It's not surprising that many people wonder why fad diets are bad even though they seem to get results. You'll find many sites on the internet that claim significant weight loss in just a few days, or even 30 pounds in 30 days. That type of weight loss is always temporary. It's usually 90% water which will given right back to you as soon as your body rehydrates, which it has to do if you're not going to suffer major health problems or die.
Other fad diets are not so obviously crash diets with crazy claims, but they are usually just over hyped diet plans that are fashionable for a while and usually make a bunch of cash for the inventor in associated product sales. In the best cases, these are good nutrition plans that will help you lose some weight, but are plans that you could of probably got for free from your doctor. In the worst cases they will prove to be very difficult to follow so you'll give up after a week.
The bad of fad diets...
1. Diets that promise you quick and easy weight loss are almost always based on eating more of just one food type and none of another food type. These don't give you the benefits that you would get if you had a balanced diet. They may suggest that you take supplements, but many supplements aren't absorbed by the body unless they are taken with foods that the diet has banned. After a few weeks, if you even stick to it that long, you may soon begin to have nutritional deficiencies.
2. Fad diets are many times boring and over restrictive. After the novelty of the first day or two, you won't enjoy your meals. You'll soon start to crave food constantly and will start to break the diet. You may even feel guilty, thinking it's your fault that you aren't losing any weight.
3. Most fad diets don't follow recommendations by the American Heart Association and similar bodies for fat levels in the diet. Many times the diet will suggest high fat foods and low carbs, which if taken long term, can result in heart disease. The promoters might tell you that the diet should only be followed for a short time. But you will probably not reach your goal weight in that time, and then what do you do? You will either continue with a plan that is not good for you, or you'll stop and probably gain back the weight you lost....and possibly gain even more weight.
4. Many fad diets don't help you incorporate enough servings of fruits and vegetables in your weight loss program, or give you the variety of foods that your body needs.
5. Quick weight loss diets are just a short temporary solution and don't help you make permanent changes to your eating habits. Permanent change is the only way to remain at your target weight once you obtain it. Fad diets just encourage yo-yo diet-binge cycles of fast weight loss and equally fast weight gain. This is much worse for your health and your self esteem than if you had simply stayed overweight all the time.
Whatever the publicity materials may say, these diets won't help you in the long term. The best way to keep your weight where you want it, is to eat a varied and healthy diet, don't overeat, exercise regularly, and stay away from fad diets.