Is Diet Coke giving you cancer?

Health & Wellness

A: Probably not.

TL;DR: Water is probably the healthiest beverage of choice, but don’t stress if you enjoy your diet drinks in moderation.

Recently the International Agency for Research on Cancer (IARC), a World Health Organization body, announced that it will classify aspartame, the artificial sweetener used in many diet drinks, as a class 2B carcinogen.

We know—this sounds scary. But like everything in life from driving a car to swimming in the ocean, it’s important to put these risks in perspective.

A “class 2B” carcinogen means that the substance “possibly” causes cancer—compared to the higher categories of 1 (causes cancer) and 2A (probably causes cancer). “Possibly” means there is some evidence that is worthy of further research, but it is far from definitive.

IARC’s determination was based on three observational studies that linked consumption of artificially sweetened beverages to increased levels of liver cancer. The FDA, which disagreed with IARC’s claims, argued that these observational studies were not strong enough to make this kind of determination. Even IARC noted that it was impossible to rule out that the increased risk of liver cancer in those observational studies was due to chance or “confounding” factors–people with higher health risks in the first place may be more likely to drink diet sodas to lose weight. Other recent review studies have concluded that artificial sweeteners do not raise the risk of cancer.

It’s also important to note that as with many things in life, “the dose makes the poison.” IARC doesn’t assess the magnitude of a risk, but rather the possibility of a risk existing. When animal studies suggest potential toxicity of a substance, it often refers to doses MUCH higher than humans could reasonably consume. Other things that can be toxic in high doses include vitamin A, caffeine, and even water. Members of another WHO body who objected to the IARC decision stated that a 150-pound person could consume around 12 diet drinks a day and still not be at higher risk of cancer. And even if the elevated cancer risk in these studies were 100% true, aspartame consumption would move the risk of liver cancer from a very low level…to a slightly higher but still very low level. These are often the types of risks we are willing to tolerate in the course of daily life. This is completely different from a well established and powerful carcinogen like smoking, which raises the risk of many cancers dramatically.

But even if the evidence for a cancer risk is not strong, there may be other reasons to think about how much aspartame you are consuming. In March the WHO recommended against using artificial sweeteners to control body weight, citing a review showing no long-term benefits. This could be because substituting diet drinks for sugary ones can reduce caloric intake in the short term, but over time it is easy to compensate. There was also some evidence of increased risk of diabetes in people who consume artificial sweeteners, but this was considered inconclusive. As mentioned above, because people at risk of weight gain and obesity-related diseases may be more likely to consume diet products in the first place, it’s often hard to determine cause and effect in these studies. Overall, the idea that diet drinks can help you reach your health and fitness goals doesn’t have a strong evidence base.

The bottom line? While it’s possible that aspartame could increase cancer risk by a small magnitude, the evidence for this is not strong. As always, you can count on us to keep following the science and updating you based on any new evidence. In the meantime, it’s a good idea to stay mindful of your aspartame consumption and assess whether these products contribute to or detract from your wider health goals. These two Nerdy Girls are partial to sparkling water, but definitely an occasional Coke Zero!

Those Nerdy Girls

Further resources:

WHO announcement on aspartame risks

“Diet Coke probably isn’t giving you cancer”- Comprehensive Summary from Health Nerd

WHO guidance on using artificial sweeteners for weight loss

New York Times coverage of WHO aspartame announcement

Review Paper: Non-sugar sweeteners and cancer: Toxicological and epidemiological evidence

Sugar- and Artificially-Sweetened Beverages and Cancer Mortality in a Large U.S. Prospective Cohort

Useful list of food and drinks that contain aspartame

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