How Unhealthy Is Diet Soda? A Dietitian Breaks It Down

As a registered dietitian, I hear some version of this admission almost every week: “I know I should quit, but I really love my diet soda.” It usually comes with a guilty look, as if that can of fizz were a moral flaw rather than a drink choice.

That guilt didn’t appear out of nowhere. Over time, diet soda has picked up plenty of cultural baggage: concerns about “chemicals,” wellness influencers calling it toxic, and a steady flow of scary headlines tying it to everything from weight gain to heart disease. It’s no surprise people feel conflicted about it.

So, is diet soda actually that bad for you? The answer is more complicated than a simple yes or no. Here’s what the research really says — and what it doesn’t — so you can decide whether that second can is worth it.

What’s in diet soda, exactly?

To understand the debate, it helps to know what you’re drinking. Diet soda gets its sweetness from non-nutritive sweeteners, which taste sweet but add few or no calories. Common examples include aspartame, sucralose, and acesulfame potassium, often labeled as acesulfame K. Some brands also use plant-based substitutes such as stevia.

Along with sweeteners, many diet sodas contain carbonated water, acids such as phosphoric or citric acid, flavorings, preservatives, and sometimes caffeine. Nutritionally, they don’t contribute much, but many people still find them genuinely satisfying.

Are artificial sweeteners safe?

Sweeteners are reviewed for safety before they reach shelves. In the United States, the FDA evaluates and approves high-intensity sweeteners as food additives before they can be used, and it sets an acceptable daily intake for each one — the amount considered safe to consume daily over a lifetime. Most people consume far less than that. Still, some concerns have persisted, especially around aspartame.

You may remember the headlines in 2023 about aspartame and cancer risk. A World Health Organization research group said aspartame may possibly raise cancer risk, but that is not the same as proving it causes cancer. The evidence was limited. At the same time, FDA guidance on safe intake did not change.

A few groups do need to be especially careful. People with phenylketonuria, or PKU, a rare inherited condition, should avoid aspartame because their bodies cannot break down one of its components. And some sugar alcohols, such as sorbitol and xylitol, can cause digestive upset in large amounts.

Is diet soda healthy?

The short answer: It depends. Research on diet soda has produced mixed results for years, and no single headline tells the whole story. Here’s where the evidence points.

Weight and metabolism

When sugary drinks are replaced with diet versions, the short- to medium-term data is generally reassuring. A 2022 network meta-analysis found that swapping sugar-sweetened beverages for low- and no-calorie sweetened drinks was linked with small reductions in body weight, body fat percentage, and liver fat. The effects were modest, but they pointed in the right direction.

A 12-month randomized trial found something similar. Among people with the most abdominal fat, those who continued drinking sugary beverages gained significantly more weight than those who switched to diet drinks or water. For most people, though, swapping one for the other did not lead to dramatic changes either way.

Heart health

This is where the picture gets murkier. A 2024 meta-analysis of 12 studies involving more than 1.2 million people found that drinking one or more diet beverages a day was associated with higher risks of death from any cause, cardiovascular disease, and stroke. An umbrella review in Advances in Nutrition flagged similarly concerning patterns for obesity, type 2 diabetes, hypertension, and cardiovascular disease incidence.

The key caveat is that these are associations, not proof of cause and effect. People who drink more diet soda may already be managing health conditions, which makes it difficult to know whether diet soda is contributing to worse health or is simply more common among people who are already at higher risk.

Gut health and blood sugar

One active area of research is how artificial sweeteners may affect the gut microbiome and blood sugar control. Some studies suggest that certain sweeteners, especially sucralose and saccharin, may change gut bacteria in ways that could affect how the body handles glucose. A study published in Cell found that some non-nutritive sweeteners — particularly saccharin and sucralose — were linked to worse glycemic responses in healthy adults, with gut microbiome effects that appeared to differ from person to person.

Diet soda vs. regular soda: Which is the better choice?

If the choice is between regular soda and diet soda, diet usually comes out ahead. The 2018 American Heart Association science advisory said diet drinks may be a reasonable stepping stone for people who regularly drink sugary beverages and want to cut back. A 2022 meta-analysis supports that idea, finding small but meaningful improvements in cardiometabolic markers when people made the switch.

That said, unsweetened drinks — especially water — appear to have advantages that diet soda does not, offering hydration with fewer unresolved questions. For the most personalized answer about whether diet soda fits your needs, a registered dietitian can help you decide what makes sense for you.

Who may want to cut back?

Diet soda is not likely to cause a health crisis for most people, but it may be worth reducing if you:

• Drink several cans a day, since the observational links appear at higher intakes
• Notice it is displacing water, milk, or other nourishing drinks
• Use it as your main caffeine source and find that it affects your sleep
• Are trying to reduce sugar cravings and want to reset your taste buds
• Have dental sensitivity or enamel concerns
• Have PKU, in which case aspartame-sweetened drinks are off the table

The bottom line

Diet soda is not something I would tell someone to drink multiple times a day, but it is also not the villain some headlines make it out to be. For someone trying to move away from sugary drinks, it can be a useful bridge. The research raises questions worth taking seriously, especially at higher intakes, but it does not prove that diet soda directly causes harm.

My advice: treat diet soda as an occasional treat, not your main source of hydration. Rely on water and other unsweetened drinks most of the time, and enjoy a can of fizz without guilt when you want one. Your overall eating pattern matters far more than an occasional Diet Coke.