Well processed carbs like bread or cereal hold on to water weight through 2.9 grams of water weight held per gram of processed carbs. So cutting processed carbs and replacing with proteins or fat would cause a decrease in held water weight but not fat unless calories eaten were actually decreased. If you ate carbs in fruit or veggies you would experience a different effect where you don't hold on to water weight. The fiber and vitamins in fruit or veggies would help you too but you wouldn't lose weight if you still didn't cut your calories back.