The so called rapid weight loss diet is one of the closest things to a crash diet. One description of a rapid weight loss diet is this - A crash diet is a diet which is extreme in its deprivations - typically severely restricting calorie intake. Meant to achieve rapid weight loss, a crash diet differs from outright starvation only slightly. Crash diets are typically unhealthy and are rarely - if ever - recommended by doctors or dietitians. Crash diets may lead to malnutrition.
When someone discontinues a crash diet, the "rebound-effect" is often experienced. This causes a person to eat far more than normal, causing them to regain both the weight that was recently lost due to the diet as well as additional weight. This is caused by an evolutionary trait of the human race that historically only took effect in times of famine: after a famine ended, people's bodies naturally craved to regain both the weight that was lost, plus additional weight as well, in order to protect themselves in case of another famine.
I think it's apparent that this type of diet is not in most peoples best interest. Let's take it a little further. It has been observed that some crash dieting or rapid weight loss can lead to Anorexia. This is defined by a loss of appetite or lack of desire to eat.
When people experience appetite loss, decreased energy consumption will subsequently lead to weight loss. When inadequate calories are consumed, the body may become weaker and less able to tolerate many challenges. As body weight decreases, cachexia sets in, and a general failure to thrive may make it more difficult to fight off illness and infection. A poor response to cancer treatments, reduced quality of life, and death may result from substantial weight loss. The spiraling effect of a patient's reluctance to eat is a source of frequent anxiety for caregivers. Weight loss due to anorexia may be temporary or may continue at a life-threatening pace if the patient continues to consume inadequate energy to sustain body weight.
The most commonly known form of anorexia is satiation following the consumption of food. This can happen in all normal humans and is known as postprandial anorexia. It's important and healthy as it prevents overeating and of course obesity. Anorexia nervosa is an eating disorder characterized by voluntary starvation and exercise stress. Anorexia nervosa is a complex disease, involving psychological and sociological components.
Needless to say, it is important to ensure that if you decide to take on a diet, make sure it is healthy and can supply the nutrients people need to carry on life in a healthy and vigorous manner. One of the most highly praised diets existing to day is the Mediterranean diet mostly due to its history and the formal research that has been undertaken. It is well worth your while to look into this particular diet further if you have the slightest doubt about undertaking a rapid weight loss diet.