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When you’re popular, people are going to talk—and Diet Coke happens to be the most popular diet cola in the entire world. So what’s behind the rumors that Diet Coke will give you cancer, make you gain weight, or flat-out kill you? Like many gossipy tales, there’s a shred of truth behind some of these notions, but they don’t come close to telling the complete story.

For starters, you should know that some of the potential health problems that have been associated with Diet Coke also apply to other diet colas, including Diet Pepsi and lesser-known brands you might find on the shelf at your local supermarket. Such concerns also pertain to a wider variety of “diet” foods—including light yogurt, and sugar-free chewing gum, as well as diet drinks that aren’t colas at all.

Here’s what the science actually says about diet cola. Spoiler: This fizzy drink is unlikely to kill you, but there are several good reasons to consider cutting back.

The aspartame-cancer connection

“When many people hear ‘diet,’ they equate it to carbonated water, but that couldn’t be further from the truth,” says Kelsey Mangano, PhD, RDN, dietitian and associate professor in the department of Biomedical & Nutritional Sciences at UMass Lowell. “The only thing that’s been removed is sugar.”

In lieu of sugar, Diet Coke and Diet Pepsi employ aspartame, an artificial sweetener that imparts sweetness sans calories. Some other diet drinks on the market use different low- or no-cal sweeteners, such as sucralose, saccharin, or stevia. 

The FDA considers all of these to be “generally recognized as safe,” but manufacturers don’t need to present significant, peer-reviewed research in order to get a food additive into that category, says Dariush Mozaffarian, M.D., DrPH, a cardiologist and director of the Food is Medicine Institute at the Friedman School of Nutrition Science and Policy at Tufts University.

Mozaffarian notes that saccharine is a coal-tar derivative and that sucralose is a lab creation that combines sugar and chlorine. But since Diet Coke and Diet Pepsi are the market leaders, let’s focus on aspartame: Turns out it was being investigated for possible use in an ulcer medication when a scientist tasted it and realized it was sweet. Trivia aside, the main present-day debate is around whether aspartame causes cancer.

In July 2023, the World Health Organization (WHO) put aspartame on a list of ingredients that are “possibly carcinogenic to humans.” Gasoline engine exhaust is in the same “possibly carcinogenic” category—but so is aloe vera. How worried you should be depends on how much aspartame you consume.

If the only source of aspartame is diet cola, you’re probably OK as far as cancer risk is concerned: An adult who weighs around 150 pounds would have to down more than 9 to 14 cans per day to get into dangerous territory, according to a joint release from the WHO, the Food and Agriculture Organization (FAO) Joint Expert Committee on Food Additives (JECFA), and the International Agency for Research on Cancer (IARC).

You might, however, also be getting aspartame from other reduced-calorie foods and drinks. That means it’s theoretically possible for your aspartame intake to get dangerously high, says Mangano. “We need more research to understand how additives like aspartame impact health,” she says.

The Coca-Cola Company, in case you were wondering, has no plans to change the Diet Coke recipe at this time. PepsiCo, interestingly, removed aspartame from Diet Pepsi in 2015 in favor of a different artificial sweetener. But consumers hated the aspartame-free version so much that the company reversed course a year later.

Other ingredients of concern in diet soda

While aspartame might be risky in high amounts, Mangano is far more concerned about other ingredients in diet soda—namely, caffeine and phosphoric acid.

Diet cola (provided you don’t pick a caffeine-free version) contains about 50 mg of caffeine, says Mangano. That’s about half as much as what you’ll find in a cup of coffee. But how many cans are you drinking, and how sensitive are you to caffeine?

If you’re sipping several cans of diet cola throughout the day, you could get close to or even exceed the FDA-recommended daily maximum of 400 mg. (If you’re pregnant, cut that number in half.) People who suffer from anxiety, insomnia, or high blood pressure are among those who might benefit from making their personal limit significantly lower or avoiding caffeine altogether, Mangano adds.

Phosphoric acid is another problematic ingredient, she says. You’ll find it in dark-colored sodas like colas (diet and regular) and root beer, and it’s detrimental to bone tissue. “Research has found that people who had the highest cola intake had the highest rate of osteoporosis,” she says. Phosphoric acid can also damage tooth enamel, she adds.

Does drinking diet soda lead to addiction and weight gain?

If you’ve heard that diet cola is “bad” for you, you may have also been told that it’s addictive or that it lead to weight gain. The truth on both is still murky.

Most diet cola, as noted earlier, contains caffeine. Caffeine does have addictive properties, but whether you can truly be addicted to it hasn’t been proven. The American Psychiatric Association says there’s no evidence to support “caffeine use disorder,” but that more research is needed.

Diet soda is also incredibly sweet, notes Mangano, and sweetness causes pleasure centers in the brain to light up. “People get a good feeling when they taste a sweet flavor” and feel motivated to seek it out again, she says. “I could see how some individuals would describe that as ‘addicted.’”

The rumor that diet soda might make you gain weight has some stronger evidence behind it, but it’s hardly conclusive. Some studies have found that adults with overweight and obesity drink more diet beverages than “normal” weight individuals, but other studies haven’t, says Mangano. A systematic review and meta-analysis published last year in JAMA Network Open concluded that using low- and no-calorie beverages in lieu of sugar-sweetened ones “was associated with small improvements in body weight and cardiometabolic risk factors.”

Even the studies that suggest obesity and diet soda go hand in hand merely indicate an association, adds Kris Sollid, RD, senior director, nutrition communications for the International Food Information Council (IFIC). Some people have theorized that consuming super-sweet diet drinks makes you crave—and eat—more caloric food, but this hasn’t been proven. It’s equally possible that overweight people are more likely to choose diet soda over caloric beverages because they’re concerned about their weight, he says.

No one has quite proven that diet drinks will help you lose weight, either. In fact, May 2023, the WHO issued a guideline advising against the use of non-sugar sweeteners (including but not limited to aspartame) for purposes of weight control. They concluded that that available evidence does not prove that use of these sweeteners leads to a reduction in body fat, and that long-term use could potentially cause problems such as an increased risk of type 2 diabetes and cardiovascular disease.

Is it OK to drink Diet Coke?

One thing that is clear, says Mozaffarian, is that regular (sugar-sweetened) soda is packed with added sugar. For that reason alone, if you’re someone who drinks a lot of soda—and you’re not willing to stop—he’d rather you opt for the diet variety. “We can’t assure you that low-calorie sweeteners are innocuous, but if you drink large volumes of soda then diet is better because of the high volume of sugar in regular soda,” he says.

For occasional soda drinkers—maybe you indulge in a glass a month or even once a week—Mozaffarian says to go with regular Coke or Pespi than roll the dice on artificial sweeteners.

That’s somewhat contrary to the advice Mangano provided. Her take: For some people who are trying to control calories, a can of diet cola once a week might be fine. She’s concerned enough about the caffeine and phosphoric acid—as well as the potential issues with aspartame—that she believes there might be some health consequences if you consume the diet stuff daily.

In an ideal world, everyone should probably scrap both diet and regular soda in favor of flavored unsweetened seltzer and other beverages that don’t contain sweeteners of any kind. Sollid thinks aiming for balance is a better goal.

“A ‘perfect’ diet probably wouldn’t have diet soda in there. But many diets that promote health can include it,” she says. “Not everything has to be organic broccoli and whole grains… It’s really about the totality of your diet. Don’t let perfect be the enemy of good.”

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