This randomized clinical trial asked whether a simple beverage swap—drinking water instead of diet beverages after lunch—can help people keep weight off after an initial weight-loss program. The study followed overweight and obese adult women (ages 18–50; BMI 27–40) who were habitual consumers of diet beverages and were attending a clinic-based weight management program. After screening, 71 participants were randomized to one of two groups: a water group that drank a 250 mL glass of water after lunch five times per week, or a diet-beverage group that continued drinking a 250 mL diet beverage after lunch five times per week. This beverage pattern was maintained through an 18-month program consisting of a 24-week weight-loss phase followed by a 12-month weight-maintenance phase.

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Both groups received the same structured lifestyle intervention; the only difference was the assigned beverage after the main meal. During the weight-loss phase, participants followed an energy-restricted plan with behavioral strategies such as goal setting and self-monitoring, plus a gradual activity target consistent with weight-control recommendations. During the maintenance phase, participants attended monthly group sessions led by a dietitian and monthly individual visits focused on sustaining energy control and physical activity. Researchers tracked body weight, BMI, waist circumference, self-reported dietary intake, and cardiometabolic markers from fasting blood samples, including glucose, insulin, HOMA-IR (insulin resistance), HbA1c, lipids, and 2-hour postprandial glucose.

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Over the 12-month maintenance period, the water group had significantly better outcomes than the diet-beverage group. Women assigned to water continued to lose weight on average during maintenance (about −1.7 kg), while the diet-beverage group was essentially weight-stable (about −0.1 kg), resulting in a meaningful between-group difference. BMI decreased more in the water group as well. Waist circumference decreased in both groups during maintenance, but differences between groups were not statistically significant. For cardiometabolic outcomes, both groups improved over time, but the water group showed greater improvements in carbohydrate metabolism during maintenance, including larger reductions in fasting insulin, HOMA-IR, and 2-hour postprandial glucose. Lipid changes improved over the full program in both groups, with no clear differences between groups during the maintenance phase.

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Overall, the findings suggest that replacing diet beverages with water after a main meal may provide a modest but measurable advantage for sustaining (and even extending) weight loss, alongside improved insulin resistance, during long-term weight management.