Does aspartame pose a health risk? Here’s what the science says.

Aspartame, an ingredient in soft drinks such as Diet Coke and Diet Pepsi, has long been one of the world’s most popular artificial sweeteners. Now a World Health Organization committee has declared that it’s a potential carcinogen.

The International Agency for Research on Cancer (IARC) announced its findings today after reviewing the scientific literature on aspartame and concluding that it might have carcinogenic effects. A number of public health agencies, including the Food and Drug Administration, have repeatedly said aspartame is safe, and public health leaders and food industry executives are worried that the news will confuse and consumers panic.

Here’s what we know about the health effects of aspartame and the science behind the IARC decision.

Why is WHO reviewing the safety of aspartame?

Aspartame has been on the market for decades. It was first approved as a sweetener by the FDA in 1974 and became widely used commercially in the early 1980s, when it was marketed as Equal (the little blue sweetener packets) and NutraSweet.

IARC’s new review was prompted in large part by two recent studies that reignited questions about the health effects of aspartame. One of those studies, the NutriNet-Santé cohort study, was published last year. It was a large observational study, involving 100,000 people, which was carried out in France. It was found that people who consumed higher amounts of aspartame were slightly more likely to develop breast cancer, obesity-related cancers and “overall cancer” compared to people who did not consume aspartame.

The other study, published in 2020, was carried out in rats and mice. It was a re-analysis of 15-year-old research from the Ramazzini Institute in Italy, which found that aspartame caused blood-related cancers such as leukemia and lymphoma in rats and mice, and that prenatal exposure to aspartame increased the rate of these cancers in rodent offspring.

How strong is the evidence?

Some experts caution that these recent aspartame studies have significant flaws. Mice and rats aren’t humans, and most animal studies, including studies by the federal government’s National Toxicology Program, have concluded the sweetener does not cause cancer.

Observational studies such as the NutriNet-Santé study can be informative, but they can only show correlations, not cause and effect. The study asked people to recall what foods they ate over the previous 24 hours and fill out questionnaires describing their diets, a practice that some critics say is unreliable. The study participants were mostly women who were on average 42 years old, mostly of healthy weight, fairly physically active and with high levels of education, making it hard to extrapolate to the broader population.

At the same time, people who consume a lot of artificial sweeteners are often different in many ways from people who don’t. In the NutriNet-Santé study, for example, people who consumed a lot of aspartame and other artificial sweeteners tended to exercise less and eat fewer fruits, vegetables and whole grains. They consumed more sugar, sodium and soft drinks, and they were more likely to smoke and have diabetes.

The researchers acknowledged that “residual confounding bias cannot be entirely ruled out.” Nonetheless, they concluded that their findings “do not support the use of artificial sweeteners as safe alternatives for sugar in foods or beverages.”

A number of other observational studies have examined whether there’s a link between aspartame and cancer, with varying results. One study by scientists at the National Cancer Institute, which also relied on food questionnaires, involved almost 500,000 people. It found that people who consumed higher amounts of aspartame-sweetened beverages did not have increased rates of brain cancer or blood cancers such as lymphoma, myeloma or leukemia compared to people who didn’t consume aspartame-sweetened soft drinks or beverages.

What do the FDA and other health organizations say about aspartame?

The FDA has long argued that aspartame is safe for the general population. In May, the agency bolstered its “aspartame and other sweeteners” webpage, a move that some health experts said was a sign that the agency was reinforcing its position on aspartame. The agency says that it monitors the scientific literature on aspartame’s safety and has reviewed more than 100 studies designed to identify possible toxic effects.

“Aspartame is one of the most studied food additives in the human food supply,” the FDA says on its website. “Scientific evidence has continued to support the FDA’s conclusion that aspartame is safe for the general population when made under good manufacturing practices and used under the approved conditions of use.”

As a result of legislation passed by the European Union in 2008, all food additives that had been approved for use in the EU before 2009 were required to undergo mandatory safety reevaluations. Aspartame was one of the first additives to be re-evaluated.

The European Food Safety Authority (EFSA) published its “full risk assessment” of aspartame in 2013, which concluded that aspartame and its breakdown products were safe for the general population.

How many products use aspartame?

Aspartame is used as a sugar substitute in a wide variety of foods, which are typically marketed with claims such as “sugar-free,” “diet” or “low-calorie.” You can find it in things such as candy, ice cream, Popsicles, flavored yogurts and other dairy products, jams and jellies, powdered drink mixes, and baked goods and breakfast cereals.

Aspartame, though, is perhaps most widely used in diet beverages — it has been a mainstay in everything from Diet Coke to Diet Barq’s root beer and Diet Snapple iced teas. About 95 percent of carbonated soft drinks that have a sweetener use aspartame, as well as about 90 percent of ready-to-drink teas, which represents a huge amount of the beverage market share.

How much aspartame can I safely consume?

The amount of aspartame in packaged foods varies from one product to the next. The FDA has established what it calls an acceptable daily intake level, or ADI, for aspartame and other artificial sweeteners, which is the amount that’s considered safe to consume on a daily basis over a person’s lifetime. The ADI is based on toxicological studies in rats. Scientists determined the highest exposure level for aspartame that was shown to cause no adverse effects in rats, and then that level was lowered tenfold to account for differences between humans and rodents, and then lowered another tenfold to provide a large safety cushion, experts say.

The FDA’s acceptable daily limit for aspartame is 50 milligrams per kilogram of body weight per day, while the EFSA has a slightly lower ADI for aspartame of 40 mg/kg per day. To reach the EFSA acceptable daily intake for aspartame, an adult who weighs 132 pounds, or roughly 60 kilograms, would have to drink about 36 cans of diet soda a day.

What’s the reputation of IARC, the group making the recommendation?

IARC is an agency within the WHO that routinely evaluates potential causes of cancer in humans. While IARC is well respected, it has received a lot of criticism for the methods it uses to classify potential carcinogens. IARC classifies the substances and behaviors that it evaluates into one of four categories:

  • Group 1: Causes of cancer in humans.
  • Group 2A: Probably carcinogenic to humans.
  • Group 2B: Possibly carcinogenic to humans.
  • Group 3: Not classifiable as to its carcinogenicity to humans.

The classification system has been ridiculed for being confusing to the general public and even to some scientists. That’s because IARC doesn’t classify things based on the magnitudes of the risk or danger they pose — but instead according to the strength of the overall research on it.

So what do plutonium, asbestos, cigarettes, salted fish, hot dogs and other processed meats all have in common? They are all listed by IARC as Group 1 carcinogens — even though the risk associated with smoking, for example, is orders of magnitude greater than the risk associated with eating processed meat.

IARC has placed aspartame into the Group 2B category of things that are “possibly carcinogenic,” which already includes 322 different agents, ranging from aloe vera, pickled vegetables and Ginkgo biloba extract, as well as lead, gasoline, methyl mercury and engine exhaust.

If aspartame does cause cancer, what’s the mechanism?

It’s not exactly clear how aspartame might cause cancer, said Marion Nestle, an emeritus professor of nutrition, food studies and public health at NYU and the author of “Soda Politics,” a book that examines the soda industry.

Aspartame is broken down in the body into two amino acids, aspartic acid and phenylalanine, which are the building blocks of proteins.

In general, aspartic acid and phenylalanine are two “completely normal, run-of-the-mill amino acids that are found in every protein,” Nestle says.

Should I stop eating products with aspartame?

If a food contains aspartame, that’s usually a sign that it’s ultra-processed — and that alone can be a good reason not to eat it, since many health experts encourage people to limit their consumption of ultra-processed foods. One clinical trial found that ultra-processed foods caused people to consume more calories and gain extra weight and body fat. Many others have linked them to heart disease, diabetes and other chronic diseases.

Nestle said she told people that it’s best not to consume foods with artificial sweeteners or a lot of added sugars, especially soft drinks. If you need a flavored drink, then try unsweetened tea, sparkling water or fruit juice that’s been diluted with water.

If you are going to consume soft drinks sweetened with aspartame or sugar, however, then do so in small quantities. “They’re a treat,” Nestle said. “They can’t be something that people drink all day long.”

Will the WHO advice on aspartame change my Diet Coke?

Experts say it’s unlikely the organization’s decision will lead many companies to stop using aspartame in their products, in part because it could upset consumers.

In 2015, PepsiCo removed aspartame from its original Diet Pepsi, in part because of controversy over aspartame’s health effects. But the company received so much backlash from customers who preferred the aspartame-sweetened version that it eventually reversed course and brought aspartame back.

In 2015, IARC declared that processed meats cause colorectal cancer, but that hasn’t seemed to have had much impact on the amount of bacon and hot dogs that Americans consume.

Should I be worried about the health effects of other sweeteners?

In May, the WHO issued new recommendations, based on a systematic review of the literature, that discouraged people from using non-sugar sweeteners to control their weight. The WHO found that not only do artificial sweeteners not help people lose weight, but they also increase the risk of Type 2 diabetes, heart disease and early death.

While much of the research on these sweeteners is observational, there have also been rigorous, randomized controlled trials that raised questions about their health effects and their ability to help with weight loss. Some experts say that consuming non-sugar sweeteners can also increase your tolerance and desire for sweet foods.

“Treat them as an element of the diet which we should discourage,” said Francesco Branca, the director of the department of nutrition for health and development at the WHO. “Sugar sweeteners do not belong to a healthy diet.”

Cutting back on sugar is a good idea, but replacing it with other sweeteners is not the best strategy, Allison Sylvetsky, an associate professor of exercise and nutrition sciences at the Milken Institute School of Public Health at George Washington University, said of the WHO recommendations: “It forces people to think back to the very basics of, ‘Okay, how can I have a healthier diet more broadly?’ Not just substituting one ingredient for another.”

Laura Reiley contributed to this report.

Do you have a question about healthy eating? E-mail [email protected] and we may answer your question in a future column.

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