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It would be OK to build towards eating a more substantive diet once there is significant improvement in the diarrhea. It is not necessary for the diarrhea to have completely resolved, but it would be better to wait until there is significant improvement.
If there is only a delay in diarrhea, but it is otherwise just as severe, it would be better to wait before advancing the diet. It would be good to be drinking fluids that have electrolytes, such as fruit juices or sports drinks, except apple juice may tend to worsen diarrhea and may contribute to the symptoms, so should be avoided.
If the significant diarrhea persists despite the Flagyl, then it may be necessary to consider further evaluation. You do not mention what evaluation has been done to this point, but persistent symptoms would be the primary reason for considering further evaluation.
If I can provide any additional information, please let me know.
Red Bull is not a sports drink. It is more of an energy drink and has a large amount of caffeine, which would not be desirable in this situation.
Common sports drinks include Gatorade and Powerade.