When you go on a crash diet you usually cut your calorie intake so much that your body 'thinks' that a time of famine has come. So it tries to save energy by slowing down your metabolism (which is opposite to what you are trying to achieve). Subjectively you feel tired, lethargic and hungry (and frustrated, of course). No one can live a happy life in a condition like that so the vast majority of people return to their previous lifestyle and gain back the weight they've lost. What's worse, after the metabolism has been slowed down by a crash diet, it may not fully return to its original level even as you resume your regular eating habits. As a result, you may gain more weight than you've lost. Then you may again go on a crash diet, lose some weight and then gain even more. This unhealthy pattern precipitated by crash diets is called weight cycling and may cause more harm than doing nothing at all.

Basic self deprivation

Basic self-deprivation is what it sounds like, i.e. when you deprive yourself of food by skipping meals, giving up foods you love, halting your meals when still hungry and so forth. This rarely works long-term. Very few people can live feeling permanently hungry or deprived. And even if they can, the quality of life may suffer.

Appetite suppression

Another approach to reducing food intake is trying to suppress appetite with drugs or supplements. It is possible to some degree, at least in the short term. One of the best known appetite suppressants is nicotine. Indeed, it is common for smokers who quit to gain weight. Many people, especially women, continue smoking partly out of the fear to gain weight. Unfortunately, the health risks of smoking far outweigh the benefits. Even if nicotine is taken by itself (e.g. as a patch), its weight control benefits, if any, are outweighed by side-effects. There are several other substances, drugs as well as botanicals, that impact the body's appetite control circuits. Each has its own profile of risks and benefits (discussed elsewhere on this site). It appears that some people get no benefit from appetite suppressants whatsoever. Others may benefit if using appetite suppressant as a part of comprehensive strategy, e.g. to jump-start a long-term weight-loss program. It is extremely rare for an appetite suppressant alone to yield long-term success.

Intelligent adjustment of lifestyle and diet composition

The best chance of reducing caloric intake is by making small, intelligently selected adjustment in your daily eating routines and lifestyle. The adjustments need to be small enough to avoid the feeling of hunger, self-deprivation, slowing of metabolism and so forth. For example, if you reduce the time you spend watching TV (or if you exercise while watching) you will almost inevitable both reduce intake and raise expenditure of calories. If you increase your intake of high-fiber foods you are likely to almost involuntarily reduce your intake of high calorie junk foods. If you don't shop when hungry you are likely to make more sensible food choices. (See also these simple steps towards weight loss.) Such small adjustments, while relatively easy to make, seem insignificant when taken separately. But when amalgamated into a set of long-term habits they can make or break your weight loss program.