During fasting periods, you should consume certain beverages that are almost exclusively zero calories and sugar-free. If there are added or natural sugars, you should avoid them. When you eat a piece of fruit, there is fiber and fruit juice. Fiber reduces the rate at which sugar is absorbed into the juice.
The fiber has been removed from the juice, so it will have the same effect on blood sugar as a soda or chocolate bar. This one is a bit controversial. Some people allow lemon water when fasting, however, fasting means consuming zero calories. A cup of water with half the juice of a lemon has 6 calories.