Aspartame, an artificial sweetener widely found in diet sodas, chewing gum and sugar-free foods, was labeled a "possible carcinogen" Thursday by the World Health Organization's International Research Agency on Cancer.
The move, based on "limited evidence" of cancer risk in humans, continues an ongoing medical debate about the safety of the ingredient, which is used in thousands of products commonly sold at grocery stores. Much of the existing research linking aspartame to cancer risk draws from studies on mice and rats, who were found to have higher rates of blood and brain cancers when exposed to high levels of the sweetener.
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There is much less research and consensus about the cancer risks for humans, making definitive claims about aspartame's safety controversial.
Some of the most common items that contain aspartame are diet drinks, chewing gum, gelatin, ice cream, cereal, dairy products such as yogurt, toothpaste and medications such as cough drops and chewable vitamins. The chemical sweetener became popular in the 1980s and has been touted as helpful for weight management, although studies in recent years suggest non-sugar sweeteners may actually cause weight gain.
Popular products that have aspartame in them include Diet Coke and Diet Snapple, Mrs. Butterworth's sugar-free syrup, Ice Breakers mints and powdered drink mixes like Crystal Light and Hawaiian Punch. Many sugar-free coffee blends, low-calorie Jell-O flavors and ice creams — from Fudgscicle pops to Klondike bars — contain the sweetener. Aspartame sweeteners also are sold under familiar brand names like Equal, Nutrasweet and Sugar Twin.
WHO's assessment puts aspartame in a risk category below two others — " possibly carcinogenic to humans" and "probably carcinogenic" — that carry stronger warnings for consumers. Other substances considered "possible carcinogens" include aloe vera, pickled vegetables and nickel.
For years, health agencies including the U.S. Food & Drug Administration have said aspartame is a safe ingredient for people to consume in moderation. WHO's new review of research on the sweetener cites "limited evidence" that the ingredient may cause cancer in humans.
"The assessments of aspartame have indicated that, while safety is not a major concern at the doses which are commonly used, potential effects have been described that need to be investigated by more and better studies," said Dr. Francesco Branca, WHO's nutrition and food safety director.
The FDA countered that the designation is misleading and could confuse people.
"Aspartame being labeled ... as 'possibly carcinogenic to humans' does not mean that aspartame is actually linked to cancer," the agency said.
The cancer risk highlighted by WHO is more of a general advisory about the need for continued research than a recommendation to eliminate aspartame consumption altogether.
"It's a slight warning to people, but it's not 'do not consume,'" Barry Popkin, a professor of nutrition at the University of North Carolina, told NBC News. "Consume moderate levels and you're OK."
WHO's recommended limit for daily aspartame remains 40 milligrams per kilogram of body weight for an adult. The FDA's limit is set at 50 milligrams per kilogram of body weight. Most people are unlikely to consume this much aspartame day-to-day.
Robert Rankin, president of the Calorie Control Council, an organization that represents the low-calorie food industry, called the WHO's review "fear mongering." He said millions of people worldwide rely on low-calorie and no-calorie sweeteners, like aspartame, to manage diabetes and prevent weight gain.
"The average 150-pound person would need to consume about 14 12-ounce cans of diet beverages or about 74 packets of aspartame-containing tabletop sweetener every day over the course of their life to raise any safety concern," Rankin said. "Obviously, that level of consumption is not realistic, recommended, nor is it aligned with the intended use of these ingredients."
The Environmental Working Group, an advocacy organization, said the FDA should still take WHO's review seriously, noting that the last time the federal agency examined the sweetener's health risks was nearly a decade ago. The FDA first approved aspartame in 1974, but later withdrew that approval over safety concerns. It was reapproved in 1981 and given an expanded range of uses in 1996.
WHO's decision to look more closely at aspartame was prompted by a study last year that assessed 100,000 people in France for possible links between cancer and artificial sweeteners, including aspartame. That study found that people who consume higher amounts of aspartame have a slightly higher risk of developing breast cancer, obesity-related cancer and "overall" cancer than those don't consume aspartame.
Experts have cautioned that the design of that study has limitations. While it may reveal a correlation, it does not prove cause and effect. The cancer rates found in the study could be linked to other behavioral traits. The researchers found that those who consumed more aspartame tended to exercise less, smoke more and eat fewer healthy foods like fruits, vegetables and whole grains, the Washington Post reported. Another large observational study, conducted by the National Cancer Institute, did not find evidence of increased risk of blood and brain cancers in people who consumed beverages sweetened with aspartame.
Given that more research is needed over time, the message to take from WHO's review of aspartame is that avoiding it in excess is the most healthy approach. Aside from cancer, the French study and others also found possible links between artificial sweeteners and a range of other health problems, from higher blood sugar levels to increased risk of heart disease. Broader questions remain about how these sweeteners fit into a balanced diet and healthy lifestyle.
"(If) you're consuming 10 Diet Cokes or 10 Diet Pepsis in a day, you shouldn't," Popkin said. "You have to cut down, because that's way in excess, and that moves toward potential carcinogen levels."