I’m going to give you a simple reasons that crash dieting (less than about 1200 calories per day) does not work in the long term: hormones. We’ve all known many people who have rapidly dropped the pounds only to quickly regain the same weight again. Dieting is painful and we all want to see results quickly, i.e. an immediate reward for our pain, so crash dieting is a natural for many people. Unfortunately, crash dieting negatively affects several key hormones and can sabotage your long term efforts.
Below I am going to cover three of those hormones below:
1. Thyroid. Imagine this: you try to shed the pounds by rapidly dropping calories and your body resets its baseline metabolism making it much more difficult for you to lose weight. Well, you don’t have to imagine it: this is exactly what the body does according to one study on obese subjects.  Dramatic calorie reduction resulted in significant alterations in thyroid function, particularly decreases in T3 output. The researchers tried to compensate by restoring T3 but still baseline metabolism could not be fully restored. Some experts believe that crash dieting negatively alter your baseline metabolism long term.
2. Testosterone. Significant drops in calories lead to significant drops in testosterone. Over time this can lead to muscle loss, which is never good for weight loss, self-image or your general health. See my link on Losing Weight the Right Way for more information.
3. Cortisol. Researchers have found in several studies that crash dieting scenarios and/or fasting lead to increased cortisol output.  Cortisol dampens testosterone, depletes muscle and can increase visceral (belly) fat. Again, none of this is good for weight loss, self-image or your general health in the long term. See my link on Visceral Fat for more information on the cortisol-visceral fat connection. CAUTION: Crash dieting is also associated with arrythmias likely for this same reason.
Rapid weight loss also negatively alters most of the key appetite hormones as I outline in my link on Hormones and Your Appetite. This requires crash dieters to display almost superhuman will power and can easily lead to rebound eating.
By the way, one area of anti-aging research is caloric restriction. Reducting calories by about 30 percent extends maximum lifespan in mice and many animals. (It doesn’t seem to work as well in humans.) How does caloric restriction do its magic in animals? Researchers believe it may work in mice by “feminizing” them. In fact, here is the title of one article: “Short-Term Calorie Restriction in Male Mice Feminizes Gene Expression and Alters Key Regulators of Conserved Aging Regulatory Pathways.”
Again, I ask if crash dieting really how you want to lose weight?
CAUTION: Rapid weight loss may be desireable for some very obese individuals: discuss with your doctor.
1) Metabolism, 1986 Apr, 35(4):289-91, “Decrease in resting metabolic rate during rapid weight loss is reversed by low dose thyroid hormone treatment”
2) The Journal of Clinical Endocrinology & Metabolism, Jun 1 1989, 68(6):1013-1018, “Fasting Alters Pulsatile and Rhythmic Cortisol Release in Normal Man”