Salut, hello, and questions :)

Hi everyone!
First of all, thank you so very much to all the ones posting so very inspiring messages on their successes on the fasting journey.
I have few questions :
I am starting tomorrow a 3 weeks juice fast, wish to loose something like 17/18 pds...
I wish to be still practicing sports while fasting, should i add some spirulina for example, in powder, to have some protein?!
I am a runner and swimmer.......
How much juice should i drink daily and is it of to add some barley grass powder to some grapefruit juice(only one i can make fresh...)?
I am thinking of 2 big glasses of that one and one or two in the evening of organic veggie juice....
I am very much looking froward to your advices, comments and more than welcoming any support or anyone wishing to be buddie on the journey :)
Love
M