May 27, 2026, 11:25 am | Read time: 6 minutes
Few groups of additives are as controversial as sweeteners. The current body of research is marked by a contradiction: Short-term clinical studies conclude that sweeteners aid in weight loss. Observational studies, however, show long-term risks for weight. The recently published SWEET study is among the largest and longest clinical studies to address this issue. The surprising result: Sweeteners perform remarkably well.
The Idea Behind the SWEET Study
The study focused on a question that has polarized nutrition debates for years: Are sweeteners a useful tool for saving sugar and controlling weight, or could they negatively affect metabolism and gut flora in the long term?
Currently, the World Health Organization (WHO) does not recommend sweeteners as a means for weight management. The reason is the conflicting study results: In clinical studies (RCTs), body weight decreased slightly—especially when sweeteners replaced sugar. However, there were no clear benefits in diabetes markers. Observational studies, on the other hand, found associations with an increased risk of obesity and type 2 diabetes.1
However, the WHO recommends limiting free sugars to less than ten percent of daily energy intake (added sugars and sugars from honey, syrups, and juices). At the same time, it is unclear whether replacing sugar with sweeteners is helpful or problematic in the long term. The SWEET study aimed to examine how prolonged use of sweeteners affects weight control, gut flora, and risk markers for type 2 diabetes and cardiovascular diseases.
Focus on Sweeteners as a Whole–Not Individual Substances
The study examined the influence of sweeteners. This included classic sweeteners like aspartame, acesulfame-K, saccharin, and steviol glycosides, as well as sugar alcohols like erythritol, sorbitol, and xylitol, slowly digestible carbohydrates, and sweet fibers. The focus was not on the isolated consumption of individual sweeteners but their use in a daily, sugar-reduced, healthy diet.
Two Groups Had to Eat Differently for a Year
The SWEET study was a randomized controlled trial. This means participants were randomly assigned to one of two groups. This study form is considered the gold standard.2
A total of 341 adults and 38 children with overweight or obesity were included. The study took place at four European centers: Copenhagen, Maastricht, Pamplona, and Athens. 203 adults and 22 children completed the one-year study.
Initially, all adults followed the same low-calorie diet for two months. The goal was a weight loss of at least five percent of the initial weight. This was followed by a ten-month weight stabilization phase. During this phase, both groups were to eat healthily and reduce sugar, with less than ten percent of energy from added sugar. In other words, the requirement was even stricter than the WHO recommendation. The sweetener group was to replace sugar-rich products with those containing sweeteners as much as possible. The control group was to avoid such products.
The study was accompanied by regular examinations, dietary logs, and questionnaires. Data collected included weight, body measurements, blood values, appetite, gastrointestinal complaints, and medication use. Urine samples served as a more objective control of sweetener consumption. In 137 adults, the gut microbiota was also analyzed, meaning the community of microorganisms in the gut. The aim was to identify potential changes in the microbiome due to sweeteners.
Sweeteners Improved Weight Loss Success
In the two-month diet phase, adults lost an average of 10.1 kilograms. After that, the focus was no longer on rapid weight loss but on what many find hardest in everyday life: maintaining success. This is where the sweetener group performed better. Those who replaced sugar with products containing sweeteners had a 1.6-kilogram greater weight loss after one year than the control group. In the subgroup where the gut microbiome was also analyzed, the sweetener group regained an average of 3.4 kilograms over the year, while the control group regained 5.6 kilograms.
The effect was stronger the more consistently participants adhered to the dietary guidelines. With particularly good adherence, the weight difference in favor of the sweetener group was up to 3.8 kilograms. The sugar intake also matched this: The sweetener group reduced their total sugar intake more than the control group and significantly lowered the consumption of sugar-rich products. This suggests that sweeteners were indeed used as a sugar substitute in this study—and not simply added on top.
For important metabolic markers like diabetes and cardiovascular risk values, researchers found no clear differences between the groups after one year.
No Significant Results for Children
For children, there was no significant sweetener effect. Although the BMI-for-age z-score improved overall during the year, it was similar in both groups. This score places a child’s BMI in comparison to children of the same age and gender. This is important because children grow, and their weight must be assessed differently from that of adults. Due to the small group size—22 children completed the study—the authors evaluated these results cautiously.
What About the Gut Flora?
In the gut microbiota, the sweetener group showed different shifts than the control group. Several bacterial groups capable of producing short-chain fatty acids were more prevalent. Short-chain fatty acids are metabolic products of gut bacteria that potentially offer health benefits. However, methane-producing microbes also increased. This aligns with reports from participants in the sweetener group of more frequent bloating and cramps.
What Do the Results Mean for Those Looking to Cut Sugar?
The study suggests that sweeteners can be useful for weight loss: not as a free pass for sweets, but as a sugar substitute in an overall healthy, sugar-reduced diet. The weight advantage was not huge but measurable—and in weight maintenance, this small difference often counts in the long run.
Those looking to reduce sugar after weight loss and maintain their weight might find it easier with sweetener-containing alternatives. However, the study does not show that sweeteners themselves make you slim. Both groups had to follow a diet. The sweetener group benefited from the overall package of their guidelines.
The gut findings are also important. Fortunately, researchers saw no evidence of a harmful effect on the microbiome—aside from the unwanted gastrointestinal complaints. However, the observed changes should be interpreted cautiously, as bacterial shifts alone cannot yet be translated into definitive health effects.
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Study Context and Possible Limitations
A clear strength is the design. It was randomized, controlled, and, with a duration of one year, long for a clinical study in the nutrition field. Additionally, several European centers were involved. It is also positive that the researchers did not only use questionnaires but also evaluated urine markers for sweetener consumption. This makes the dietary information more credible.
Nevertheless, the study has limitations. The dropout rate was 40 percent, higher than planned. As a result, the number of participants at the end was smaller than intended. The study also took place during the COVID-19 pandemic, which complicated tracking and organization.
Conclusion
The SWEET study shows: Sweeteners are not a weight-loss trick, but they can help with maintaining weight. Those who replace sugar with sweetener-containing products after a diet can better maintain weight loss. Although the gut microbiome changed, fortunately, there were no clear indications of disadvantages in important metabolic values.