The meme that drinking diet sodas causes weight gain has been ricocheting around the Internet for years. Generally, this claim has been based on observational studies in which researchers find that fat people tend to drink more diet drinks than do skinny people. This has always seemed to me to be a case of post hoc ergo propter hoc, i.e., after this, therefore because of this.
I know that I will annoy some epidemiologists, but a large proportion of observational studies seem only slilghtly more respectable than casting horoscopes. Why? Because, even with the best will in the world, it's nearly impossible to eliminate confounders so that a real causal relationship might be revealed.
A new randomized trial study in the journal Obesity (and funded by beverage companies) followed more than 300 people through a weight loss program. The only difference is that half were asked to drink at least 24 ounces of water per day and other half to drink 24 ounces of non-nutritive sweetened (NNS) beverages per day.
The program involves 12 weeks of losing weight followed by 9 months of weight maintenance. The study found that those consuming diet drinks lost an average of 14.2 pounds whereas those drinking water dropped an average of 10 pounds. The researchers chose 12 weeks as the length of the study because prior work shows that weight loss slows considerably after six months in a treatment program. (I know from personal experience that that is so.)
The study further notes:
Based on the design of this study we are unable to say, what is the mechanism for the greater weight loss in the NNS group compared to the water group. Weekly hunger scores were significantly lower among the NNS group than the water group although the absolute changes were small. While it is plausible that the NNS participants were more likely to adhere to the dietary recommendations due to less hunger than the Water group we cannot conclude this based on this study. Some authors have suggested that use of NNS may increase appetite for sweet foods and disrupt regulation of energy balance. Weight loss results for the present study suggest that NNS consumption did not increase energy intake from other foods compared to water. This is consistent with other studies that have not found increased consumption of sweet or high energy foods while using NNS. Further studies will be needed to ascertain the mechanism(s) that may be responsible for the weight loss results.
Look, it may turn out that the observational studies suggesting that drinking diet sodas make people fat are true, but I wouldn't bet on it. It has always seemed much more plausible that obese people drink diet soda because they don't want to get even fatter by consuming the extra calories in sugar-sweetened drinks. Sometimes interestingly counterintuitive claims are bunk. So diet soda drinkers unite! You've nothing to lose but your excess avoirdupois!