's cancer research arm, the International Agency for Research on Cancer (IARC), had conducted a safety review of aspartame. Off the back of that review, according to Reuters, it was preparing to label the sweetener as "possibly carcinogenic to humans" - meaning it has the potential to cause cancer. This indicates there is some, albeit limited, evidence linking aspartame to the disease. The IARC's decision is set to be made public on July 14. 

The review was conducted to assess whether or not aspartame is a potential hazard based on all the published evidence, a person familiar with the matter told The Guardian. However, it does not take into account how much of a product a person can safely consume.

The IARC has two more serious groups above "possibly carcinogenic", which is group 2B: "probably carcinogenic to humans", or group 2A, and "carcinogenic to humans", or group 1. Other 'possible carcinogens', along with aspartame, include aloe vera extract, pickled vegetables, gasoline, and working as a dry cleaner.  

"Really they're just being conservative because they don't want to declare something as safe if it isn't safe," Dr Slade Matthews, a lecturer in Pharmacology at the University of Sydney, told The Project. 

"It doesn't mean they're saying it's definitely established as carcinogenic, because it indicates that their evidence is quite weak."

Examples of known carcinogens to humans, included in group 1, include tobacco, alcoholic beverages, asbestos, coal, and processed meat. Professions that include exposure to carcinogens include firefighting, iron and steel founding, painting and chimney sweeping.

Examples of probable carcinogens to humans, included in group 2A, include red meat, frying in high temperatures, non-arsenical insecticides and night shift work. Professions that include exposure to probable carcinogens include hairdressing and barbering.

However, when broken down in the body, aspartame does produce methanol, a toxic alcohol - although a very small amount. 

According to the Centers for Disease Control and Prevention (CDC), methanol occurs naturally in humans, animals, and plants. Foods such as fresh fruits and vegetables, fruit juices, fermented beverages, and diet soft drinks containing aspartame are the primary sources of methanol in the human body. Most methanol poisonings occur as a result of drinking beverages contaminated with methanol or from drinking methanol-containing products, it says. 

And according to New Zealand's Nutrition Foundation, the amount of methanol produced is often less than the amount from foods in which methanol occurs naturally, such as bananas, citrus fruit and some vegetables. 

A study in 2006 suggested that very high doses of aspartame increased the risk of certain cancers in rats, but critics of the study pointed out that the amount fed to the rats was equivalent to eight to 2083 cans of diet soda. 

But the alcohol is widely used in a number of everyday goods and products, and Dr Matthews warns not to be overly concerned about methanol consumption via the likes of Diet Coke. For example, there's about the same amount of methanol in a can of Coke as a glass of orange juice, he noted.

"The fundamental maxim of toxicology is it's the dose that determines the likelihood and severity of a toxic effect - and there's only a tiny bit of methanol [in aspartame]," he told The Project. 

"My calculation, based on a 50kg person, is that it'd [take] about 11 cans [to produce a toxic effect]. But 11 cans - that's no good. It's not going to be any good for your kidneys if you go around drinking 11 cans of soda. I think you can have a can or two a day with no problems," he added. 

If the sweetener is officially declared a possible carcinogen on Friday, the decision will likely prove controversial. In a statement to The Guardian, Frances Hunt-Wood, the secretary general of the International Sweeteners Association,  expressed concerns about the reports and warned that the IARC is not a food safety body.

"Aspartame is one of the most thoroughly researched ingredients in history, with over 90 food safety agencies across the globe declaring it is safe, including the European Food Safety Authority, which conducted the most comprehensive safety evaluation of aspartame to date," she said. 

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