Board Thread:Questions and Answers/@comment-99.7.173.84-20130107024812/@comment-3329304-20130107130005

It is clear that a queen, like all other chiropterans need to feed blood regularly for maintaining good health. Both in anime and manga version, it has been stated that Hagi's blood awakens the chiropteran Saya, in other words, it gives her more nutrition than normal human blood. This might be because a chevalier himself receives the blood of queen. So, your both suggestions are complementarily effective.