You’ll lose more weight on a low-carb diet, but that is because you lose a lot of water weight
It is true that you lose an appreciable amount of water weight when you begin a low-carbohydrate diet. A quick science lesson: The hormone insulin stimulates the kidneys to retain fluid. When you cut carbs, your body produces less insulin, causing your kidneys to flush out excess water. In addition there is a decrease in the sugar stored in your muscles, which is stored with water.
Despite these additional losses from H2O, you still will lose more body fat following a low-carbohydrate diet compared to a low-fat diet. A 2010 study published in the Journal of Strength & Conditioning Research showed that adults who combined weight training and a low-carb diet lost more than 15 pounds of body fat in 12 weeks, while those on low-fat diets didn’t lose even half of that. So in actuality, you lose more weight on a low-carb diet because you are losing more fat.