This is a tricky one, because some people who go on gluten-free diets do actually lose weight, at least at the beginning. However, cutting out gluten isn't the reason. Gluten-abstainers who lose weight probably are the ones who're cutting out processed foods that are higher in calories and fats, and eating more fruits, vegetables, legumes and lean meats. As a result, they consume fewer calories,.
But people who eliminate gluten by switching to processed gluten-free foods aren't going to get that benefit, especially if they consume gluten-free cakes, cookies and other sweets [sources: Cleveland Clinic, UW Health]. As Dr. Kelly Thomsen, a gastroenterologist at Vanderbilt University Medical Center, told ABC News in 2014: "Some gluten-free foods contain extra sugar or calories to make them more palatable – to make up for the loss of the gluten."
The misconception might be due to the fact that people with celiac disease are usually thin, but this is more because they're unable to absorb nutrients from food that contains gluten. Once they go on a gluten-free diet, they often gain weight.