We know that consumption of beverages containing sugar increases the risk of obesity, metabolic syndrome, and type 2 diabetes, so it is often advised to consume diet drinks instead.  However, it has been suggested that even diet beverages may be associated with an increased risk of type 2 diabetes.  As such, a recent study studied the effect of diet beverages vs water as part of a weight management program in women with overweight or obesity and type 2 diabetes.
The study, published in Diabetes, Obesity & Metabolism, randomized 81 women who were habitual consumers of diet beverages, to consume either water after lunch five times per week instead of her usual diet drink, or to continue to drink diet beverages after lunch, during a 6 month weight loss program which consisted of dietary modifications and exercise.
They found that women who switched up diet drinks to water had a 1.2kg greater decrease in body weight compared to women who continued to drink diet beverages, as well as greater improvements in fasting blood sugar, fasting insulin levels, and post meal blood sugars.
While the differences in the study were small, it is interesting to consider how diet drinks could be detrimental to weight loss efforts.  It has been proposed that artificial sweeteners may raise the hedonic desire for sweet, energy dense foods. The women drinking diet beverages did consume a few more calories per day than those drinking water, so this may be a plausible mechanism of the differences seen in this study.  There may also be an effect of sweeteners on gut bacteria, which we are learning have an important impact on energy balance and metabolic homeostasis.
It should also be noted that there is conflict in the research in this area, with other studies showing no effect on weight between water vs diet beverages or even a benefit of diet beverages over water.

However, based on the current study, it seems that water may be best.
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