Crash diets are designed to achieve a quick loss of weight. Unfortunately most of this weight loss is not the desired loss of body fat. It is mainly the loss of water and muscle tissue. As soon as you eat properly again, the water comes straight back but the loss of muscle is less forgiving. Muscle is a very metabolically active tissue, which means that your body has to burn calories just to sustain it. This means that the more muscle tissue you have, the faster your metabolism (RMR) will be, and therefore, the more calories your body has to burn just to keep you alive. If you are losing muscle through crash dieting, your metabolism is becoming slower and slower and therefor you would have to eat even less than you were before to lose weight. On top of this, your body also thinks it is roaming around in the wilderness without any food so it goes into survival mode – which is exactly what you would want it to do if you really were stuck without food for days. Wanting you to eat, your brain will trigger all of your hunger responses, making you think you are hungrier than what you are. This survival mechanism alone makes it very difficult to stick to your diet. Eventually, it all just seems too hard and you give up. But the worst is not over. Now with a slower metabolism, you quickly regain the weight that you lost. And if you keep eating the way you did before the diet (or in most cases, worse), you end up with even more body fat than when you started!
Your objective is not to lose weight, but rather to lose body fat and live according to a healthy lifestyle. Resistance exercise is the best way to increase lean muscle tissue, and at the very least prevent muscle loss, which will keep your metabolism from slowing down whilst consuming fewer calories (but not too few!).