For the first time, a semi-independent committee for the World Health Organization said Thursday that it’s determined that aspartame, a popular artificial sweetener found in thousands of products like diet sodas and sugar-free gum, should be categorized as “possibly carcinogenic to humans.”

But as alarming as the designation might sound, this label does not mean your diet soda causes cancer.

The designation means that some of the research reviewed by WHO’s International Agency for Research on Cancer (IARC) shows that there may be a possible link between aspartame and liver cancer, but that science is by no means conclusive, like it is for a substance like asbestos or tobacco.

Aspartame is considered one of the most studied food additives in existence. Multiple regulatory bodies like the US Food and Drug Administration have repeatedly said that aspartame is safe for human consumption if used within certain guidelines. In fact, a separate WHO committee of experts also did a risk assessment on aspartame and said Thursday that WHO’s own guidelines do not need to change.

While some scientists and food and beverage manufacturers worry that WHO’s label of “possibly carcinogenic” will confuse consumers, the agency said its hope is that this designation will prompt scientists to do even more research on aspartame and a possible link, if any, to cancer.

The FDA said on its website Friday that it disagreed with IARC’s conclusion about aspartame being a possible carcinogen to humans. It took issue with the studies the committee relied on to come to its conclusion, saying in an email to CNN that the research had “significant shortcomings.”

“FDA scientists do not have safety concerns when aspartame is used under the approved conditions,” the FDA said.

American Beverage, an association representing the non-alcoholic beverage industry, said in a statement that “There is a broad consensus in the scientific and regulatory community that aspartame is safe. It’s a conclusion reached time and time again by food safety agencies around the world.”

The association said safety is always the highest priority of its industry.

“The fact that food safety agencies worldwide, including the FDA, continue to find aspartame safe makes us confident in the safety of our products.”

What is aspartame?

Aspartame is one of the most commonly used artificial sweeteners in the world.

It has been on the market for decades. The Calorie Control Council, an international association that represents the low- and reduced-calorie food and beverage industry, says aspartame is found in about 6,000 products globally.

Aspartame can show up in places you might not expect, like in toothpastes or medications, but it’s more often seen on the labels of products marketed as “diet” or “sugar free.”

Sodas like Diet Coke, Coke Zero and Pepsi Zero Sugar have aspartame, as do many low-calorie coffee sweeteners like Equal and NutraSweet, and some juices. In food, aspartame is often in no-sugar salad dressing, low-calorie ice cream, gelatins and puddings like Jell-O Sugar Free Instant Pudding. It’s also in sugar-free gum like Extra.

Although WHO said in a separate decision in May that people shouldn’t rely on non-sugar sweeteners for weight control, aspartame is often used in “diet” drinks because it has fewer calories than regular sugar. Compared with regular table sugar, aspartame is also about 200 times sweeter, so products don’t need as much of it.

If you put a package of Equal in your coffee, it would have about the same sweetness as 2 teaspoons of regular sugar. The packet of Equal has 4 calories, but two teaspoons of sugar have 32 calories.

Initial confusion about aspartame

There’s been some confusion about aspartame from the beginning.

In 1974, the FDA approved the use of aspartame in some foods and beverages, but the decision was suspended for a few years because of some contradictory studies, objections to its approval and questions about the initial studies themselves. Some scientists were concerned when an early animal study showed that aspartame may have caused brain tumors in rats.

It wasn’t until 1981, after a thorough investigation, that the FDA finally allowed the marketing of aspartame in dry foods. That year, the FAO/WHO Expert Committee on Food Additives determined the parameters for an acceptable daily intake.

Aspartame guidelines

WHO’s guidelines haven’t changed since 1981: a daily maximum of 40 milligrams of aspartame per kilogram of body weight. The US recommendations are slightly more generous; in 1983, the FDA set the guideline at 50 milligrams per kilogram of body weight.

A person would have to drink a lot of soda or eat a lot of food with aspartame in it to consume that much.

AmericanBeverage says diet sodas typically contain an average of 100 milligrams of aspartame per can.

With the WHO recommendation of 40 milligrams of aspartame per kilogram of body weight per day, someone who weighs the American average of 83 kilograms or 184 pounds could drink up to 33 cans a day and stay within the limits. The US recommendation of 50 milligrams per kilogram would permit more than 40 cans for an 184-pound person.

How the classification process works

Hundreds of studies have lookedat aspartame and any possible health effects.

Even with all the research saying it’s safe, as science advances, the FDA and other regulatory bodies have continued to re-examine exactly how safe it is. The latest review was the first time WHO’s independent cancer expert panel had decided to officially review the evidence that has been gathered over the years.

IARC, WHO’s cancer research arm, takes that science and does a hazard assessment. The committee looks at items such as chemicals, viruses and food additives to determine if any can cause cancer. It does this kind of review in five-year cycles, says Dr. Mary Schubauer-Berigan, the acting head of the IARC Monographs Programme. In 2015, this panel put processed meat like sausage and hot dogs in the same cancer category as cigarettes.

For its review of aspartame, the committee looked at lab data, data from animal studies and human data. One of the 2022 studies from France of 100,000 adults showed that people who consumed food or beverages with a lot of artificial sweeteners, including aspartame, had a slightly higher cancer risk than those who consumed fewer sweeteners. It was an observational study, so it could not determine whether the sweetener was the reason for the higher the cancer risk. The study also relied on people to recall what they ate or drank and record it, and such self-reported research is sometimes considered less reliable.

Another study from 2020 in mice and rats reanalyzed research from Italy’s Ramazzini Institute and found that aspartame caused leukemia and lymphoma in the animals. But scientists point out that humans are not rats or mice, and most animal studies of aspartame done by the National Toxicology Program have not seen a cancer link.

At a news conference Wednesday, Schubauer-Berigan urged scientists to do more and better research on the topic.

“Despite consistent positive findings in these three studies, the working group concluded that chance bias and confounding could not be ruled out with reasonable competence, and thus they concluded that the evidence was limited,” she said. “The working group concluded as well that there was limited evidence for cancer in experimental animals, based on a set of studies in mice and rats. There was also limited mechanistic evidence that aspartame exhibits key characteristics of carcinogens.”

Dr. Francesco Branca, director of WHO’s Department of Nutrition and Food Safety, who was part of the group that did the risk assessment of aspartame, said his committee came to a similar conclusion.

“The main conclusion of the panel was that there is no convincing evidence from experimental or human data that has aspartame as adverse effects after ingestion, within the limits established by previous committee, which is the acceptable daily intake of 40 milligram per kilogram body weight. So basically, the committee reaffirmed the acceptable daily intake level of zero to 40 milligrams per kilogram body weight,” he said.

The studies give conflicting results, he said, and it was not possible to demonstrate any general toxic effect.

“It was also not possible to have any consistent or convincing evidence from the animal studies,” Branca said.

The committee analyzed the 12 animal studies, and the majority gave negative results. Those that found a cancer connection were limited in the design and in the quality of the interpretation of the data. The epidemiological studies in humans saw some cancer effects, including liver, breast and lymphoma, and some effects on type 2 diabetes, but the studies were observational, meaningthere was no proof of cause and effect.

“It could not be ruled out that there are effects that confound the results, particularly the estimate of exposure,” Branca said.

What it means in the real world

Asked whether people should consider drinking full-sugar sodas over those with aspartame, Branca noted that there’s a third option, not because of any cancer risk but because of general concerns about obesity.

“If consumers are faced with a decision on whether to take cola with sweeteners or one with sugar, I think there should be a third option considered, which is to drink water instead,” he said. It’s important to limit the number of sweet foods and beverages people eat, regardless of real or artificial sugar, he said.

Branca emphasized this was particularly important for young children, who studies have shown consume too many sweetened products. Avoiding anything sweet is a healthier option.

Some scientists who weren’t involved in the WHO decisions are concerned that calling something possibly carcinogenic could unnecessarily alarm people.

“While there is a lot of attention has been paid to this, the concerns are probably being overblown,” said Dr. Ryan Marino, a specialist in medical toxicology at University Hospitals in Cleveland.

Dr. Qi Sun, an associate professor of nutrition and epidemiology at Harvard’s T.H. Chan School of Public Health, said consumers don’t necessarily need to be worried.

“I feel the evidence is pretty sparse to say either way to say ‘aspartame is cancerous’ or to suggest that aspartame is not as carcinogenic,” he said.

But Dr. Thomas Galligan, principal scientist for food additives and supplements with the science advocacy organization Center for Science in the Public Interest, said his group has long advised consumers to avoid aspartame because of possible cancer concerns. The center has been pushing IARC to review aspartame for many years, and he said the declaration should “raise alarm bells for consumers” and for the FDA.

The FDA has no obligation to review its decisions about aspartame based on the WHO decision, but Galligan said that by US statute, no amount of cancer risk from food additives is acceptable.

“No matter how big or how small, and that’s the law of the land right now,” he said. “We really think everybody should be taking this seriously.”

Dr. Allison Sylvetsky, an associate professor in the Department of Exercise and Nutrition Sciences at the Milken Institute School of Public Health, said she thinks people should be asking a bigger question about noncaloric sweeteners in general.

“Even if they’re safe, there’s a lot of questions about whether they’re really helpful for their intended use in terms of helping with weight management or helping with preventing a variety of diseases. I think if people are concerned, the most prudent option is just choose unsweetened alternatives,” she said. “Aspartame is not something that you need to have as a part of your diet. It doesn’t have any nutritional value.

“Choosing unsweetened alternatives is probably the one way to go,” she added.