Diets can do more harm than good if you just follow along mindlessly without considering the long-term effects. There’s all kinds of diets out right now that ask that for many days in a row, you eat nothing but the one food (cabbage soup, grapefruit), or one food group (raw vegetables and fruit), or one type of food (juice, protein).
That, or you’re encouraged to drastically cut your daily calorie intake without considering what you truly need to consume on a day-to-day to function at your best.
The promise: You'll drop pounds fast, kick-starting a long-term healthy-eating plan; you'll reset your metabolism; you'll clear out the “toxins", fats and other nasty things that are bloating you up.
The problem: When you eat only one kind of food, you fail to eat all the other kinds, so you miss out on protein, fats, fiber and other essential vitamins and nutrients. Also, when you drastically cut calories, your body adapts by learning how to do more with less. It lowers your metabolic rate, holds on desperately to fat stores and takes longer to execute basic functions like breathing; it literally slows down to survive.
The risks: After a few days, your calorie-starved body may break down muscle mass for energy. This can throw off your metabolism and make future calories (i.e., the ones you'll consume after the fast/diet) harder to burn off.
Furthermore, what are leaving your body aren't only “toxins” because your body will also naturally flush out excess nutrients and chemical substances.
Bottom line: Learn to eat nutritious, balanced meals as your main “diet". If you want to do a cleanse, (unless it is supervised by a credible professional), make sure it’s a short-term cleanse (1-3 days) that involves all kinds of nutrients and not just one or only a few kinds.
Also, if you’re going to reduce calories, consult with a credible professional to ensure you’re choosing a method that is sustainable for you, and the process in itself allows your body to not only survive, but to truly thrive for the long-term.