Researchers Followed Hundreds Of Adults For 2 Years To See If Diet Soda Helps Keep Weight Off

There are a few little luxuries in my life that I simply can’t live without. Chief among them is my Coke Zero. In fact, I’m not fussy; make that any sugar-free soda. I’m not even ashamed to admit it, although I do recognize that these drinks have a bit of a reputation problem. Diet sodas are often promoted as a simple way to cut calories and sugar, but the artificial sweeteners they contain have also been blamed for everything from triggering cravings and contributing to weight gain to other, potentially more serious health problems. This raises the question of whether these sugar-free drinks are actually helping people make healthier choices or doing more harm than good. A major new study has brought the debate back into the spotlight, comparing diet drinks with water when it comes to weight management. The researchers described it as “the longest and most comprehensive randomized controlled trial to date examining non-nutritive sweetened (NNS) drinks.” Naturally, with a sugar-free Red Bull in hand, I had to dive in and get to the bottom of it. Happy Pixels / Pexels (not the actual photo) A major new study published in the British Journal of Nutrition offers what could be some reassuring news for diet soda fans. The EffectS of non-nutritive sWeetened beverages on appetITe during aCtive weigHt loss (SWITCH) trial, led by the University of Liverpool in England and funded by the American Beverage Association, aimed to “provide robust evidence” on how beverages containing non-nutritive sweeteners (NNS) compared with water for long-term weight management. The trial included 493 adults aged 18 to 65, all with a body mass index (BMI) between 27 and 35, placing them in the overweight or obese categories. The study had two phases: 12 weeks of active weight loss followed by 40 weeks of supported weight maintenance, then a further 52 weeks of unsupported weight maintenance. During the first year, participants received nutrition counseling, behavioral support, and physical activity recommendations. Of the original participants, 220 voluntarily continued into the second year, when they no longer received this support. Throughout the study, half the participants were assigned to drink 22.3 ounces of water a day, while the other half received the same amount of beverages containing non-nutritive sweeteners. That’s roughly two cans’ worth a day. The drinks included various diet sodas, low-calorie fruit drinks, and flavored waters, most of which contained aspartame, acesulfame potassium, or sucralose. At the end of the full 104-week study, participants in the diet-drink group had lost an average of 10.6 pounds (around 4.8 kilograms), compared with 8.2 pounds (around 3.7 kilograms) in the water group. The difference between the groups wasn’t statistically significant. The researchers concluded that “water and NNS beverages showed equivalence regarding weight change from baseline.” Among those who continued into the second year, both groups maintained their weight reductions, even though they no longer received weight maintenance support. They also saw reductions in waist and hip measurements, alongside improvements in body composition and several metabolic markers. Sugar intake fell in both groups, too, although the reduction was slightly greater among participants drinking NNS beverages. magnific / Magnific (not the actual photo) Alas, not quite. The findings suggest that diet drinks can be used without hindering weight loss as part of a structured program. But that’s not the same as showing that artificial sweeteners themselves promote weight loss. As Alix Turoff, a dietitian who was not involved in the study, told Fox News Digital, artificially sweetened beverages do not have “a magical metabolic effect.” Instead, it comes down to the calories saved by making the switch. “If someone normally drinks 250 calories of regular soda and switches to zero-calorie soda, then they’re saving calories,” she explains. As Professor Jason Halford, one of the study’s authors, concluded: “For people trying to manage their weight, non-nutritive sweetened beverages offer an effective alternative to sugar-sweetened drinks—and perform equivalently to water over two years.” Andrea Piacquadio / Pexels (not the actual photo) “When it comes to sugar,” the American Heart Association (AHA) says, “you can definitely have too much of a good thing. And most people in the U.S. do.” In fact, the organization estimates that the average American consumes around 17 teaspoons of added sugar a day—almost double the recommended daily limit for men and nearly triple that for women. Sugar-sweetened beverages—soft drinks, fruit drinks, and sports and energy drinks—are a pretty significant contributor here, accounting for around 24% of added sugar intake. And you don’t have to drink them excessively to rack up the sugar, either. A single 12-ounce can of regular soda contains approximately 10 teaspoons (42 grams) of added sugar, nearly twice the AHA’s recommended daily limit for women. Given that excessive added sugar consumption is associated with serious health consequences, including an increased risk of heart disease, stroke, and type 2 diabetes, replacing sugary drinks with low- or zero-calorie alternatives might seem like a sensible way to cut back. And there’s certainly interest in doing so. According to a 2023 survey by the International Food Information Council, 28% of Americans said they regularly and intentionally used low- or no-calorie sweeteners, while more than half believed products containing them could improve their health, according to Alexandria Hardy, an early intervention nutrition therapist. But the research and guidance on non-nutritive sweeteners isn’t quite as straightforward as the calorie count on a can of diet soda might suggest. As dietitian Lauren Manaker explains, that’s where “things get murky.” stockking / Magnific (not the actual photo) “Diet sodas derive their sweetness from artificial or low-calorie sweeteners, such as aspartame, sucralose, and acesulfame potassium (Ace-K). The research on these sweeteners is, to put it mildly, all over the place,” Manaker continues. Part of the uncertainty stems from questions about whether sweeteners offer lasting benefits for weight management. In 2023, the World Health Organization (WHO) recommended against using what they call non-sugar sweeteners (NSS) for weight control, citing a lack of convincing evidence that they provide long-term benefits for reducing body fat. “Replacing free sugars with NSS does not help with weight control in the long term. People need to consider other ways to reduce free sugars intake, such as consuming food with naturally occurring sugars, like fruit, or unsweetened food and beverages,” said Francesco Branca, the WHO's Director for Nutrition and Food Safety. “NSS are not essential dietary factors and have no nutritional value. People should reduce the sweetness of the diet altogether, starting early in life, to improve their health.” The evidence informing the WHO’s guidance included short-term randomized controlled trials, many lasting three months or less, as well as observational studies that had linked sweetener consumption with health concerns. However, observational studies cannot establish that sweeteners cause these outcomes, and shorter trials offer limited insight into their long-term effects. The SWITCH trial adds another dimension to the debate. Following participants for up to two years, it found that beverages containing non-nutritive sweeteners performed comparably to water for weight management. The researchers also found no clinically meaningful differences in blood pressure or cholesterol between the groups. As Professor Joanne Harrold, who co-led the research, said: “These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomized trial. We hope this evidence informs future WHO guidance and public health policy.” But while the SWITCH findings offer some reassurance about the outcomes measured, they don’t resolve every question about sweeteners and long-term health. The WHO also highlighted observational studies suggesting possible associations between non-sugar sweeteners and health concerns, including obesity, type 2 diabetes, cardiovascular disease, and mortality. However, such findings don’t establish that sweeteners cause these conditions. The WHO noted that the observed associations could be influenced by participants’ existing health characteristics and complex patterns of sweetener use, which was one reason its recommendation was conditional. The WHO recommendation was also not a declaration that sweeteners are unsafe. It was guidance on their use for weight control, based on the available evidence about long-term benefits and potential health outcomes. DC Studio / Magnific (not the actual photo) Regulatory bodies such as the US Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA) have assessed approved sweeteners and established acceptable daily intake (ADI) limits. These specify how much of a sweetener a person can consume daily over a lifetime without appreciable health risk. A 2019 review published in Advances in Nutrition explains that these assessments consider factors such as expected consumption, cumulative exposure, and toxicology data. Approved sweeteners are generally considered safe when consumed within established limits. As the UK’s National Health Service explains, people generally don’t need to track their daily intake, as regulations account for how and in what quantities sweeteners are used in food and drinks. One sweetener that has attracted particular attention is aspartame, commonly used in diet soft drinks as a low-calorie alternative to sugar. In 2023, the International Agency for Research on Cancer (IARC) classified it as “possibly carcinogenic to humans,” based on limited evidence. However, this classification identifies a potential cancer hazard rather than establishing the risk associated with typical levels of consumption, as the World Health Organization (WHO) explained. In other words, the classification does not, by itself, mean that drinking diet soft drinks containing aspartame at typical levels poses a demonstrated cancer risk. Following its assessment, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) maintained the acceptable daily intake for aspartame at 40 milligrams per kilogram of body weight per day (around 18 milligrams per pound), concluding that the available evidence did not warrant changing the existing limit. To put that limit into perspective, the WHO noted that an adult weighing 70 kg (154 lb) would need to consume more than 9–14 cans of diet soft drink containing 200–300 mg of aspartame each day to exceed the ADI, assuming they consumed no aspartame from other sources. But staying within the recommended limit doesn't mean every question about sweeteners and long-term health has been answered. The Cancer Association of South Africa (CANSA) notes that while some observational studies have linked sweeteners such as aspartame and acesulfame-K to increased cancer risk, systematic reviews and meta-analyses have generally found no association. Because observational findings cannot establish causation, more long-term research is needed to clarify any potential risks. magnific / Magnific (not the actual photo) Although non-nutritive sweeteners are considered safe and well tolerated within established limits, questions remain about their potential effects on metabolic health. The evidence is mixed: Some studies have linked sweetener consumption to obesity, metabolic syndrome, and type 2 diabetes, while others have found no such associations. One area of interest is the gut microbiota, the community of microorganisms living in the digestive tract. Some research suggests that certain sweeteners may alter gut bacteria in ways that affect how the body regulates blood sugar. However, the findings vary by sweetener and study, and the long-term health implications remain unclear. As Manaker explained, “Some research links high consumption of certain artificial sweeteners to changes in gut bacteria, which can affect everything from digestion to immune health; other preliminary data suggest some sweeteners don’t impact the gut microbiota.” Researchers have also questioned whether consuming something sweet without the energy normally associated with sugar could affect appetite. One hypothesis is that sweetness without calories might disrupt the body’s associations between sweet tastes and energy intake, potentially affecting hunger and encouraging people to eat more later. However, this remains a hypothesis rather than an established effect of diet drinks. The SWITCH trial offers some reassurance here. As Turoff explained to Fox News Digital, “The study did not find significant between-group differences in hunger or sugar intake at the end of the trial, which is relevant because one common concern is that artificial sweeteners may increase appetite or lead to compensatory eating.” Zoshua Colah / Unsplash (not the actual photo) How should we weigh the potential benefits of diet drinks against the questions that remain about their long-term effects? Ultimately, there’s no single right choice for everyone. As dietitian Lindsey DeSoto writes, “There’s no clear ‘winner’ between diet and zero sugar soda. The best choice is the one that fits your taste preferences, health goals, and overall eating pattern.” Regular soda contains added sugar and calories, and frequent consumption of sugary drinks is associated with its own set of health risks. At the same time, a sugar-free label doesn't automatically make a product healthy. As Manaker explains, “If artificial sweeteners don't sit well with you, an occasional regular soda, paired with a nutrient-rich diet, can fit just fine.” For those looking to cut back on sugar or manage their weight or blood sugar, however, Manaker says diet soda has the edge: “zero calories, no sugar rush.” It can satisfy a craving for something sweet and carbonated without adding sugar or calories to your daily intake. But choosing between regular and diet soda also doesn't have to be an all-or-nothing decision. Your preference may depend on the drink, the sweetener used, or your individual dietary goals. The SWITCH trial found that beverages containing non-nutritive sweeteners performed comparably to water for weight management over two years. But comparable results for weight management don't mean diet drinks can replace water as your main source of hydration. And if you’re already drinking mostly water, this study doesn’t give you a reason to change course. In fact, the dietitians cited here largely agree on one thing: water should remain your go-to drink, while soda, whether diet or regular, should be consumed in moderation. As Manaker puts it, it’s best enjoyed “like a slice of cake: for fun, not as a staple.” So, will I be giving up my Coke Zero? Not just yet.