Celebs are fans of these diets because they do work to slim you down quickly. But they work because they are so restrictive—when you cut out certain food groups from your diet, it’s hard to make up for those lost calories by eating more of other foods groups. That’s why you lose the weight: you’re eating fewer calories. Following these diets can help you kick-start your diet and motivate you to transition and stick to a more balanced, healthy diet. But following them for too long means you’ll miss out on key nutrients.