Today there are several techniques to lose weight. While some are unhealthy and should not be counted in the first place, there are others which can be done but not on a routine basis, juice fasting is one of those techniques.
Even though this method is not great in the long run, but sometimes can be followed to do an absolute detox. 
A juice detox wants you to absorb fruit and vegetable juices for a day with no food consumption. This method is farthest in its path but can be practised once in a while to refine your body. Juice detoxing or juice fasting is not a good way to lose weight as it is for a short period and once you get back to regular food, you will again gain all the weight. 
During this, you can also have water and tea. While weight loss is one aim, the nutrient is another. An occasional juice fast can help get rid of toxins and lose weight for that day or two days, till you get back to normal food.
One major disadvantage of a juice refine or fast is the lack of fibre. Since most of the fibre is located in its pulp, you may end up being constipated during a juice fast. Secondly, they are farthest in nature and in no way help your body if you do them uniformly for the weight loss.

Also, see:

5 Simple methods to balance your diet

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