There is such a ting as a lemonade diet. It is part of a larger program known as the Master Cleanse diet. Please, of course, consult with you physician before undertaking it.
I believe what you are referring to is The Master Cleanse aka "The Lemonade Diet". I did do the Master Cleanse for 3 days. I quit because my face broke out in hives! But not to worry, I was later...
Yes, lemon is the main ingredient in lemonade. If you think about it, lemonade is the lemons' liquid point. Lemon and lemonade are both sour (depending on how much sugar is added to the lemonade).