Diet soda is regarded with suspicion by many people, but supposed harmful effects have never been backed up by science. Past studies have linked diet sodas with weight gain, but critics (including me) point out that those studies show correlation, not causation.

However, a new study from researchers in the UK and Iran, and published in the journal Diabetes, Obesity, and Metabolism provides the most compelling evidence that drinking diet soda has a harmful effect on weight loss efforts.

Researchers followed 81 participants (overweight and obese women with type-2 diabetes) over 24 weeks in which they followed the same integrated weight-loss program. Half consumed a single 8-oz diet soda after lunch, five days per week, and the other half only consumed water. Both groups were free to consume water at any other time during the day, but no other diet soda or other diet beverages.

At the end of the 24-week period, the group that had consumed only water lost significantly more weight: 14 pounds, on average, compared to 11.5 pounds among the diet soda drinkers.

Early Results with Dramatic Implications

Researchers have not proposed a mechanism by which diet sodas may limit weight loss (or potentially cause weight gain) but this result is striking, particularly in that those participants in the diet soda group consumed only forty ounces per week. That’s the equivalent of two single-serve bottles, less than a single can per day, and the result was an 18% decrease in weight lost over 24 weeks.


Certainly more research needs to be done, but this suggests that diet soda (and other diet beverages) may harm weight loss efforts in dramatic fashion. As a real fan of Diet Coke myself (some would say addict) it’s hard to imagine giving it up entirely. That said, if you trying to lose weight you may want to consider eliminating diet beverages entirely.


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