Water vs Diet Soda: Which is Better for Weight Loss?


Many people think drinking diet beverages is a great way to lose weight.

However, most dietary recommendations say pure water is best. Recently, a team of scientists from Iran and the UK delved into this issue.

Here is a detailed summary of the results, recently published in the American Journal of Clinical Nutrition.



Sugar-sweetened beverages have long been linked to weight gain.

For this reason, beverages with low-calorie sweeteners have become a popular alternative for those who wish to maintain or lose weight.

Although the evidence is mixed, some studies actually suggest that diet beverages may be associated with weight gain as well.

Article Reviewed

A group of researchers from Iran and the UK compared the effectiveness of diet beverages and water on weight loss.

Effects on weight loss in adults of replacing diet beverages with water during a hypoenergetic diet: a randomized, 24-wk clinical trial.

Basic Study Design

This was a 24-week, randomized weight loss trial in 62 healthy, overweight and obese women. The participants ranged in age from 18 to 50 years.

Before the study started, all of the participants regularly consumed diet beverages.

The participants were randomly assigned to one of two groups:

  • Diet beverage group: Participants were provided with 250 ml (8.5 oz) of a diet beverage and were told to drink it after lunch, 5 times per week.
  • Water group: Participants in this group stopped drinking diet beverages and replaced them with equal amounts of water, provided by the study staff.

However, it is important to note that the contents of the diet beverage are not specified.

This study was designed to promote weight loss of 7–10%, during the research period. This involved a calorie-restricted diet that was high in carbs (60%), but low in fat (23%) and protein (17%).

Additionally, participants had professional support throughout the study, and were encouraged to gradually increase physical activity to 60 minutes, 5 days of the week.

The researchers measured body weight, insulin, blood sugar and blood lipids at the beginning, middle and end of the study.

Bottom Line: This was a randomized weight-loss trial in overweight and obese women. It compared the effectiveness of drinking diet beverages or replacing them with water.

Finding 1: Water is More Effective for Weight Loss

Participants in both groups achieved significant weight loss during the study.

However, drinking pure water resulted in a 13.6% greater weight loss than diet beverages. However, the difference was small, amounting to 2.6 pounds, or 1.2 kg.

The chart below shows the difference in weight loss and body mass index (BMI) between groups.


Overall, drinking water led to greater weight loss and improvements to BMI, compared to drinking diet beverages.

Bottom Line: The study showed that drinking pure water leads to greater weight loss, compared to drinking diet beverages.

What Do Other Studies Say?

In general, these results are supported by studies showing that increased water intake can help with weight loss.

However, there are mixed results in studies that compared water and diet beverages. For example, one randomized controlled trial found that diet beverages caused significantly greater weight loss, compared to water.

Another trial found that weight loss was similar among those who drank water or diet beverages (7).

The cause of these inconsistencies is unknown, but they might be due to differences in the type of diet beverage or a lack of compliance.

Despite these disparities, the present study is significant because it is different from previous trials.

This study is the first to examine the effectiveness of replacing diet beverages with water, during a weight loss program, where participants were obese, regular drinkers of diet beverages.

Bottom Line: The study showed that replacing diet beverages with water led to greater weight loss. However, other studies have found no difference, and some have even found diet beverages to lead to more weight loss than water.

Finding 2: Water Caused Greater Improvements in Blood Sugar Control

Drinking pure water, rather than diet beverages, caused significantly greater improvements in blood sugar control.

The chart below shows the improvements in insulin and blood sugar levels 2 hours (2-hbs) after eating 75 grams of glucose.


Insulin resistance also improved significantly.

Additionally, although fasting blood sugar levels improved overall, the difference between groups was not statistically significant.

Bottom Line: Overall, the weight loss program caused significant improvements in blood sugar control. Drinking water caused greater improvements in fasting insulin and insulin resistance.

Finding 3: Water and Diet Beverages Had Similar Effects on Blood Lipids

The weight loss program caused substantial improvements in blood lipids in both groups.

The chart below shows the changes in both groups.

weight loss

These improvements were not significantly different between groups.

Bottom Line: The blood lipid profile improved in both groups. However, there was no significant difference between groups. In general, this indicates that water and diet beverages may have similar effects on blood lipids.


The study appears to have been well designed and executed.

However, the paper itself has one major limitation. It fails to provide information about the contents of the diet beverage used in the study, which makes interpreting the results difficult.

There are many different artificial sweeteners in the food supply. Different types may have different effects.

Also, according to self-reported food records, calorie intake and carb intake reduced slightly more in the water group, compared to the diet beverage group.

This may explain some of the differences between groups.

Bottom Line: The study appears to have been designed well. However, the paper fails to specify the contents of the diet beverage, which makes interpreting the results difficult.

Summary and Real-Life Application

In short, this study indicates that pure water promotes significantly greater weight loss, compared to diet beverages.

Additionally, drinking water caused greater improvements in insulin sensitivity, indicating improved metabolic health.

However, it is impossible to reach any conclusions based on this study alone, because other studies have shown completely different effects.

At the end of the day, this may depend on the individual.

If you like diet drinks, and they help you stick to a healthy diet, then there is probably no need to stop drinking them.

But if you are drinking them, and you have trouble losing weight or want to speed things up, removing these drinks may or may not help.

It’s definitely something that is worth experimenting with, but the science is not settled on this issue yet.

Written by Aline Pilani

Hey, I’m Aline Pilani. I am a certified personal trainer and nutritionist. I have spent the last 10 years of my life helping people losing weight, increase their health and confidence, and I truly want to do the same for you.