Unfortunately there are at least 2 different things that a person might mean when they talk about a "fast diet."  The first thing they might mean is that they want to lose weight quickly. Or, they want a diet that produces fast results. In other words, they are looking for a fast weight loss diet.
The other thing that people might be referring to is the idea of doing a fast in order to diet and lose weight. Fasting can mean that a person does not eat any food for a certain period of time, and only drinks water or in some cases only drinks juice.

Now regardless of what the person means when they say they want to do a "fast diet," either meaning for the term will produce the same outcome. This is because doing a fast really does produce rapid weight loss! Now there are a number of reasons that this idea is not very popular, especially in the eastern part of the world.

The idea of fasting is generally looked down upon in the eastern world. We tend to tell people who are sick that they need to eat. We encourage eating. But fasting is actually a natural part of being sick, and can help people to recover faster. Animals fast naturally when they are ill, without having to think about it. In our modern world, food has become too convenient. We no longer have to go catch our dinner, so when we get sick, we just keep shoveling the food in. Many of us do realize that we have an inclination to eat less though when we are ill. Fasting frees up our bodies energy in order to allow it to try and heal itself.

I do not have direct experience with water fasting and I cannot recommend it for weight loss. What I do have experience with though is with juice fasting, usually relatively short fasts of about 3 to 5 days. These have taught me a great deal about how to lose weight quickly and that is one of the biggest benefits of doing a juice fast: it will teach you how to eat right. For one thing, you will realize just how little you really need to eat. Second of all, after depriving yourself of all food for a few days, you will come to appreciate simple foods again without relying on junk food or sweets to satisfy you. A simple piece of fruit becomes a delicious feast again after you are done fasting.

This makes juice fasting much more than a simple slim fast diet where you end up returning to the same unhealthy eating habits when you are done with the diet. Juice fasting has the power to change your long term eating habits if you are mindful about it and continue to keep juice fasting every once in a while. Many people believe that fasting sounds like torture but it is actually very easy to do.

On a juice fast, your stomach might be hungry for a few hours before it realizes that you are just going to keep feeding it juice. Then it does an amazing thing: it turns itself off. It does this to conserve energy and this is what allows you to heal faster if you happen to be ill. The other extremely huge bonus from this is that you will NOT feel hungry. No one believes this at first until they actually experience it, but the fact of the matter is that anyone who goes on a juice fast will cease to have hunger by the second day. This becomes a glorious gift because it frees up a lot of mental energy and you realize how much time you used to spend obsessing over food in the past. This is part of the mental changes that occur that has the power to seriously change your relationship with food. Even if you did not lose any weight on the fast it would still benefit you because it would train you how to eat healthier by showing you how very little you really need to exist on.

As far as fast diets go you cannot get much faster than a juice fast...the only way to do anything faster is to do a water fast of course. But even this might be misleading because during a juice fast you can still exercise and burn a ton of calories, and this is where the real fast weight loss comes in at. I tend to run 6 miles per day and during a juice fast this running routine really makes the pounds drop off fast. You can't really maintain that level of exercise on a water fast so really I believe that juice fasting is the quickest way to drop weight if you can also burn about 500 extra calories each day through exercise.