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 help with weight loss. Observational studies show long-term risks for weight. The recently published SWEET study is one of 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 aid to save sugar and control 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 of weight management. The background is the discrepant study situation: In clinical studies (RCTs), body weight decreased slightly—especially when sweeteners replaced sugar. However, there were no clear advantages 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 helps in the long term or could be problematic. 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 Overall—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 the 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. Then began 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 records, and questionnaires. Among other things, weight, body measurements, blood values, appetite, gastrointestinal complaints, and medication use were recorded. Urine samples served as a more objective control of sweetener consumption. In 137 adults, the gut microbiota was additionally analyzed, meaning the community of microorganisms in the gut. The background is that potential changes in the microbiome due to sweeteners were to be identified.
Sweeteners Improved Weight Loss Success
In the two-month diet phase, adults lost an average of 10.1 kilograms. After that, it was no longer about rapid weight loss but about 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 higher 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 during 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 implementation, the weight difference in favor of the sweetener group was even up to 3.8 kilograms. The sugar intake also matched this: The sweetener group reduced their total sugar more than the control group and significantly lowered the consumption of sugar-rich products. This suggests that sweeteners were actually used as a sugar substitute in this study—and not just added on top.
For important metabolic markers like diabetes and cardiovascular risk values, the researchers found no clear differences between the groups after one year.
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No Significant Results for the Children
For the children, there was no significant sweetener effect. Although the BMI-for-age z-score improved overall over the year, it was similar in both groups. This score ranks a child’s BMI compared to children of the same age and gender. This is important because children grow, and their weight must be assessed differently than 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, different shifts were observed in the sweetener group compared to the control group. Several bacterial groups that can form 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 who more frequently experienced bloating and cramps.
What Do the Results Mean for Those Who Want to Save 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 terms of weight maintenance, this small difference often counts in the long run.
Those who want to reduce sugar and maintain their weight after weight loss 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 finding is also important. Fortunately, the researchers saw no evidence of a harmful effect on the microbiome—aside from the unwanted gastrointestinal complaints. However, the observed changes must be read cautiously, as bacterial shifts alone cannot yet be translated into definite health effects.
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Study Assessment 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 made tracking and organization more difficult.
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.