'Crash diets' are what anyone trying to lose weight dreams of - a diet through which weight loss is rapid, and generally easy. Usually the term is used to refer to the situation where food consumption is simply stopped for a few days in order that the wieght is lost quickly, although often also can refer to various 'fad' diets - such as the 'cabbage soup diet, and 'low carb diet'. The lure of these diets is obvious: the period for which one has to make an effort concerning food and eating is short, yet apparently effective.
Many of these diets lead to weight loss in the short term, which fuels the hope of further weight loss, which in turn can encourage further crash dieting. However, the weight loss from a crash diet is not fat loss, but a loss primarily of water, followed by the body's short term energy reserves of glycogen. This means that weight lost can all potentially be replaced in one meal. Not only, therefore, do crash diets give an idea of weight loss, followed by failure; but they also increase the propensity to store more fat as the body's metabolic rate is lowered. During a crash diet, the body goes into 'starvation' mode: a sudden decrease in calorific intake causes a disproportionate decrease in the hormone leptin, which stimulates hunger and the storage of fat. In this way, crash diets will make it harder to lose weight in the future.
A yo-yo dieting system, such as one caused by crash dieting and subsequent weight gain, is naturally detrimental to the mental health and self esteem of the person involved. To effectively lose weight, it is necessary to consider a long term time scale: to lose one kilogram of fat, about 7000 calories must be 'burnt' off. This will take at least a week. Of course, to simply lose weight and not fat, faster diets will work; but most people on a diet want to lose fat, not lean tissue. Crash dieting, therefore, although popular, is not an effective way to lose fat, is detrimental to self esteem, and does not encourage the switch to a healthier lifestyle needed to maintain a steady weight.