The results of a new study published in the journal Obesity imply that diet soda is a more effective weight loss tool than water. The findings are controversial, and what is more, the study was funded by the American Beverage Association. But does this mean the results are biased? We investigate.

The new study, from researchers at the University of Colorado and Temple University in Philadelphia, PA, is not the first to investigate how diet sodas may influence weight loss. But previous research on this subject has been mixed.

In 2013, Medical News Today reported on a review of the available research on the subject by Prof. Susan Swithers, an expert in psychology and neuroscience from Purdue University in West Lafayette, IN. Prof. Swithers came to the conclusion that diet drinks containing aspartame, sucralose and saccharin increase the likelihood of excessive weight gain, metabolic syndrome, type 2 diabetes and cardiovascular disease.

Some researchers have hypothesized that diet beverages actually increase appetite, which is thought to be responsible for the weight gain observed in participants of these studies.

However, a 2013 study published in the American Journal of Clinical Nutrition found that diet drinks do not increase appetite.

The new study included 303 overweight participants, all of whom were taking part in a weight loss and exercise program and all of whom were regular consumers of diet drinks.

Randomized into two groups, one group was instructed not to consume any diet drinks and to drink at least 24 oz of water daily during the study period. The other group could continue to drink diet sodas.

After 12 weeks, the researchers found that those in the diet drink group had lost 14.2 lb on average. What is surprising is that this is about 4 lb more than the people in the group instructed to drink mostly water, who only lost an average of 10 lbs.

But why did this happen? The researchers themselves confess they are not sure, explaining that because of the design of the study they are unable to identify the mechanism for the greater weight loss in the diet soda group.

Not only the results, but also the methodology of the research, have prompted criticism from other researchers. “This paper is fatally flawed, and leaves us with little science to build on,” Prof. Swithers told NPR’s The Salt nutrition blog. She points out that the study does not detail what the non-diet drink group consumed, beyond water.

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The study does not provide detailed information on what – in addition to water – the control group consumed in lieu of diet drinks.

As these participants were regular soda drinkers, it could be that they replaced their diet soda intake with other sweetened drinks, in addition to the water they were asked to drink.

“Did they switch to regular sodas? [Did they] add sugar instead of artificial sweeteners to their coffee or tea?” she asks. “This paper tells us nothing about the long-term health consequences that should be our real focus.”

Medical News Today asked Dr. John Peters, one of the authors behind the new study and Chief Strategy Officer at the University of Colorado’s Anschutz Health and Wellness Center, why the team made the decision to only use regular consumers of diet soda as subjects and whether this could have confounded the results.

“It is well known that many people do not like the taste of diet drinks and we didn’t want this to cloud interpretation such that people in that group would not be compliant with the treatment,” he replied.

Other critics have queried the researchers’ decision to publish the results early – just 12 weeks into the year-long overall study period. As a result, the study cannot say at this stage what the long-term effects of diet drinks on weight may be.

“Most weight loss trials not using drugs have shown that most of the weight loss ever achieved occurs within the first 3-5 months,” Dr. Peters told us. “We don’t know what the 1-year results will show. Nearly every weight loss trial shows that people regain some amount of weight after active weight loss stops. The question we will address is whether this weight regain is any different between treatment groups.”

But the most contentious area of the study remains the fact that it was funded by the beverage industry, leaving the public with a suspicion that this has distorted the results in the industry’s favor.

“You are big food and you lie to the American people,” blasted one reader in the comments section of NPR’s report. The author of the blog also cited a 2007 meta-analysis in PLOS Medicine that reported a correlation between industry funding of health studies and results from those studies favoring the industry’s position.

Given the nature of the funding, we asked Dr. Peters if he anticipated the public backlash.

“We actually weren’t surprised that some people would be skeptical,” he answered. “That is why we posted the design on before we started the study to be completely transparent about what we were doing. And we had a third-party clinical trials organization monitor the data.”

Dr. Peters – who has also worked as a consultant to The Coca-Cola Company outside of this research – explains that his team made an agreement with the American Beverage Association that, whether the findings turned out to be positive or negative, the results would be published.

He also considers there to be some advantages to industry funding of research:

I am skeptical that this kind of study would be funded by a non-industry organization. That may be why this is the first prospective randomized trial to look at the question of diet beverages and weight loss. We thought it was important to get the data because of the belief that because obesity is associated with diet beverage consumption that it must be causing appetite disruption and weight gain.”

In conclusion, the study asserts that people who are engaged in a weight loss strategy should not be discouraged from drinking diet sodas if they desire to do so. The authors propose further studies to examine the mechanism responsible for the weight loss results.