Highlights:
- Analysis of three large prospective cohorts suggests that replacing sugar-sweetened beverages with diet drinks or water may help limit long-term weight gain.
- Increasing intake of low/no calorie sweetened beverages was not associated with weight gain, but rather, with slightly less weight gain over time.
- Findings are in line with evidence from randomised controlled trials showing that replacing sugary drinks with low/no calorie sweetened alternatives can modestly reduce energy intake and body weight.
Low/no calorie sweetened beverages such as diet sodas are widely consumed as an alternative to sugar-sweetened drinks. However, their role in weight management has long been debated as a result of conflicting and inconsistent evidence.
Randomised controlled trials (RCTs) generally show small but consistent reductions in body weight when low/no-calorie sweeteners replace sugars. By contrast, observational studies have produced mixed findings: some report positive associations with weight gain, while others find neutral or protective associations. These discrepancies may partly reflect methodological issues such as reverse causation.1
A recent analysis of three major U.S. prospective cohort studies with repeated dietary and weight measures provides new evidence on how changes in beverage consumption are associated with long-term weight change.2
What the new study looked at
The researchers investigated how changes in consumption of low/no calorie sweetened beverages, sugar-sweetened beverages (SSBs), and water were associated with changes in body weight and body mass index (BMI) over time.
The study analysed data from three large ongoing U.S. prospective cohort studies: the Nurses’ Health Study, Nurses’ Health Study II, and the Health Professionals Follow-up Study. Researchers followed 143,409 adults aged 26–65 for 24 to 32 years, repeatedly collecting information on diet, lifestyle, and body weight. By tracking within-person changes in beverage intake and weight every four years, the researchers aimed to capture a more realistic picture of long-term weight change than studies relying solely on baseline habits.
Key findings
Generally, people tend to gain weight over time. On average, participants gained 1.3kg every four years. However, increasing consumption of low/no calorie sweetened beverages was associated with less weight gain over time. Specifically, increasing diet drink intake by three servings per week was linked to a net weight change of about -0.18 kg over a four-year period. Similar patterns were observed for BMI. Mediation analysis showed that this association was partially explained by reducing energy intake from SSBs.
When participants replaced SSBs with either diet drinks or water, the association with weight change was much larger. Replacing three servings of SSBs per week with the equivalent amount of diet beverages was associated with 1.39 kg less weight gain over four years. A similar effect was observed when sugary drinks were replaced with water, while replacing low/no calorie sweetened beverages with water was associated with a slight favourable outcome, although the difference was small. Overall, the results consistently showed that reducing sugary drink intake was the most important factor, regardless of whether the replacement was diet beverages or water.
The associations were strongest among people living with overweight or obesity and those with higher SSB intake, suggesting that replacing sugary beverages with either diet drinks or water may be particularly beneficial for individuals at higher risk of weight gain and for higher SSB consumers.
Why these findings matter
Excess consumption of sugary drinks remains a major contributor to added sugar intake worldwide and is strongly linked to weight gain and cardiometabolic diseases. Identifying realistic alternatives that people are willing to adopt is therefore a key public health question.
This study suggests that diet beverages may serve as a useful transitional substitute for people trying to reduce their intake of sugary drinks. While the weight differences associated with increasing diet drink intake alone were small, replacing sugary beverages with diet drinks was associated with meaningful reductions in weight gain particularly in individuals with overweight or obesity.2
Putting the findings into context
The findings by Espinosa and colleagues are particularly relevant to ongoing discussions about current dietary guidelines, including the WHO recommendation on non-sugar sweeteners.3 The WHO highlighted in its guideline the importance of studies that include multiple, sequential exposure assessments that account for varying consumption patterns, and reduce susceptibility to reverse causation – people already trying to lose weight may switch to diet drinks as a sugar reduction and weight management strategy.
By analysing changes in consumption over time rather than only baseline intake, Espinosa et al. attempt to address some of these limitations. The findings reject the claims that diet beverages may be associated with weight gain, aligning more closely with evidence from RCTs suggesting that replacing sugary drinks with low/no calorie alternatives can help reduce calorie intake and support weight control.2
The new study also builds upon a recent umbrella review of meta-analyses of both RCTs and prospective cohort studies, which revealed a significant methodological divide: while simple analyses of cohort studies with prevalent (baseline) exposure assessment often associate low/no calorie sweeteners with increased risks of obesity, diabetes and cardiovascular disease, bias-adjusted analyses showed the opposite. Specifically, when low/no calorie sweeteners were used to replace sugar and reduce overall calories, they were associated with lower body weight, obesity, and cardiometabolic risk in adjusted cohort analyses.4
Take-home message
The study highlights a consistent pattern: reducing sugar-sweetened beverage consumption is key for limiting long-term weight gain. Replacing sugary drinks with either diet beverages or water was associated with less weight gain over time. While diet beverages are unlikely to be a stand-alone solution for weight management, they may serve as a practical alternative for people trying to reduce their sugar intake, particularly when used in place of sugary drinks.