Is crash dieting, yo-yo dieting or fasting, good for your health? I do not think so.
This is not healthy because when your weight goes up and down a lot, it can cause you to have heart problems. Quick weight loss tends to come back quickly also, and usually you end up weighing more than you did to begin with. This kind of weight gain elevates your levels of cholesterol and can also damage your vital organs.

In one study of people who went on crash diets of 500 calories or less a day 1/3 of them were found to develop gall stones.

The 14-year Farmington heart study showed that people whose weight changed a lot or often had a higher death rate than people who maintained their weight. The people who sudden or often weight changes had a greater risk of coronary heart disease. The study showed that weight fluctuation seemed to pose a great risk of heart disease and premature death.

You need to avoid crash diets and very low calorie diets because they cause the metabolism to slow down, resulting in fewer calories being burned. This happens because the body thinks it is being starved and wants to save fat to survive. It is also unhealthy to only exercise after eating more than you usually do, because this causes pulled muscles, fractures, shin splints and other such problems. This kind of dieting also weakens the immune system.

Crash diets always make you feel deprived, crazed, out of control and always hungry. This is usually followed by a knock your socks off binge. Fasting followed by overeating. Fad diets which ever is on the market followed by eating more than you should. Diet pill to not feel hungry to cravings for food you have been depriving your body of. Thus creating a vicious circle until you smarten up and eat a balanced diet on a regular basis, not depriving yourself and eating a little of each food group to give your body the vitamins and mineral it needs to keep your body healthy.
This kind of dieting can also cause mood swings.

What we do to our bodies has consequences and years of fasts and dieting and years of emotional eating have their effect. It makes you have fragile health; makes women go through premature and illness induced menopause and get osteoporosis.

What and how you eat matters, not only to the size of your body but to its well being.
Due to the theory of cause and effect, depending on the strength of ones constitution, their genetic background, their environment, and their inner life, the food thy eat will skew their biochemistry in a particular direction, which will then affect the course of their life. This is not a prediction or a fear-based assumption it is simply cause and effect.

Sources for my argument on why people should not crash diet, yo-yo diet and or fast are:

The book "When You Eat at the Refrigerator, Pull Up a Chair.
By Geneene Roth

Weight Watchers Magazine and Diet pamphlets

Everyday Health Tips
2000 Practical Hints for Better Health and Happiness
By the editors or Prevention Magazine Health Books.

Prescription for Nutritional Healing
Second edition
James F. Balch M.D.
Phyllis a Balch C.N.C

On the other hand crash diets, fasting and yo-yo dieting is better for your health than being obese.

In general overweight people do not live as long as people of normal weight, and the only way to stay at a normal weight is to diet. The increased death rate of obese people is mostly due to three types of diseases - circulartory system diseases, hypertension-high blood pressure, coronary heart disease and stroke. Other diseases contributed to being obese are diabetes and digestive systems diseases.

These kinds of diets and fasting will help people to maintain their weight once they reach their goal weight.

Fasting and dieting for short periods of time can be good for you in many cases it also helps to clean-out your system. It lowers your cholesterol when you are fasting as well as cleans out your colon. Blood purification is another good reason to fast, over time our bodies as a result of pollution in the air and chemicals in our food and water and other means build up in our bodies. The body needs to shed these toxins