I understand the need for some skepticism about this industry-funded study. However, the carbonation in diet soda (or diet pop, depending on the part of the country you happen to be in) can make a person feel full and reduce the amount of food they eat compared to a person drinking water. This is a possible mechanism for the results.
Industry-Funded Study Finds Diet Soda Consumption Didn't Hinder Weight Loss — Physician’s First Watch
Industry-Funded Study Finds Diet Soda Consumption Didn't Hinder Weight Loss
By Kelly Young
Patients may ask about a study in Obesity finding that people who drank diet soda while in a weight-loss program lost more weight than those who drank water. The study was funded by the American Beverage Association.
Roughly 300 adults (mean BMI, 33) who regularly drank diet soda were randomized to either continue drinking diet soda (24 ounces daily) or switch to water. Both groups attended weekly behavioral weight-loss meetings.
At 12 weeks, participants in the diet soda group had lost significantly more weight than those in the water group (5.95 kg vs. 4.09 kg). Weekly hunger scores were slightly lower in the diet soda group.
The authors conclude: "These results strongly suggest that [diet sodas] can be part of an effective weight loss strategy and individuals who desire to consume them should not be discouraged from doing so because of concerns that they will undermine short-term weight loss efforts."