If you intend to do a juice or water fast, there are generally three stages that your body will go through during the fast.
Stage 1 This is sometimes the hardest part for many people because the body it making changes in order to cope with the new climate it finds itself in. First off your system will change the pH of your stomach in order to make it more alkaline. It is also during this stage that the digestive tract is cleansed - some people still get daily bowel movements, while others don't get any at all. It is during this phase of the fast that you will notice the most weight loss, as water, minerals and water-soluble vitamins are excreted. Protein will also be lost - up to 75 grams a day to begin with, but then settling down to around 20 grams a day. During this stage you may experience hunger, headaches, sweating and light-headedness.
Stage 2 During the second stage, the body really gets to work and the liver starts purging itself of its chemical and toxic load. It does this by moving them to the bloodstream for later elimination. As the poisons are thrown off into the bloodstream you may experience nausea, diarrhea, muscle aches, shortness of breath, nervousness, etc. You will probably feel like you have flu. Your tongue is also likely to be coated, and you may have bad breath, body odor and skin eruptions. To you body, it is just as if you have recently ingested these poisons. Often you need to get better before you get worse.
Stage 3 This stage is where the deep tissue cleansing begins. Toxins are removed from the blood and cellular ash is flushed from the kidneys. You may get similar symptoms to stages 1 and 2 of the fast but should also get bursts of energy, as your body heals itself. During this phase your organs will be detoxifying and renewing. It's hard to know what to expect during this stage, as a lot depends on how toxic your body was to begin with. This is the stage where many people end up with vast amounts of energy, while others may feel worse than ever.
Warning Fasting should never be undertaken without access to a physician. Even if you have fasted in the past, it's not possible to be objective about your condition while you are fasting. You need to keep your own safety in mind at all times.