A committee of 25 international experts has determined that aspartame may "possibly" cause cancer in people, according to a report released Thursday by the World Health Organization.

This new classification, which is based on a review of "limited evidence," does not change the recommended limit on the daily intake of the artificial sweetener.

"Our results do not indicate that occasional consumption should pose a risk to most consumers," said Dr. Francesco Branca, director of the Department of Nutrition and Food Safety at the WHO, during a press conference in Geneva. He said the problem is for "high consumers" of diet soda or other foods that contain aspartame. "We have, in a sense, raised a flag here," Branca said, and he called for more research.

But the U.S. Food and Drug Administration says it disagrees with this new classification, pointing to evidence of safety. In a written statement, an FDA official told NPR that aspartame being labeled by the WHO "as 'possibly carcinogenic to humans' does not mean that aspartame is actually linked to cancer."

The WHO has long set the acceptable daily intake, or ADI, of aspartame at a maximum of 40 milligrams per kilogram of body weight per day. So, a person who weighs 60 kilograms (about 130 pounds), could consume up to 2,400 milligrams per day, which is roughly equivalent to 12 cans of Diet Coke — much higher than most people consume.

While the WHO is not changing the acceptable daily intake, Branca says "we're just advising for a bit of moderation." If people consume aspartame as a way to avoid sugar and control weight, "the benefit is not there," Branca says.

Based on a review from 2022 showing there's no clear consensus on whether sweeteners are effective for long-term weight management, the WHO now recommends against the use of non-sugar sweeteners to control body weight.

Aspartame was approved for use as a sweetener in the U.S. in 1974. Coca-Cola began blending the artificial sweetener into Diet Coke in the 1980s and popularized the zero-calorie drink with splashy ad campaigns, promoting the taste of it. But for all its popularity, there have long been skeptics and critics, and in recent years, small studies suggest that artificial sweeteners may increase food cravings in some people and alter the microbiome. In addition, a few recent studies point to potential cancer risks, which is why the World Health Organization set out to review all the data.

Two separate World Health Organization committees examined the evidence on aspartame. The International Agency for Research on Cancer used a classification system to rank the potential of aspartame to cause cancer in humans, landing on 2B, which translates to "possibly carcinogenic to humans."

The agency found "limited" evidence that aspartame may cause liver cancer, based on a review of several studies that used intake of artificially sweetened beverages as a proxy for aspartame exposure. It also reviewed the evidence from a large French study, the NutriNet-Santé study, published in 2022, that found people who consumed the most aspartame had about a 15% increased risk of cancer, including breast and obesity-related cancers, compared with people who didn't consume aspartame.

The research agency concluded that these were "high-quality" studies, however it could not rule out that the findings weren't due to chance, bias, or "confounding variables,'' meaning it wasn't sure that the increase in cancer was due to aspartame. It could be explained by other lifestyle habits or exposure to other carcinogens. "Thus, the evidence for cancer in humans was deemed "limited" for liver cancer and "inadequate" for other types of cancers, according to the analysis published in Lancet Oncology.

A second committee, the Joint Expert Committee on Food Additives, or JECFA, also reviewed the evidence and concluded that "the evidence of an association between aspartame consumption and cancer in humans is not convincing," according to a summary released by the WHO. The group pointed to inconsistent evidence and determined the acceptable daily intake levels should remain in place.

In its written response, the FDA said it disagrees with the conclusion that studies support classifying aspartame as a possible carcinogen to humans. "FDA scientists reviewed the scientific information included in the [International Agency for Research on Cancer's] review in 2021 when it was first made available and identified significant shortcomings in the studies," an FDA spokesperson wrote in an email. "We note that JECFA did not raise safety concerns for aspartame under the current levels of use and did not change the Acceptable Daily Intake ...."

Scientists have called for more long-term research, pointing out that it can take decades for cancer to develop after exposure to carcinogens. "I think there's actually been very little long-term research, surprisingly," says Dr. William Dahut, chief scientific officer at the American Cancer Society.

People want a simple yes or no answer on whether aspartame consumption can increase their cancer risk. "We don't have the evidence yet," he says. Most of the studies in people have not actually tracked the amount of aspartame people consume over time, so there's a gray area.

One link that warrants further evaluation is whether aspartame increases inflammation in the body, which could increase the risk of cancer. "We are actually doing our own research in that area," Dahut says.

Dahut says the possible link to cancer from aspartame is far less clear than it is for things like obesity and smoking, but he says it makes sense to be cautious about your intake. "Since there is a possible link, it is certainly reasonable to limit one's intake until more definitive studies are available," Dahut advises.

The American Beverage Association, a lobbying group that includes The Coca-Cola Co., PepsiCo and Keurig Dr Pepper, says the decision by the WHO to leave in place the previously established "acceptable daily intake" reinforces the position of the FDA. "Aspartame is safe," says Kevin Keane, interim president and CEO of American Beverage, in response to the World Health Organization review of aspartame.

There's conflicting evidence on whether diet soda helps people manage their weight or cut back on calories. Studies have gone both directions. Though the WHO analysis points to a lack of long-term benefits, some studies have shown that swapping caloric beverages for zero-calorie alternatives can be helpful.

"For people who are presently consuming diet soda, the worst possible decision would be to switch to regular sugar-sweetened soda," says physician Walter Willett of the Harvard T.H. Chan School of Public Health. Sugary drinks can raise the risk of obesity and Type 2 diabetes. "The best beverages for daily consumption are water, coffee and tea," Willett says.

Willett finds the evidence linking aspartame to cancer in people to be weak, and despite the uncertainties over long-term consequences, he does see a role for diet soda for people trying to manage their weight and limit sugar intake. He likens diet soda to a nicotine patch: "Possibly helpful for some people to transition from dependence, but not the best long-term solution."

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