Extreme diets promise drastic and rapid weight loss using severe calorie deficits and dietary restrictions. Of course you’ll shed pounds like crazy if all you have is smoothies or cabbage soup. But in the end, it’ll only harm your health—and your weight.
Your body is clever. When you deprive it of food, it goes into survival mode to conserve energy. As soon as you start eating normally again, it gets a signal to store as much energy as possible to prepare for the next starvation period. As a result, you will gain back all the weight you've lost, and maybe even more.
Not to mention that by completely forbidding yourself from enjoying your favorite treats, such as chocolate chip cookies, you risk going on a cheating binge and gulping down the whole box…