Katy Perry made a showstopping appearance at iHeartRadio ... Katy explained that she always begins her day with a glass of water and apple cider vinegar followed by celery juice.

This puts Perry's 'clean' food diet ... calories that support overall health and metabolism." But Katy's daily dose of apple cider vinegar and celery juice is still up for debate.

After selling out shows around the world for her upcoming The Lifetimes Tour, Katy Perry will launch an extensive run in the ...