Read up on the different types of vegetarians there are. Take note of the potential nutritional deficiences of each one and the way in which it can be solved.
Study the different types of gastrointestinal diseases a person could have and the prescribed diet for each disease.
Make a study guide of the different food items that correspond to a particular diet. For example, oatmeal can be served to a patient on a high fiber diet while pasta would be good for a patient on a low residue diet.
Be aware of when a patient should not eat anything due to a gastrointestinal disorder or surgical procedure.
Read up on total parenteral nutrition. Know when it should be used, its associated complications and how to deal with any such complication if they come up.
Be aware of how to insert feeding tubes and how to provide nutrition through feeding tubes. Know what precautions need to be taken before giving feedings like checking tube placement and listening for bowel sounds.