"But it's not a great way to lose weight, because you'll gain it all back -- you yo-yo. It's just like the Atkins diet. The weight you lose is water weight." Slender Quick Just don't look at water fasts or juice diets as a weight-loss solution. As with the Atkins diet, restricting carbohydrates causes you to lose weight -- but you'll gain it all back, says Dillard.