What would be the best way to design a table in Mysql that handles read items in an RSS feed reader?
This looks like a join table on Items and Users.  You may find putting ID columns these two tables and using them in the join table works well.  Feed would be a parent for Items, and you could make the primary key if Items a compound key including Feed Id.

For the table in question you likely want indexes on:


User_Id, Feed_Id, Item_Id (Primary key searchable for user's data)
Feed_Id, Item_Id  (Foreign key from Items compound key)
User_Id  (Foreign key from Users, optional as included in the first index)